INFS 328 Systems Analysis and Design

Transcription

1 INFS 328 Systems Analysis and Design Session 13 Systems Analysis and Construction Tools Lecturer: Prof. Ellis Edwin Badu, Dept. of Information Studies Contact Information: College of Education School of Continuing and Distance Education 2017

2 Session Overview As stated in session 1, a number of tools and techniques are used by systems analysts and designers to analyse and synthesise systems functions or units. Two were briefly described. This session will demonstrate to you how to construct some of the tools for analysing and designing systems namely; systems flow chart, data flow diagram, entity relationship diagram, entity life history and decision tables. Slide 2

3 Session Outline The key topics to be covered in the session are as follows: Systems Flow Chart Data Flow Diagram Entity Relationship Diagram Entity Life History Decision Table Slide 3

4 Topic One SYSTEMS FLOW CHART Slide 4

5 Systems Flow Chart SYSTEM FLOW CHART System flow chart is a physical modeling tool that has various symbols to identify input and output operations, represent data or files and show media such as disks, documents and reports. It shows the overall structure of an Information System. It is used mainly as documentation on legacy systems Slide 5

6 Systems Flow Chart Advantages of Using System Flow Chart It defines procedures for new operations It highlights physical media used in systems, the various work stations through which data pass as well as the sequence of activities. Helps to avoid duplication Helps with the allocation of resources of staff to various jobs in the organization Slide 6

7 Systems Flow Chart Advantages of Using System Flow Chart Uses very much in systems where the information flow entails a large number of documents because it can show all input files, processing and output for a system. It is able to show origination, processing and destination of each document and the procedures employed by users. It is flexible and versatile Procedures can be compared to see which is best and it is very useful for feasibility study particularly when it is used to establish that automation is beneficial Slide 7

8 Systems Flow Chart DISADVANTAGES It provides little details on how processes are actually accomplished does not furnish the details of programs for a system. It reduces an entire program or set of programs into a single box. Slide 8

9 Symbols Systems Flow Chart There are many symbols used in systems flow charts. These are standard set of symbols developed by the American National Standards Institute (ANSI). Flow charts may also be drawn using software like Visio Professional, Corel Flow, System Architect, VISO Enterprise, Visible Analyst and ROSE. Symbol shapes indicate their meanings. Slide 9

10 Systems Flow Chart Symbols Slide 10

11 Systems Flow Chart Symbols Slide 11

12 Systems Flow Chart Symbols Slide 12

13 Systems Flow Chart Slide 13

14 Systems Flow Chart Slide 14

15 Systems Flow Chart Slide 15

16 Topic Two DATA FLOW DIAGRAM Slide 16

17 Data Flow Diagram Dataflow Diagram s (DFD s) The basic purpose of dataflow diagrams is to describe the flow of data between entities, processes and data stores. Analysts use dataflow diagrams to understand the flow of data into, out of, and within the organisation and to provide a basic understanding of how a system works. The highest level DFD is called a context diagram (or Level 0 DFD). This defines the system boundary and shows how all information enters and leaves the system. DFDs can be used on both manual and computerised systems, and can b e used to model the existing system in order to highlight any gaps in the current data flow or logic. The four symbols used in dataflow diagrams are as follows. Slide 17

18 Dataflow Symbols: Data Flow Diagram Dataflow - A dataflow indicates the movement of data from one location in the system to another location. A dataflow could be a letter, a verbal message, a telephone call an or a fax (i.e. it may or may not involve the transfer of a physical document). Slide 18

19 Dataflow Symbols: External Entity Data Flow Diagram An entity is either the destination or the source of data, which is external to the system. It may be people, groups or another organisation that either provide data to or receive data from the system. Slide 19

20 Dataflow Symbols: Data Store Data Flow Diagram Data stores are where data is held within the system and which receive dataflow. Examples of data stores are data files (manual or computerised), reports, documents and transaction records. Slide 20

21 Dataflow Symbols: Data Process Data Flow Diagram Data processes are processing activities carried out from a data store or which produce data for a data store. Again, data processes can be computerised or manual. A process simply uses data from storage and performs some kind of operation upon that data such as sorting, or re-calculating the data, then sends the processed data back to storage or as output to an external entity. Slide 21

22 Features of DFDs Data Flow Diagram As it is not possible to show all of the organisation s processes within one context diagram, it is necessary to explore DFDs in order to show increasingly detailed levels of processing. This decomposition has the following purposes: Shows clearly where data is stored Defines which processes change data Ensures data analysis is complete and thorough Slide 22

23 Features of DFDs Data Flow Diagram Provides a basis for software specification by indicating the processing functions required by systems software. Each DFD will take one of the processes within a higher level DFD and draw further DFDs of its internal processes. This is repeated until a level is reached whereby the process has been described in sufficient detail to design a computer program from the process description. Slide 23

24 Data Flow Diagram Slide 24

25 Questions Activity: The stores department of X Ltd send purchase requisition orders (PRO) to the purchasing department. The purchasing department clerk checks the purchase requisition, and if incorrect it is returned to the stores department for correction. If correct, the requisition is accepted and an order is processed using an existing file of approved suppliers. The order is then processed and sent to the supplier. The original requisition order document is put on file and a copy of the order is created and filed. Goods are delivered by the supplier and checked on receipt and received. The supplier then sends an invoice. The invoice is compared with the requisition order on file and if the invoice is queried it is returned to the supplier. Accepted invoices are passed to accounts who pay the invoice. Completed orders are filed. Prepare a DFD for the above process. Slide 25

26 Questions Slide 26

27 Topic Three ENTITY RELATIONSHIP DIAGRAM Slide 27

28 Entity Relationship Diagram Entity relationship modelling (ERM) Entity relationship modelling is a tool used within data analysis, and is structured around three basic concepts. An entity This is an item (person, product, activity, job, department or business) that is important to an organisation and about which information must be stored. For example, customers and suppliers are entities as are employees. Slide 28

29 Entity Relationship Diagram Attributes An attribute is a fact or characteristic of an entity which the business records. For example, the attributes recorded about an employee could include name, address, qualifications, department and current salary. Slide 29

30 Entity Relationship Diagram Relationship These are the logical links between entities. The degree of relationship between entities may be one of three, as shown below. 1. One-to-one relationship (1:1) means that the entity only relates to one other entity. That is, a student can only enrol on one degree. Slide 30

31 Entity Relationship Diagram Relationship 2. One-to-many relationship (1:N) means that an entity can relate to one or more other entities. One degree will contain many modules of study. Slide 31

32 Entity Relationship Diagram Relationship 3. Many-to-many relationship (M:N) means that a number of entities may relate to a number of other entities. These various modules are likely to be taught by a number of lecturers. Slide 32

33 Activity: Questions Z Ltd has many departments (containing a number of staff). Each department has only one manager, and these managers form the Management Board. The managers are all salaried and therefore are on the monthly payroll. The other staff within the department can either receive a salary (monthly) or can be paid wages (received weekly). Demonstrate the above using an ERM diagram. Slide 33

34 Answer to Activity Slide 34

35 Topic Four ENTITY LIFE HISTORY (ELH) Slide 35

36 Entity Life History (ELH) ELH is also sometimes referred to as an entity life cycle analysis. An entity life history is a representation of the processes that occur in the life of each individual entity, and is designed to show the way in which information within a system changes over time. An ELH shows what happens to an entity between its creation and its termination. The entity can go through three phases of development: recreation, amendment and termination. The important function of ELH analysis is the identification of the events and functions of an entity that cause the entity to change, rather than the analysis of the entity itself. Slide 36

37 Entity Life History (ELH) Slide 37

38 Entity Life History (ELH) There are three main symbols used within ELH diagrams; there is a rectangular box, within which can be placed either an asterix or a small circle. The top level shows the entity itself, and at each subsequent level the boxes read from left to right (in order of create, amend, delete). At the lower levels, the boxes represent events which occur within the life of the entity. If an event affects an entity many times, this can be shown by an asterix. For example, the above shows that the student will study numerous modules during the life cycle. Similarly, testing will consist of a number of examinations. The boxes with small circles in the top right hand corner indicate alternatives for particular events. For example, students may be learning or being tested but not at the same time. Similarly, for an examination, the student can either pass or fail, but not both. Slide 38

39 Topic Five DECISION TABLES Slide 39

40 Decision Tables Decision tables are used to describe the processing logic of a system. The most useful application of decision tables is in a situation where there may be a number of alternative conditions to evaluate. A decision table contains four quadrants: the conditions quadrant, the conditions entry quadrant, the action quadrant and the action entry quadrant. Slide 40

41 Decision Table Quadrants Decision Tables Slide 41

42 Method of Construction Decision Tables The decision table begins with the posing of a question. Starting with a basic example: do we offer credit facilities to our customers? From the question, establish the conditions and then complete the condition quadrant of the table. The conditions need to be formulated into a question format that can be answered by yes or no answers only. Slide 42

43 Decision Tables Method of Construction Thus, for the above example, the conditions could be as follows: Customers with orders under GH 100 do not receive credit facilities Customers with order valued between 100 euros and249 Ghana Cedis receive credit for one year. Existing customers rate of interest is 5 percent per annum, and new customers rate of interest is 7 per cent Existing customers with orders above GH 250 are offered credit terms at 7 per cent for one year, but new customers with orders over GH 250 must undertake a credit check procedure. Slide 43

44 Method of Construction Decision Tables Conditions order under GH 100? Order between 100 GH 249? Existing customer? Actions Conditions Entry Action Entry Slide 44

45 Method of Construction Decision Tables The next stage is to fill in the condition entry quadrant. The condition entry should contain the answers in yes or no format that will cover every possible condition in the condition quadrant. The condition entry quadrant is completed in the form of columns of combinations of yes and no answers. The number of columns is determined by 2 to the power of n, in being the number of questions in the conditions quadrant. In this case therefore, the number of columns s 23=8 Slide 45

46 Method of Construction Decision Tables The next stage is to methodically complete the condition entry quadrant by applying the halving rule. Conditions order under GH 100? Order between 100 GH 249? Existing customer? Actions Y Y Y Y N N N N Y Y N N Y Y N N Y N Y N Y N Y N Slide 46 Actions Entry

47 Method of Construction Decision Tables Now eliminate any impossible columns from the condition entry quadrant Conditions order under GH 100? Order between 100 GH 249? Existing customer? Actions Y Y Y Y N N N N Y Y N N Y Y N N Y N Y N Y N Y N Slide 47 Action Entry

48 Decision Tables Method of Construction In this question, the first two columns are impossible situations and can therefore be eliminated. The next stage is to complete the action quadrant, by listing the possible alternative actions that can be carried out. Conditions order under GH 100? Order between GH 249? Existing customer? Actions No credit given Credit given 1 year at 5% Credit for 1 year at 7% Credit check Slide Y Y N N N N N N Y Y N N Y N Y N Y N Action Entry

49 Decision Tables Method of Construction Now complete the action entry quadrant by considering each possible combination of conditions, and place an x in the action entry quadrant to indicate action to be taken. Conditions order under GH 100? Order between GH 49? Existing customer? Actions No credit given Credit given 1 year at 5% Credit for 1 year at 7% Credit check Y Y N N N N N N Y Y N N Y N Y N Y N XX X XX X Slide 49

50 References Checkland, P. (1999). System Thinking, System Practice. Chiches: John Wiley. O Brien, J. A. (2003). Introduction to Information Systems: Essentials for E-Business Enterprise. Boston: Irwin O Leary, I. and O leary, T. I. (2004). Computing Today. Boston: Mc Craw-Hill Rowley, J. (1990). The Basics of Systems Analysis and Design for Information Managers. Ludin: Clive Bingley Whitten, J. et al (2000). Systems Analysis and Design Methods. 6th ed., Boston: Mc Craw-Hill Slide 50