data and how to use What if analysis. We will explain how to change parameters and how to use SyntheSim feature.

Transcription

1 This lecture is about learning how to use import external data and how to use What if analysis. We will explain how to change parameters and how to use SyntheSim feature. 1

2 This lecture will show how to import data in models created with Vensim PLE. Vensim PLE does not support importing data from Excel or other spreadsheet and database programs, but has a feature that lets you import and use data stored in a specially structured file. The structure of this file and the process for importing data will be presented. Then, I will explain the substance of What if analysis and how it is performed in Vensim, followed by a step by step procedure for using SyntheSim. 2

3 There are two types ofauxiliary variables in Vensim. The first one are defined as explicit functions. The formulas for these variables are entered using the keyboard or the function menu in Vensim. The other one are defined as implicit functions and are entered using the Lookup function. If the amount of data necessary to define the Lookup function is big and is available in a file, it is convenient to enter these date directly from the file. 3

4 It is sometimes necessary external data to be entered in the model. The professional version of Vensim provides a possibility the data to be entered from Excel, but that is not offered in Vensim PLE. However for data that show dependence on time, Vensim PLE provides a way for them to be imported. 4

5 This can be done by using.dat file that can be edited with a text editor. The first line in the file should contain the name of the variable in Vensim, and the rows that follows should represent the time values and the correspondent value of the variable separated by a number of spaces. Note that Vensim does not allow the time to repeat several times, or all values for the time need to be different. Once the file is created it can be imported by selecting from.dat format option in the Model menu. Vensim imports the file by creating a correspondent.vdf file. Vensim will report whether the file was successfully converted and how many values were read. 5

6 In order to be able to use the imported data, Vensim requires the name of the file to be specified. This can be done by going to the Set mode (click on the Set button). Then, you will see an empty field next to the method used to do the calculations (in the left side of the main toolbar). Write the name of the file there and then the simulation can be performed. 6

7 In this example the data for the atmospheric CFC are imported. The CFC.dat file contains the year and the amount of this gas for years 1938 to Once these data are imported Vensim reports that the dataset was converted. Once this is done the graph of the atmospheric releases of CFC can be seen. 7

8 When creating and running models it is often of interest to see how the behaviour changes when certain value in the model change. This kind of testing is called What if analysis. You can perform this kind of analysis only if the constants in the model are presented as special types of variables and can be considered as parameters. Hence, it is a good practice to keep them that way and not to include them in the formulas that define other variables. What if analysis can be performed by using the equation panel, changing the values ofthe constants and running the simulation again. However, Vensim offers faster and better way to do that. You can choose one out of two options: the set button or the SyntheSim button. 8

9 The Set button has the shape of a man set to run and is located left from the field with the name of the.vdf file for storing the results of the simulation. By clicking the button you will obtain blue boxes around all parameters (constants in the model ). The blue boxes allows you to easily change the value a certain parameter. Then you can change the name of the.vdf file and simulate to store the data that are obtained from simulating the model with the changed value of the parameter. You can repeat simulation for different values and observe how they affect the behaviour of the variables of interest. 9

10 This slide shows en example of using the Set button to see how the bank balance will change when different values for the interest rate are used. The graphs shown are with different colours. When the interest rate is bigger the bank balance increases faster and the amount of money earned for the same time period is bigger. 10

11 SyntheSim is a feature in Vensim that provides you with an opportunity to experiment with your model and perform What if analysis in a very easy way. By clicking on the button with the shaded running man you will enter SyntheSim mode and obtain slightly different appearance of your sketch. At the same time the Set button is changed into a red Stop button. By clicking on this button you can exit the SyntheSim mode. 11

12 In SynteSim mode the diagrams of all variables are presented in small rectangles below the name of the variable (assuming that some simulation run was performed before). If you move the cursor close to the graph of a certain variable, you will be able to get the same graph in a bigger window. In addition you will get sliders shown under each parameter (constant) in the model with the value of the constant shown on the holder of the slider. 12

13 There are two types of sliders. The one that is active has arrows which are coloured, while the others that are passive have white arrows. You can make some slider becoming active by clicking on it. The active slider is the one to be used to change the value of a parameter. The change can be made by moving the slider in the desired direction. However, it is convenient first to click on it. A popup window appears asking you to enter the minimum and the maximum and minimum value for the parameter you want, as well as the increment for the change. By filling these values you have more control when moving the slider. You will observe how the BOTG for all variables change when you move the slider. 13

14 Themain toolbar changes when you are in SynteSim mode. This slide shows how it looks like and explains the purpose of each button on the toolbar. 14

15 Although the Lookup function is not a constant, Vensim provides a possibility to introduce changes in it, too. When working in SyntheSim mode, the variables defined using Lookup function appear as blue boxes. By clicking on the box the Lookup function panel pops up and you can move the dots in the graph or change the values in the table. Then the simulation can be run to obtain new output values. This is one more convenience that Vensim offers for performing What if analysis. 15