System Analysis and Design methods

CH / NSS ICT / Dec. 2012 System Analysis and Design methods Systems Flowcharts (p.123-125 Text Book D2) ♦ ♦ ♦ A system flowchart is a diagram and ...
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CH / NSS ICT / Dec. 2012

System Analysis and Design methods Systems Flowcharts (p.123-125 Text Book D2) ♦





A system flowchart is a diagram and overview of a complete system. It will show: i.

the tasks to be carried out in the new systems, whether manual or by the computer

ii.

the devices (disk drives, tape drives, terminal, etc.) that are to be used in the system

iii.

the media used for input, storage and output

iv.

the files used by the system.

Frequently a pictorial representation of how the system will work is easier to understand and take in than a length text. Manual Operation Represent manual performance e.g. collect application forms

Process Main process of the system e.g. computer programs

Manual Input Key in information into the system e.g. job of data entry clerks

Magnetic Disk Random access data storage e.g. database in harddisks

Punch Card Old media for data input and output e.g. time cards

Magnetic Tape Sequential access data storage e.g. data backup in tapes

Merge Combine sets of data into one e.g. relate two database into one

Online storage Remote data storage with online access e.g. file server in the server room.

Display Unit Media for displaying information e.g. VDU

Report / Document Hardcopy paper output e.g. bank statement

Sort Sort a set of data

Communication Link Transmission of data with outside world e.g. satellite communication

Exercises: Match following descriptions with the corresponding systems flowcharts below. (1) A sequential payroll master file held on tape is updated from a sequential transaction file using the ‘Grandfather-father-son’ method of update, and pay-slips are produced. B (2) A transaction file held on tape is sorted on to a disk file, and a report produced in the new sequence. F (3) A collection of input documents is batched, and the batches keyed in and stored on disk. E (4) A transaction file held on tape is validated, with valid transactions being copied to a disk file and an error report produced which gives details of invalid transaction. A (5) A customer order is keyed in and the stock master file checked to as certain whether sufficient stock is available. The customer order is stored on an indexed customer order file. D (6) Electricity bill payments are read by an OCR device and stored on disk. These transactions are then used to update the indexed customer master file. C

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CH / NSS ICT / Dec. 2012 A.

C.

B.

D.

E.

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F.

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Example 1. A library has the following check out procedure: A person wants to borrow a book and takes the book to a check out counter. He gives the librarian the borrowing card and the book. The librarian uses bar code reader to get both information of the borrower and the book from the related database. If there is no error, it stores relevant information to a transaction file of the borrowed books. The due date notice is then printed for the borrower. If error occurs, an error report is generated for the librarian. Simplify the above text into a system flowchart. General Approach i.

Always bear in mind that a system flowchart shows what the system would do, not how the system works.

ii.

So ignore any unimportant information which does not involve of data flow. sentence ‘A person wants to ... counter.’

iii.

Convert the main data process into relevant system flowchart symbols.

iv.

Name the symbols but don’t be too detailed. e.g. Don’t specify which hardware or software is using or who does the specific jobs.

v.

Join the symbols with arrows which represent the flow of data.

vi.

Condition cannot be represented in a system flowchart. Therefore, just show what the system would produce. Don’t label the data flow arrows with ‘Yes’, ‘‘No’

vii.

No looping should be in a system flowchart. i.e. no cycle of data flow could be found.

E.g. The

System flowchart for example 1:

Wants to borrow a book

Book’s database

Collect borrowing information

User bar code reader to input information

SYSTEM ANALYSIS AND DESIGN TOOLS

Borrower’s database Borrowed books

Validate borrowing information

Error report

Perform check-out transaction

Due date notice

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Example 2. ABC College records the examination results of the students as follows: After all examination papers are marked by the teachers, the secretary input the information into the exam results database. Before updating the database, the system will automatically check for any abnormal input. If these inputs are found, the system will alert the secretary to confirm update or not. Simplify the above text into a system flowchart.

System flowchart for example 2:



Factors affecting the systems flowchart design: i.

Is this batch or real-time system?

ii.

How is the data to captured and input to the system?

iii.

What file organization is therefore appropriate?

iv.

What kind of storage media will be used?

v.

What will be the output from the system?

vi.

What are the processing steps to be carried out?

Structure Chart (p. 125-127 Text Book D2) ♦

It is a graphical representation of the relationships between different modules of a system.



A higher level module that sends data to a lower level module is known as the calling module, while the lower module is known as the called module.



Lines connecting the modules indicate the calling structure and the little arrows show the data or control information passed between the modules.

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The data, represented by an arrow with a circle at one end that passes in and out of a module are called data couples.



The arrow with a darkened circle is a control couple, which contains internal information, e.g. validation result.

Data Flow Diagram (p. 128-134 Text Book D2) ♦

It shows how the data moves through the system, and what data stores are used. It does not define what type of data storage is used. Entity - data source or data destination  people who receive data or output information

Process - an operation performed on the data. The two lines are optional; the top section of the box can be used to label the process, the middle to give a brief explanation, and the bottom to say where the process takes place. Data store - this could, for example, represent a file held on disk or magnetic tape, a batch of input documents or a report. Data flow - the arrow represents move of data between entities, processes or data stores. The arrow should be labelled to describe what data is involved. ♦

The processing elements from one diagram may be decomposed so that further basic symbols are used, and the amount of detail becomes more and more comprehensive as you delve deeper into the system.



Level 0 DFD (Context diagram). It contains only one process that represents the entire system.



Level 1 DFD. The only process in level 0 DFD is decomposed into lower level processes and data stores.



The simplest of data flow diagram might consist of a query from the user of a stock control system (Level 0 DFD). The user provides the stock number and the result of the processing might be information regarding ‘a description and level of the stock’. The idea is shown in fig. 3(a), and the processing can easily be developed into a further data flow diagram shown in fig. 3(b).

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CH / NSS ICT / Dec. 2012 User provides Data to process Stock number User

Stock report request

Level 0 DFD (Context diagram)

Description of stock User receives data from stock inquiry Stock report request

Stock description data

Validation Process

Valid stock data

Stock file processing

Stock description

Stock number User

Select report output format

Description of stock report

Level 1 DFD Output formatting data ♦

Another DFD example. Suppose that we wish to analyze what happens in an office where Sue, the accounts clerk, processes requests for the payment of expenses in a company. A typical scenario would be that an employee brings along a filled-in expenses sheet, and then Sue verifies the signature, transfers the information into the computer system, and makes sure that a particular budget is not over spent. If successful, then a cheque must be raised, signed by the chief accountant, then put in a tray ready for collection via Sue. The following data flow diagram illustrates the typical transactions. Process forms Expenses Forms

Unprocessed forms

Balance verified

Expenses Forms

Sue checks signatures

Employee signature rejected Rejected forms

Awaiting collection

Happy Employee

signed cheque identity

Sue verified employee

Signature verified

Rejected forms

Balance

Rejected forms

Sue checks balance

Insufficient funds

Sue states reasons for rejection

Balance verified

Awaiting signature

Happy Employee cheques

Rejected forms

Accounts

Unsigned cheques

Sue prepares cheques

Unsigned cheques

Processed cheques

Chief account signs

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Data Dictionary (p. 134-136) ♦

It is a collection of description of the data objects or items in a system.



It contains names, description, types contents and associated processes of data.



The data contents are decomposed into small data structures or data elements.



 Booktitle  Author  E.g. Book record   Publisher  ISBN



Data Structure Conventions

Name   Sex  Author   Date of birth  Date of death

Symbol

Meaning

Example

=

Is composed of

Telephone = Area code + Local number

+

And

Date of birth = Day + Month + Year

{}

Repetitive elements

HK ID = Letter +

[]

Either / or

Sex = [ Male / Female ]

()

Optional element

Author = Name + Sex + Data of Birth + (Date of death)

7 1

{digit}

Gantt Chart (p. 137-138) ♦

The duration of each activity is represented by the length of a bar as follows:



When drawing a Gantt chart, the project manager or systems analyst must:



i.

identify the tasks involved.

ii.

estimate the time required for each task.

iii.

list the task from top to bottom.

iv.

identify that some tasks must be finished before the others.

The advantages of using a Gantt chart: i.

Make a realistic assessment of the progress and the completion time.

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CH / NSS ICT / Dec. 2012 ii.

Concentrate on the resources to be allocated to each activity.

iii.

Show the dependencies of various activities.

iv.

Show the concurrent activities.

Critical path analysis (Reference Only, p. 313 - 315 Ray Bradley) ♦

It is a branch of operation research, i.e. the use of mathematical and scientific methods in the management decision-making role in modern industry.



The analysis basically divides up a project into smaller self-contained activities (each of which may be further subdivided) and assigns the time and resources that are necessary to complete each activity.



Then the project as a whole is represented by special graphs. Fig. 5 shows a simple example on preparing and cooking a meal.



The numbers in fig. 5 are known as using the nodes to represent the shown in fig. 6.



From the graphs, by analyzing the times through the various paths, the longest time from the start to the finish of the project may be determined, and the path with the longest time is known as the critical paths . The critical path would then determine the length of time that would needed to complete the entire project.



If the project was to be completed in a shorter time, some of the times on this critical path must be cut down. Prepare Fillet (a)

2

Fry fillet 3 until brown Mix paté until smooth

nodes events

Roast fillet

. We can now construct what is called a network and branches to represent activities , as

4

Leave until cold

13

Prepare fillet (b)

5

Prepare and roll out pastry

14

6

Bake in oven until done 15

1

Prepare broccoli

7

Cook broccoli

Prepare carrots

8

Cook carrots

Prepare asparagus

9

Cook asparagus

Prepare gravy

10

Cook gravy

Prepare roast

5m

2

11

5m

Part boil

3

10 m

12

4

Serve 16 dinner

Roast potatoes

30 m

13

10 m

2m

5

14

7m

6

40 m 15

1

3m ♦

2m

7

10 m

3m

8

20 m

2m

9

15 m

2m

10

3m

11

10 m

12

3m  16

50 m

If this is done, then a new critical path may be created.

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The above techniques are also known as Review Technique



The critical path from the fig. 5 is the route through the network that takes the longest amount of time, as this will be the shortest time to complete the project. From fig. 5 the times for each activity into account then the time through each path can be found as follows:



From the following diagram it can be seen that the critical path is . 16

PERT

,

Program .

Evaluation

and

1 – 2 – 3 – 4 – 13 – 14 – 15 –

Total time of 1st path = 5 + 5 + 10 + 30 + 10 + 40 + 3 = 103 minutes Total time of 2nd path = 2 + 0 + 10 + 40 + 3 = 55 minutes Total time of 3rd path = 7 + 0 + 10 + 40 + 3 = 60 minutes Total time of 4th path = 2 + 10 + 3 = 15 minutes Total time of 5th path = 3 + 20 + 3 = 26 minutes Total time of 6th path = 2 + 15 + 3 = 20 minutes Total time of 7th path = 2 + 3 + 3 = 8 minutes Total time of 8th path = 3 + 10 + 50 + 3 = 66 minutes

Decision Table (Reference Only, p. 310 – 313 Ray Bradley) ♦

The decision tables show the content of decisions, combinations of conditions and combinations of actions.



A decision table consists of the condition stub, the condition entry, the action stub and the action entry. In the decision table these four elements form quadrants, thus



: : condition stub condition entries : : : : action stub action entries : : The condition entries’ and action entries’ quadrants together constitute one or more decision rules, which run vertically through the two quadrants. These rules indicate the actions that are to be taken according to particular combinations of conditions. (Example shown in fig. 8)

Figure 8 Decision table with four redundancies Ordering Procedure 1

2

4

5

7

9

10

12

13

A. Customer wholesale? B. Qty. sufficient at East Coast? C. Qty. sufficient at West Coast? D. Call from East Coast?

Y Y  Y

Y  Y N

Y Y N N

Y N Y Y

 N N 

N Y  Y

N  Y N

N Y N N

N N Y Y

E. Ship from West Coast F. Ship from East Coast G. Bill at wholesale price H. Bill at retail price I. Backorder

. X X . .

X . X . .

. X X . .

X . X . .

. . . . X

. X . X .

X . . X .

. X . X .

X . . X .

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HIPO Diagram (Hierarchy-Input/Process/Output Diagram) (Reference Only, p. 307 Ray Bradley) ♦

The disadvantage of flowcharts is that they depict sequential and linear systems very well, but not necessarily the more complex, even parallel, procedures that can be constructed in computing. HIERARCHY DIAGRAM 1.0

3.0

2.0

2.1

3.1

4.0

3.2

5.0

4.1

4.2

Legend

Description Section

OVERVIEW DIAGRAM 4.2 Process

5.2

5.3

1 2 3 4 5

DETAIL DIAGRAM

3.0 Input

5.1

Output

From 4.1 Input

1.

1.

2.

2.

Process

3.

3.

4.

4.

Output

5. To 5.1 ♦

In order to make programming a more organized and structured activity rather than an inductive one detailed, HIPO diagrams were introduced. (Example shown in fig. 7.)



The intention is to demonstrate the relationships between input and process, and process and output rather than emphasize on the structure of the system.

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