VOSviewer Manual. Nees Jan van Eck and Ludo Waltman. 23 October Manual for VOSviewer version 1.6.6

Transcription

1 VOSviewer Manual Nees Jan van Eck and Lud Waltman 23 Octber 2017 Manual fr VOSviewer versin 1.6.6

2 Table f cntents 1 Intrductin Terminlgy User interface Main panel Netwrk visualizatin Overlay visualizatin Density visualizatin Zming and scrlling Optins panel Infrmatin panel Overview panel Actin panel File tab Items tab Analysis tab Create Map wizard File types Map and netwrk files Map files Netwrk files Crpus and scres files Crpus files Scres files Thesaurus files Cluster clrs, verlay clrs, and density clrs files Cluster clrs files Overlay clrs files

3 4.4.3 Density clrs files Advanced tpics Using cmmand line parameters Making a map available nline Increasing the availability f memry References

4 1 Intrductin VOSviewer is a sftware tl fr creating maps based n netwrk data and fr visualizing and explring these maps. The functinality f VOSviewer can be summarized as fllws: Creating maps based n netwrk data. A map can be created based n a netwrk that is already available, but it is als pssible t first cnstruct a netwrk. VOSviewer can be used t cnstruct netwrks f scientific publicatins, scientific jurnals, researchers, research rganizatins, cuntries, keywrds, r terms. Items in these netwrks can be cnnected by c-authrship, cccurrence, citatin, bibligraphic cupling, r c-citatin links. T cnstruct a netwrk, data frm Web f Science, Scpus, PubMed, RIS, r Crssref JSON files can be used. Visualizing and explring maps. VOSviewer prvides three visualizatins f a map: The netwrk visualizatin, the verlay visualizatin, and the density visualizatin. Zming and scrlling functinality allws a map t be explred in full detail, which is essential when wrking with large maps cntaining thusands f items. Althugh VOSviewer is intended primarily fr analyzing biblimetric netwrks, it can in fact be used t create, visualize, and explre maps based n any type f netwrk data. VOSviewer has been develped in the Java prgramming language. Because Java is platfrm-independent, VOSviewer runs n mst hardware and perating system platfrms. VOSviewer can be dwnladed frm It can be used freely fr any purpse. This manual pertains t versin f VOSviewer. The manual is rganized as fllws. We first intrduce sme terminlgy in Chapter 2. We then discuss the user interface f VOSviewer in Chapter 3, and we explain the file types used by VOSviewer in Chapter 4. Finally, we cnsider a number f advanced tpics in Chapter 5. Fr additinal infrmatin abut VOSviewer, we refer t a paper that we have written (Van Eck & Waltman, 2010). This paper prvides a general intrductin t VOSviewer. It als discusses in cnsiderable detail the technical implementatin f specific elements f the sftware. Similar infrmatin, including a step-by-step tutrial, can als be fund in a mre recent bk chapter (Van Eck & Waltman, 2014). 3

5 2 Terminlgy When wrking with VOSviewer, it is imprtant t understand the terminlgy used by the sftware. We nw intrduce this terminlgy. Maps created, visualized, and explred using VOSviewer include items. Items are the bjects f interest. Items may fr example be publicatins, researchers, r terms. A map nrmally includes nly ne type f item. It is fr example uncmmn t have a map that includes bth publicatins and terms. Between any pair f items there can be a link. A link is a cnnectin r a relatin between tw items. Examples f links are bibligraphic cupling links between publicatins, cauthrship links between researchers, and c-ccurrence links between terms. A map nrmally includes nly ne type f link. Als, between any pair f items, there can be n mre than ne link. Each link has a strength, represented by a psitive numerical value. The higher this value, the strnger the link. The strength f a link may fr example indicate the number f cited references tw publicatins have in cmmn (in the case f bibligraphic cupling links), the number f publicatins tw researchers have c-authred (in the case f c-authrship links), r the number f publicatins in which tw terms ccur tgether (in the case f cccurrence links). Smetimes the links between items all have a strength f ne. VOSviewer then des nt shw the strength f a link. Items and links tgether cnstitute a netwrk. Hence, a netwrk is a set f items tgether with the links between the items. 1 Items may be gruped int clusters. A cluster is a set f items included in a map. 2 Clusters are nn-verlapping in VOSviewer. In ther wrds, an item may belng t nly ne cluster. Clusters d nt need t exhaustively cver all items in a map. Hence, there may be items that d nt belng t any cluster. Clusters are labeled using cluster numbers. If there is nly ne cluster, this cluster usually has cluster number 1, if there are tw clusters, these clusters usually have cluster numbers 1 and 2, and s n. 1 In the literature, a netwrk is smetimes referred t as a graph. Likewise, an item is smetimes called a nde r a vertex, a link is smetimes called an edge, and the strength f a link is smetimes called an edge weight. These terms are nt used by VOSviewer, but they may be used by ther sftware tls fr netwrk analysis and netwrk visualizatin. 2 In the literature, a cluster is smetimes referred t as a cmmunity, but this term is nt used by VOSviewer. 4

6 Items may have varius attributes in VOSviewer. If items have been assigned t clusters, cluster numbers are an example f an attribute. Of special imprtance are the weight and scre attributes. These attributes are represented by numerical values. Weight attributes are restricted t nn-negative values. Scre attributes d nt have this restrictin. A weight f an item shuld in sme way indicate the imprtance f the item. An item with a higher weight is regarded as mre imprtant than an item with a lwer weight. 3 In the visualizatin f a map, items with a higher weight are shwn mre prminently than items with a lwer weight. A scre attribute may indicate any numerical prperty f items. 4 Hwever, since weight attributes are already used t indicate the imprtance f items, it is recmmended t use scre attributes t indicate ther prperties. Scre attributes are cnsidered nly in the verlay visualizatin f a map (see Subsectin 3.1.2), nt in the netwrk visualizatin and the density visualizatin. Items may have multiple weight and multiple scre attributes. The weight and scre attributes that are used in the visualizatin f a map can then be selected in the ptins panel f VOSviewer (see Sectin 3.2). There are tw standard weight attributes, referred t as the Links attribute and the Ttal link strength attribute. Fr a given item, the Links and Ttal link strength attributes indicate, respectively, the number f links f an item with ther items and the ttal strength f the links f an item with ther items. Fr example, in the case f c-authrship links between researchers, the Links attribute indicates the number f c-authrship links f a given researcher with ther researchers. The Ttal link strength attribute indicates the ttal strength f the c-authrship links f a given researcher with ther researchers. In additin t the standard Links and Ttal link strength attributes, items may als have custm weight attributes. Custm weight attributes are discussed in Subsectin Mre precisely, VOSviewer assumes weight attributes t have a rati scale. In ther wrds, if an item has a weight that is twice as high as the weight f anther item, the frmer item is regarded as twice as imprtant as the latter item. 4 Unlike weight attributes, scre attributes are nt assumed t have a rati scale. Scre attributes may als have an interval r an rdinal scale. In ther wrds, the minimum requirement fr scre attributes is that it makes sense t cmpare tw items based n whether ne item has a higher scre than the ther item. 5

7 3 User interface The main windw f VOSviewer is shwn in Figure 1. It cnsists f the fllwing five panels: Main panel. This panel presents a visualizatin f the currently active map. Zm and scrll functinality can be used t explre the map in full detail. Optins panel. This panel can be used t make adjustments t the visualizatin f the currently active map presented in the main panel. Infrmatin panel. This panel presents descriptins f items in the currently active map. Overview panel. This panel presents an verview f the currently active map. A rectangular frame indicates the area in the map that is shwn in the main panel. Actin panel. This panel can be used t perfrm different kinds f actins, such as creating a new map, pening r saving an existing map, making a screensht, and updating the layut r the clustering f a map. VOSviewer prvides three visualizatins, referred t as the netwrk visualizatin, the verlay visualizatin, and the density visualizatin. As can be seen in Figure 1, the Netwrk Visualizatin, Overlay Visualizatin, and Density Visualizatin tabs in the main windw f VOSviewer can be used t switch between the visualizatins. A status bar is available at the bttm f the main windw f VOSviewer. The status bar prvides infrmatin abut the currently active map. It shws the number f items in the map, the number f clusters t which the items have been assigned, the number f links between the items, and the ttal strength f the links. Mrever, when the muse pinter is mved ver an item in the main panel, the status bar prvides infrmatin abut this item. Likewise, when the muse pinter is mved ver a link between tw items, the status bar prvides infrmatin abut this link. In the next sectins, the five panels in the main windw f VOSviewer are discussed in mre detail. 6

8 Figure 1. Main windw f VOSviewer. The numbers designate (1) the main panel, (2) the ptins panel, (3) the infrmatin panel, (4) the verview panel, and (5) the actin panel. 3.1 Main panel As can be seen in Figure 1, the main panel presents a visualizatin f the currently active map. Zm and scrll functinality can be used t determine the area in the map that is shwn in the main panel. Three visualizatins are available in the main panel: The netwrk visualizatin, the verlay visualizatin, and the density visualizatin. T illustrate the different visualizatins, we use a map f a c-citatin netwrk f 232 jurnals in the fields f ecnmics, management, and peratins research (fr mre details, see Van Eck & Waltman, 2010). The map is available in the files jurnal_map.txt and jurnal_netwrk_sparse.txt, which are distributed tgether with VOSviewer. T pen the map in VOSviewer, click the Open buttn n the File tab in the actin panel, select the VOSviewer map file jurnal_map.txt and the VOSviewer netwrk file jurnal_netwrk_sparse.txt, and click the OK buttn. 7

9 3.1.1 Netwrk visualizatin In the netwrk visualizatin, items are represented by their label and by default als by a circle. The size f the label and the circle f an item is determined by the weight f the item. The higher the weight f an item, the larger the label and the circle f the item. Fr sme items the label may nt be displayed. This is dne in rder t avid verlapping labels. The clr f an item is determined by the cluster t which the item belngs. Lines between items represent links. By default, at mst 500 lines are displayed, representing the 500 strngest links between items. An example f the netwrk visualizatin is shwn in Figure 2. The distance between tw jurnals in the visualizatin apprximately indicates the relatedness f the jurnals in terms f c-citatin links. In general, the clser tw jurnals are lcated t each ther, the strnger their relatedness. The strngest c-citatin links between jurnals are als represented by lines. Figure 2. Netwrk visualizatin Overlay visualizatin The verlay visualizatin is identical t the netwrk visualizatin except that items are clred differently. There are tw ways in which items can be clred in the 8

10 verlay visualizatin. If items have scres, the clr f an item is determined by the scre f the item, where by default clrs range frm blue (lw scre) t green (average scre) t red (high scre). On the ther hand, if items have user-defined clrs (specified using the red, green, and blue clumns in a VOSviewer map file; see Subsectin 4.1.1), the clr f an item is determined by the user-defined clr f the item. If items have neither scres nr user-defined clrs, the verlay visualizatin is nt available. An example f the verlay visualizatin is shwn in Figure 3. A clr bar is shwn in the bttm right crner f the visualizatin. The clr bar is shwn nly if clrs are determined by scres f items. The clr bar indicates hw scres are mapped t clrs. In the verlay visualizatin shwn in Figure 3, clrs indicate impact factrs f jurnals. Fr example, jurnals clred blue have an impact factr belw 1.0, jurnals clred green have an impact factr arund 1.5, and jurnals clred red have an impact factr abve 2.5. Figure 3. Overlay visualizatin. 9

11 3.1.3 Density visualizatin There are tw variants f the density visualizatin. We first discuss the item density visualizatin, fllwed by the cluster density visualizatin. The Item density and Cluster density radi buttns in the ptins panel can be used t switch between the tw variants f the density visualizatin. We refer t Van Eck and Waltman (2010) fr a discussin f the technical implementatin f the density visualizatin. In the item density visualizatin, items are represented by their label in a similar way as in the netwrk visualizatin and the verlay visualizatin. Each pint in the item density visualizatin has a clr that indicates the density f items at that pint. By default, clrs range frm blue t green t red. The larger the number f items in the neighbrhd f a pint and the higher the weights f the neighbring items, the clser the clr f the pint is t red. The ther way arund, the smaller the number f items in the neighbrhd f a pint and the lwer the weights f the neighbring items, the clser the clr f the pint is t blue. An example f the item density visualizatin is shwn in Figure 4. Figure 4. Item density visualizatin. 10

12 The cluster density visualizatin is available nly if items have been assigned t clusters. The cluster density visualizatin is similar t the item density visualizatin except that the density f items is displayed separately fr each cluster f items. In the cluster density visualizatin, the clr f a pint in the visualizatin is btained by mixing the clrs f different clusters. The weight given t the clr f a certain cluster is determined by the number f items belnging t that cluster in the neighbrhd f the pint. Like in the item density visualizatin, the weight f an item is taken int accunt as well. An example f the cluster density visualizatin is shwn in Figure 5. Figure 5. Cluster density visualizatin Zming and scrlling T facilitate the detailed explratin f a map, VOSviewer ffers zm and scrll functinality. In the main panel, zming and scrlling can be dne in the fllwing three ways: Using the muse. T zm in, mve the muse upwards while keeping the right muse buttn pressed. The ther way arund, t zm ut, mve the muse dwnwards while keeping the right muse buttn pressed. As an alternative, 11

13 the muse wheel can be used t zm in and ut. T scrll thrugh a map, mve the muse while keeping the left muse buttn pressed. Using the navigatin buttns in the tp right crner f the main panel (see Figure 1). Use the plus and minus buttns t zm in and ut. Use the arrw buttns t scrll thrugh a map. Using the keybard. Use the plus and minus keys t zm in and ut. Use the arrw keys t scrll thrugh a map. 3.2 Optins panel The ptins panel can be used t make adjustments t the visualizatin f the currently active map presented in the main panel. The netwrk visualizatin, the verlay visualizatin, and the density visualizatin each prvide smewhat different ptins. Sme f the ptins are nt always available. The ptins panel shws nly the ptins that are relevant fr the currently active map. When the netwrk visualizatin r the verlay visualizatin is selected, the fllwing ptins may be available: Visualizatin. Scale. This slider determines the size f the labels f items in the visualizatin f the currently active map. It als determines the thickness f the lines used t display links between items. Weights. When items have multiple weight attributes, the Weights drp dwn list determines the currently selected weight attribute. This weight attribute then determines hw prminently items are shwn in the visualizatin f the currently active map. The higher the weight f an item, the larger the label f the item in the visualizatin. Scres. This drp dwn list is available nly if the verlay visualizatin is selected. When items have multiple scre attributes, the Scres drp dwn list determines the currently selected scre attribute. This scre attribute then determines the clring f items in the visualizatin f the currently active map. By default, the higher (lwer) the scre f an item, the clser the clr f the item is t red (blue). Labels. Size variatin. The higher the weight f an item, the larger the label f the item in the visualizatin f the currently active map. The Size variatin slider determines the strength f this effect. 12

14 Circles and Frames. These radi buttns determine hw items are represented in the visualizatin f the currently active map. If the Circles radi buttn is selected, items are represented by their label and by a circle. If the Frames radi buttn is selected, items are represented by their label displayed within a rectangular frame. Max. length. This text bx determines the maximum length f a label displayed in the visualizatin f the currently active map. If the length f the label f an item exceeds the maximum length, nly the first part f the label is displayed. Fnt. This drp dwn list determines the fnt that is used t display the labels f items in the visualizatin f the currently active map. The availability f fnts in the Fnt drp dwn list depends n the use f CJKV (Chinese, Japanese, Krean, and Vietnamese) characters in the labels f items. If the labels f items d nt include CJKV characters, a number f fnts are available, with the Open Sans fnt being selected by default. If the labels f items d include CJKV characters, nly the SansSerif fnt is available. The SansSerif fnt ensures that CJKV characters are displayed crrectly. Lines. Size variatin. The strnger the link between tw items, the thicker the line that is used t display the link in the visualizatin f the currently active map. The Size variatin slider determines the strength f this effect. Min. strength and Max. lines. In the visualizatin f the currently active map, links between items can be displayed using lines. The Min. strength and Max. lines text bxes determine, respectively, the minimum strength f links displayed in the visualizatin and the maximum number f links displayed in the visualizatin. If the number f links that have the required minimum strength exceeds the maximum number f links, nly the strngest links are displayed. Clred lines. This check bx determines whether links are displayed using gray lines r clred lines. Curved lines. This check bx determines whether links are displayed using straight lines r curved lines. 13

15 Clrs. Cluster Clrs. This buttn is available nly if the netwrk visualizatin is selected. The buttn ffers fur ptins: Edit. This ptin is the default chice. Chse this ptin t edit the current cluster clrs in the Edit Cluster Clrs dialg bx. Imprt. Chse this ptin t imprt cluster clrs frm a cluster clrs file (see Subsectin ). Exprt. Chse this ptin t exprt the current cluster clrs t a cluster clrs file (see Subsectin 4.4.1). Restre riginal. Chse this ptin t restre the riginal cluster clrs. Overlay Clrs. This buttn is available nly if the verlay visualizatin is selected and if items have scres. The buttn ffers fur ptins: Set range. This ptin is the default chice. Chse this ptin t bring up the Set Overlay Clrs Range dialg bx. This dialg bx can be used t change the minimum and the maximum scre that determine hw the scres f items are mapped t clrs. By default, scres less than r equal t the minimum scre are mapped t blue, scres equal t the average f the minimum and the maximum scre are mapped t green, and scres greater than r equal t the maximum scre are mapped t red. The Set Overlay Clrs Range dialg bx can als be used t determine hw the scres f items are nrmalized. By default, n nrmalizatin is applied. If the Divide by mean ptin is selected in the Nrmalize scres drp dwn list, the scres f items are nrmalized by dividing each scre by the mean scre f all items. 5 If the Subtract mean ptin is selected, the scres f items are nrmalized by subtracting frm each scre the mean scre f all items. Finally, if the Subtract mean and divide by st. dev. ptin is selected, the scres f items are nrmalized by first subtracting frm each scre the mean scre f all items and by then dividing by the standard deviatin f the scres f all items (i.e., a z-scre nrmalizatin). Wrking with 5 This ptin is available nly if the scres f items all have the same sign. In ther wrds, the scres f items must either all be nn-negative r all be nn-psitive. 14

16 nrmalized scres is an advanced feature f VOSviewer that shuld be used nly in special cases. Fr mst purpses, we recmmend nt t nrmalize scres. Imprt. Chse this ptin t imprt verlay clrs frm an verlay clrs file (see Subsectin ). Exprt. Chse this ptin t exprt the current verlay clrs t an verlay clrs file (see Subsectin 4.4.2). Restre riginal. Chse this ptin t restre the riginal verlay clrs. Black backgrund. This check bx determines whether the visualizatin f the currently active map has a white r a black backgrund clr. When the density visualizatin is selected, the fllwing ptins may be available: Visualizatin. Scale. This slider determines the size f the labels f items in the visualizatin f the currently active map. Weights. When items have multiple weight attributes, the Weights drp dwn list determines the currently selected weight attribute. This weight attribute then determines hw prminently items are shwn in the visualizatin f the currently active map. The higher the weight f an item, the larger the label f the item in the visualizatin. The selected weight attribute is als used in the calculatin f the density f items. Labels. These ptins are identical t the ptins prvided when the netwrk visualizatin r the verlay visualizatin is selected (except that the Circles and Frames radi buttns are nt available). Density. Kernel width. This slider determines the value f the kernel width parameter. We refer t Van Eck and Waltman (2010) fr mre infrmatin abut this parameter. Item density and Cluster density. These radi buttns determine whether the item density visualizatin r the cluster density visualizatin is selected. 15

17 Clrs. Density Clrs. This buttn is available nly if the item density visualizatin is selected. The buttn ffers three ptins: Imprt. This ptin is the default chice. Chse this ptin t imprt density clrs frm a density clrs file (see Subsectin 4.4.3). Exprt. Chse this ptin t exprt the current density clrs t a density clrs file (see Subsectin 4.4.3). Restre riginal. Chse this ptin t restre the riginal density clrs. Cluster Clrs. This buttn is available nly if the cluster density visualizatin is selected. The buttn ffers fur ptins: Edit. This ptin is the default chice. Chse this ptin t edit the current cluster clrs in the Edit Cluster Clrs dialg bx. Imprt. Chse this ptin t imprt cluster clrs frm a cluster clrs file (see Subsectin 4.4.1). Exprt. Chse this ptin t exprt the current cluster clrs t a cluster clrs file (see Subsectin 4.4.1). Restre riginal. Chse this ptin t restre the riginal cluster clrs. White backgrund. This check bx is available nly if the cluster density visualizatin is selected. The check bx determines whether the cluster density visualizatin has a black r a white backgrund clr. 3.3 Infrmatin panel The infrmatin panel is available nly if items have a descriptin attribute. Descriptin attributes may fr example have been made available using the descriptin clumn in a VOSviewer map file (see Subsectin 4.1.1). The infrmatin panel presents descriptins f items in the currently active map. When the muse pinter is mved ver an item in the main panel, the descriptin f the item is shwn in the infrmatin panel. 3.4 Overview panel The verview panel presents an verview f the currently active map. Each item in the map is represented by a small clred dt. A rectangular frame is displayed in 16

18 the verview panel t indicate the area in the currently active map that is shwn in the main panel. By left-clicking in the verview panel, it is pssible t mve t a different area in the currently active map. This new area is then shwn in the main panel. 3.5 Actin panel The actin panel can be used t perfrm different kinds f actins. The panel cnsists f three tabs: The File tab, the Items tab, and the Analysis tab. These tabs are discussed in Subsectins 3.5.1, 3.5.2, and The Create Map wizard, which can be accessed frm the File tab, plays an imprtant rle in VOSviewer. This wizard is discussed separately in Subsectin File tab The File tab can be used t perfrm a number f basic actins. The fllwing buttns are available n the File tab: Map. Create. Use this buttn t create a new map. The buttn brings up the Create Map wizard. This wizard is discussed in detail in Subsectin Open. Use this buttn t pen an existing map. The buttn brings up the Open Map dialg bx. T pen a map, a VOSviewer map file and ptinally als a VOSviewer netwrk file (see Sectin 4.1) can be prvided. Alternatively, a GML file r Pajek files can be prvided. 6 Save. Use this buttn t save the currently active map. The buttn brings up the Save Map dialg bx. A map can be saved in a VOSviewer map file and a VOSviewer netwrk file (see Sectin 4.1). Alternatively, a map can be saved in a GML file r in Pajek files. Screensht. This buttn ffers fur ptins: Save. This ptin is the default chice. Chse this ptin t save a screensht f the visualizatin f the currently active map presented in the main panel. The screensht resembles the visualizatin f the 6 GML (graph mdeling language) files are supprted by varius sftware tls fr netwrk analysis and netwrk visualizatin, fr example by Gephi, a ppular sftware tl fr netwrk visualizatin available at Pajek is a well-knwn sftware tl fr scial netwrk analysis (De Ny, Mrvar, & Batagelj, 2011). It is available at VOSviewer supprts Pajek netwrk, matrix, partitin, and vectr files. 17

19 currently active map as clsely as pssible. Hwever, if the Optimize labeling check bx in the Screensht Optins dialg bx (see belw) is checked, the visibility f labels in the screensht is ptimized. This means that sme labels nt visible in the visualizatin f the currently active map may be visible in the screensht. Screenshts can be saved in a number f graphic file frmats. Fr mst purpses, we recmmend the PNG frmat. Sme frmats, such as EPS, PDF, and SVG, use vectr graphics t save a screensht. This has the advantage that the screensht can be resized withut lss f quality. Print. Chse this ptin t print a screensht f the visualizatin f the currently active map presented in the main panel. Cpy t clipbard. Chse this ptin t cpy a screensht f the visualizatin f the currently active map presented in the main panel. The screensht is cpied t the clipbard and can fr example be pasted int a Wrd dcument r a PwerPint presentatin. Optins. Chse this ptin t bring up the Screensht Optins dialg bx. This dialg bx can be used t change sme settings related t screenshts. The Scaling drp dwn list determines the reslutin (i.e., the number f pixels) f a screensht. The reslutin is calculated relative t the reslutin f the main panel. Using a scaling f 100%, screenshts have the same reslutin as the main panel. Using the default scaling f 200%, screenshts have a reslutin that is twice as high (i.e., twice as many pixels hrizntally and vertically) as the reslutin f the main panel. The Scaling drp dwn list has n effect n screenshts that are saved in a file frmat that uses vectr graphics. The Optimize labeling check bx determines whether the visibility f labels in a screensht is ptimized. Optimizing the visibility f labels means that sme labels nt visible in the visualizatin f the currently active map presented in the main panel may be visible in a screensht. The Include brder check bx determines whether a brder is included arund a screensht. Inf. Manual. Use this buttn t pen the VOSviewer manual. This requires an internet cnnectin. 18

20 Abut VOSviewer. This buttn brings up the Abut VOSviewer dialg bx. This dialg bx prvides infrmatin abut VOSviewer. It shws the versin number f VOSviewer, a cpyright ntice, a license text, a link t the VOSviewer website, a list f sftware libraries used by VOSviewer, the versin number f Java, and the amunt f memry that is used by VOSviewer Items tab The Items tab presents a list f items in the currently active map. By default, a list f all items in the map is presented. Hwever, a filter can be used t restrict the list t a subset f the items in the map. T d s, enter a filter string in the Filter text bx. This yields a list f all items with a label that cntains the filter string. The Grup items by cluster check bx determines hw items are listed. If the check bx is unchecked, items are simply listed alphabetically. If the check bx is checked, items are first gruped by cluster and then listed alphabetically within each cluster. By duble-clicking n an item n the Items tab, it is pssible t zm in n the item in the visualizatin f the currently active map presented in the main panel Analysis tab The Analysis tab can be used t update the layut and the clustering f the currently active map. This is dne using the VOS layut technique and the VOS clustering technique. 7 The Analysis tab can als be used t change the parameters f these techniques. The fllwing ptins are available n the Analysis tab: Nrmalizatin. Use the nrmalizatin methd drp dwn list t determine hw the strength f the links between items is nrmalized. Nrmalized link strengths are used as input fr the VOS layut technique and the VOS clustering technique. The fllwing ptins are available in the nrmalizatin methd drp dwn list: N nrmalizatin. If this ptin is selected, n nrmalizatin is perfrmed. We generally d nt recmmend this ptin. 7 Tgether these tw techniques prvide a unified framewrk fr layut and clustering. We refer t Van Eck, Waltman, Dekker, and Van den Berg (2010), Waltman, Van Eck, and Nyns (2010), Waltman and Van Eck (2013), and Van Eck and Waltman (2014) fr mre infrmatin abut these techniques. 19

21 Assciatin strength. If this ptin is selected, the assciatin strength methd is used fr nrmalizing the strength f the links between items. Apart frm a multiplicative cnstant, this methd is identical t Eq. (6) in Van Eck and Waltman (2009). This ptin is selected by default. Fractinalizatin. If this ptin is selected, the fractinalizatin methd is used fr nrmalizing the strength f the links between items. Apart frm a multiplicative cnstant, this methd is identical t Eq. (13) in Van Eck and Waltman (2009). LinLg/mdularity. If this ptin is selected, nrmalizatin is perfrmed in the same way as in the LinLg layut technique and the mdularity clustering technique. Fr mre infrmatin abut these techniques, we refer t Newman (2004) and Nack (2007, 2009). Layut. Attractin and Repulsin. These parameters influence the way in which items are lcated in a map by the VOS layut technique. The Attractin parameter must have an integer value between -9 and +10. The Repulsin parameter must have an integer value between -10 and +9. The value f the Repulsin parameter must be lwer than the value f the Attractin parameter. Fr mst purpses, ur recmmendatin is t set the Attractin and Repulsin parameters t values f, respectively, 2 and 1. Values f 2 and 0 r values f 1 and 0 smetimes als yield gd results. Use default values. This check bx determines whether default values f the attractin and repulsin parameters are used when a new map is created using the Create Map wizard (see Subsectin ). If the check bx is checked, default parameters values are used. These default parameters values depend n the type f map that is created. 8 If the check bx is nt checked, the parameter values specified in the Attractin and Repulsin text bxes are used. Advanced Parameters. This buttn brings up the Advanced Layut Parameters dialg bx. This dialg bx can be used t change a number f 8 The repulsin parameter has a default value f -1 when creating a map f a c-authrship netwrk f authrs. It has a default value f 0 when creating a map f a c-ccurrence netwrk f keywrds r a map f a citatin netwrk f dcuments. In all ther cases, the repulsin parameter has a default value f 1. The attractin parameter always has a default value f 2. 20

22 mre advanced parameters f the VOS layut technique. The fllwing parameters are available: Randm starts. This parameter determines the number f times the ptimizatin algrithm f the VOS layut technique is run. Each time the ptimizatin algrithm is run, a different layut may be btained. The best layut btained in all runs f the ptimizatin algrithm is used as the final layut. The higher the value f the Randm starts parameter, the higher the quality f the final layut that is btained. Max. iteratins. This parameter determines the maximum number f iteratins perfrmed by the ptimizatin algrithm f the VOS layut technique. The higher the value f the parameter, the higher the quality f the layut that is btained. In general, the default value f the parameter wrks well and des nt need t be changed. Initial step size, Step size reductin, and Step size cnvergence. These are technical parameters f the ptimizatin algrithm f the VOS layut technique. The parameters must have values between and 1. In general, the default values f the parameters wrk well and d nt need t be changed. Randm seed. This parameter determines the seed f the randm number generatr used by the ptimizatin algrithm f the VOS layut technique. The seed must be a nn-negative integer. Update Layut. This buttn is available nly if infrmatin is available abut the links between the items in the currently active map. Use this buttn t update the layut f the currently active map using the VOS layut technique. Clustering. Reslutin. This parameter determines the level f detail f the clustering prduced by the VOS clustering technique. The parameter must have a nnnegative value. The higher the value f the parameter, the larger the number f clusters prduced by the VOS clustering technique. We recmmend t try ut different values fr the Reslutin parameter and t use the value that yields the mst apprpriate level f detail fr ne s purpses. Min. cluster size. This parameter determines the minimum size f clusters prduced by the VOS clustering technique. Each cluster prduced by the 21

23 VOS clustering technique is expected t include at least the minimum number f items specified by this parameter. The Min. cluster size parameter can be used t simplify the clustering results btained frm the VOS clustering technique by getting rid f small and uninteresting clusters. Merge small clusters. This check bx determines hw the VOS clustering technique handles small clusters. Small clusters are clusters that d nt have the minimum cluster size specified by the Min. cluster size parameter. If the check bx is checked, small clusters are merged int larger clusters. If the check bx is nt checked, small clusters are discarded and their items have n cluster assignment. Advanced Parameters. This buttn brings up the Advanced Clustering Parameters dialg bx. This dialg bx can be used t change a number f mre advanced parameters f the VOS clustering technique. The fllwing parameters are available: Randm starts. This parameter determines the number f times the ptimizatin algrithm f the VOS clustering technique is run. Each time the ptimizatin algrithm is run, a different clustering may be btained. The best clustering btained in all runs f the ptimizatin algrithm is used as the final clustering. The higher the value f the Randm starts parameter, the higher the quality f the final clustering that is btained. Iteratins. This parameter determines the number f iteratins perfrmed by the ptimizatin algrithm f the VOS clustering technique. The higher the value f the parameter, the higher the quality f the clustering that is btained. In general, the default value f the parameter wrks well and des nt need t be changed. Randm seed. This parameter determines the seed f the randm number generatr used by the ptimizatin algrithm f the VOS clustering technique. The seed must be a nn-negative integer. Update Clustering. This buttn is available nly if infrmatin is available abut the links between the items in the currently active map. Use this buttn t update the clustering f the currently active map using the VOS clustering technique. 22

24 Rtate/flip. Rtate. Use this buttn t rtate the currently active map. The Degrees t rtate parameter determines the number f degrees by which the map is rtated. Flip Hrizntally. Use this buttn t flip the currently active map in hrizntal directin. Flip Vertically. Use this buttn t flip the currently active map in vertical directin Create Map wizard The Create Map wizard can be used t create a new map. There are a number f ways in which a new map can be created. The Create Map wizard ffers a guide thrugh the varius ptins in a number f steps. A summary f the main ptins prvided by the wizard is presented in Table 1. 9 As can be seen in Table 1, a map can be created based n netwrk data, bibligraphic data, r text data. The chice between these three types f data is made in the first step f the Create Map wizard, as shwn in Figure 6. When a map is created based n netwrk data, VOSviewer needs t be prvided with infrmatin abut the items in the netwrk and the links between the items. This infrmatin can be made available in a VOSviewer netwrk file, pssibly cmplemented with a VOSviewer map file (see Sectin 4.1). Alternatively, the infrmatin can be made available in a GML file r in Pajek files. There are n restrictins n the nature f the items and the links in a netwrk. The items and the links will typically be f a biblimetric nature, but this is nt a requirement. 9 The terminlgy used in Table 1 is identical t the terminlgy that is used in the Create Map wizard. Elsewhere in this manual, we smetimes use a different terminlgy. Dcuments may be referred t as publicatins, surces as jurnals, and authrs as researchers. 23

25 Table 1. Summary f the main ptins prvided by the Create Map wizard fr creating a new map. Data Files Links Items Netwrk data VOSviewer files GML files Pajek files Bibligraphic data Web f Science files C-authrship Authrs Scpus files Organizatins PubMed files Cuntries RIS files C-ccurrence Keywrds Crssref JSON files Citatin Dcuments Surces Authrs Organizatins Cuntries Bibligraphic cupling Dcuments Surces Authrs Organizatins Cuntries C-citatin Cited references Cited surces Cited authrs Text data VOSviewer files C-ccurrence Terms Web f Science files Scpus files PubMed files RIS files Crssref JSON files 24

26 Figure 6. First step f the Create Map wizard. When a map is created based n bibligraphic data r text data, first a netwrk is cnstructed based n the data that is made available t VOSviewer and then a map is created based n the cnstructed netwrk. As can be seen in Table 1, many different types f netwrks can be cnstructed. Text data can be used t cnstruct a netwrk f c-ccurrence links between terms. As discussed in mre detail belw, terms are identified in the text data using natural language prcessing algrithms. Bibligraphic data can be used t cnstruct a netwrk f c-authrship, cccurrence, citatin, bibligraphic cupling, r c-citatin links. 10 Depending n the type f link, different types f items are supprted, as shwn in Table 1. Bibligraphic data can be made available t VOSviewer in Web f Science, Scpus, PubMed, RIS, r Crssref JSON files. These files can als be used t make text data available t VOSviewer. Text data is extracted frm the title and/r abstract fields 10 A citatin link is a link between tw items where ne item cites the ther. Citatin links are treated as undirected by VOSviewer. Hence, n distinctin is made between a citatin frm item A t item B and a citatin in the ppsite directin. A bibligraphic cupling link is a link between tw items that bth cite the same dcument. A c-citatin link is a link between tw items that are bth cited by the same dcument. 25

27 in these files. Alternatively, text data can be made available in a VOSviewer crpus file, pssibly cmplemented with a VOSviewer scres file (see Sectin 4.2). Belw, a number f tpics related t the Create Map wizard are discussed in mre detail. File types fr bibligraphic data VOSviewer supprts five file types fr bibligraphic data: Web f Science files, Scpus files, PubMed files, RIS files, and Crssref JSON files. We nw discuss the use f each f these file types in mre detail. Web f Science can be accessed at A subscriptin is required. When dwnlading data frm Web f Science, make sure that the Web f Science Cre Cllectin database is selected. Chse the Save t Other File Frmats ptin, and chse either the Plain Text r the Tab-delimited file frmat. Althugh VOSviewer supprts bth file frmats, we recmmend the use f the tab-delimited frmat. When asked which data elements t dwnlad, chse the Full Recrd and Cited References ptin. Dwnlading cited reference data is necessary fr identifying citatin, bibligraphic cupling, and c-citatin links between items. Scpus can be accessed at A subscriptin is required. T dwnlad data frm Scpus, chse the CSV exprt ptin. (D nt chse the Dwnlad ptin!) Make sure that the data is dwnladed in a CSV file and that all data elements are included. PubMed can be accessed at T dwnlad data frm PubMed, chse the Send t ptin, chse File as the destinatin, and chse MEDLINE as the file frmat. Data dwnladed frm PubMed cannt be used fr identifying citatin, bibligraphic cupling, and c-citatin links between items. Hence, when wrking with PubMed data, sme ptins in the Create Map wizard will nt be available. The RIS file frmat is a general file frmat fr bibligraphic data. It is supprted by a large number f sftware tls, such as EndNte, Mendeley, and Publish r Perish. Like PubMed data, data in a RIS file cannt be used fr identifying citatin, bibligraphic cupling, and c-citatin links between items. Crssref can be accessed thrugh an applicatin prgramming interface (API). The dcumentatin f this API is available at Data can be dwnladed thrugh the API in a Crssref JSON file. Instead f wrking directly with the Crssref API, it is als pssible t prvide a set f DOIs t 26

28 VOSviewer. VOSviewer will then use these DOIs t dwnlad data fr the crrespnding dcuments thrugh the Crssref API. An example f the use f Crssref data in VOSviewer is presented in a blg pst available at This blg pst als highlights sme limitatins f Crssref data. Finally, we nte that Web f Science and Scpus impse dwnlad restrictins. Files dwnladed frm Web f Science and Scpus may cntain data fr at mst 500 and 2000 dcuments, respectively. In rder t wrk with data fr a larger number f dcuments, the data needs t be dwnladed in multiple batches. Each batch is saved in a separate file. T facilitate wrking with data dwnladed in multiple batches, VOSviewer is able t handle multiple Web f Science files r multiple Scpus files when creating a map. Similarly, VOSviewer is als able t handle multiple Crssref JSON files. This is cnvenient because in a single call t the Crssref API data can be dwnladed fr at mst 1000 dcuments. Thesaurus files When a map is created based n bibligraphic data r text data, there ften is a need t perfrm data cleaning. A VOSviewer thesaurus file can be used fr this purpse. When creating a map based n bibligraphic data, a VOSviewer thesaurus file can be used t merge different variants f a surce title, an authr name, an rganizatin name, a cuntry name, r a cited reference. This may fr example be useful when the name f a researcher is written in different ways in different dcuments (e.g., with the first initial nly r with all initials). A VOSviewer thesaurus file can then be used t indicate that different names in fact refer t the same researcher. When creating a map based n text data, a VOSviewer thesaurus file can be used t merge terms. This may be useful nt nly fr merging synnyms (e.g., h-index and Hirsch index ), but als fr crrecting spelling differences (e.g., behavir and behaviur ). In additin, it may als be useful fr merging abbreviated terms with full terms (e.g., JIF and jurnal impact factr ). A thesaurus file can als be used t ignre terms. Fr example, when wrking with titles and abstracts f scientific publicatins, ne may want t ignre general terms such as cnclusin, methd, and result. We refer t Sectin 4.3 fr a technical discussin f VOSviewer thesaurus files. 27

29 Full cunting vs. fractinal cunting When a map is created based n bibligraphic data, the Create Map wizard ffers a chice between tw cunting methds. By default, full cunting is used. The alternative is t use fractinal cunting. T illustrate the difference between the tw cunting methds, we cnsider an example in which a c-authrship netwrk is cnstructed. As shwn in Figure 7, we have fur authrs, labeled A1, A2, A3, and A4, and three dcuments, labeled D1, D2, and D3. D1 is authred by A1, A2, and A3, D2 is authred by A1 and A3, and D3 is authred by A2 and A4. The netwrks cnstructed using full and fractinal cunting are presented in Figure 8. We nte that the nly difference between the tw netwrks is in the strength f the links. Figure 7. Authrship links between fur authrs and three dcuments. Figure 8. C-authrship netwrk cnstructed using full (left) r fractinal (right) cunting. In the full cunting netwrk, the link between A1 and A3 has a strength f 2. This indicates that A1 and A3 have c-authred tw dcuments, namely D1 and D2. 28

30 The ther links have a strength f 1, indicating that fr each f these links the assciated authrs have c-authred ne dcument. The idea f fractinal cunting is t reduce the influence f dcuments with many authrs. When fractinal cunting is used, the strength f a c-authrship link between tw authrs is determined nt nly by the number f dcuments cauthred by the authrs but als by the ttal number f authrs f each f the cauthred dcuments. In the case f fractinal cunting, when an authr has cauthred a dcument with n ther authrs, this yields a strength f 1 / n fr each f the n c-authrship links. The ttal strength f the n c-authrship links then equals 1. This is different frm the full cunting case, in which each f the n cauthrship links has a strength f 1, resulting in a ttal strength f the n cauthrship links f n. As can be seen in Figure 8, in the fractinal cunting case, the link between A2 and A4 has a strength f 1.0. A2 has c-authred D3 with A4. Because there are n ther authrs invlved, this results in a c-authrship link with a strength f 1 / 1 = 1.0. A2 has c-authred D1 with A1 and A3. In this case, A2 has tw c-authrs, which results in tw c-authrship links with a strength f 1 / 2 = 0.5 each. The link between A1 and A3 has a strength f 1.5. A1 and A3 have c-authred D1 and D2, resulting in c-authrship links with strengths f 0.5 and 1.0, respectively, which yields a ttal strength f 1.5. We have nw explained the difference between full and fractinal cunting in the calculatin f the strength f c-authrship links. There are similar differences between the tw cunting methds in the calculatin f the strength f cccurrence, bibligraphic cupling, and c-citatin links. We refer t Perianes- Rdriguez, Waltman, and Van Eck (2016) fr a mre in-depth discussin f the differences between full and fractinal cunting, including a discussin f the advantages and disadvantages f each cunting methd. Identificatin and selectin f terms When a map is created based n text data, terms first need t be identified in the text data and the terms t be included in the map then need t be selected. The natural language prcessing algrithms used in the term identificatin stage assume the text data made available t VOSviewer t be in English. Other languages are nt supprted. The term identificatin stage cnsists f the fllwing five steps: Remval f cpyright statements. In many cases, text data prvided t VOSviewer cnsists f titles and abstracts f scientific publicatins. Abstracts 29

31 may include a cpyright statement. VOSviewer remves these cpyright statements. We nte that this step is carried ut nly if the Ignre cpyright statements check bx in the Create Map wizard is checked. Sentence detectin. A sentence detectin algrithm is applied t the text data. Using this algrithm, the text data is split up int sentences. VOSviewer uses the sentence detectin algrithm prvided by the Apache OpenNLP library. Part-f-speech tagging. A part-f-speech tagging algrithm is applied t the text data. Using this algrithm, each wrd is assigned a part f speech, such as verb, nun, adjective, prepsitin, and s n. VOSviewer uses the part-fspeech tagging algrithm prvided by the Apache OpenNLP library. Nun phrase identificatin. Nun phrases are identified in the text data. VOSviewer defines a nun phrase as a sequence f ne r mre cnsecutive wrds within a sentence such that the last wrd in the sequence is a nun and each f the ther wrds is either a nun r an adjective. T identify nun phrases, VOSviewer cnsiders nly the lngest pssible nun phrases that can be fund in a sentence. Shrter nun phrases embedded within lnger nes are nt cnsidered. Fr example, in the sentence The artificial neural netwrk is highly accurate, VOSviewer identifies artificial neural netwrk as a nun phrase, while it des nt identify neural netwrk and netwrk as nun phrases. We nte that in VOSviewer a nun phrase cannt include a prepsitin. This fr example means that degrees f freedm and United States f America are nt identified as nun phrases. Nun phrase unificatin. Unificatin f nun phrases is accmplished by remving mst nn-alphanumeric characters, by remving accents frm characters, by cnverting upper case characters t lwer case, and by cnverting plural nun phrases t singular. Plural t singular cnversin is dne by examining the last wrd in a nun phrase. If the last wrd is a plural nun, it is cnverted t singular. The term identificatin stage described abve yields a set f nun phrases, r terms, that have been identified in the text data made available t VOSviewer. In the secnd stage, starting frm the set f identified terms, a selectin f terms is made. The selectin is made by excluding terms with a small number f ccurrences (e.g., by default, terms with fewer than 10 ccurrences are excluded), by excluding terms with a lw relevance scre, and pssibly als by manually excluding certain terms. The selected terms are included in the map that is created. The use f relevance scres t select terms requires sme further explanatin. When a map is created based n text data, ne typically des nt want t include 30