THE SYSTEM LIFE CYCLE

For every system that is developed, there is a lifecycle:

• Analyze the problem
• Specify the requirements for a solution and alternatives
• Design a solution to match the specification
• Implement the design
• Assure quality of the end product
• Maintain the end product
• Plan and control the above

When you design a system, you must first analyze the problem you are going to solve. You would look at current solutions, use questionnaires, interviews, etc.

The next step is to figure out what you need to implement the chosen solution(s). You also need to figure out whether the requirements are feasible.

For a solution, the ideas of system design need to be considered such as defining the input, process, and output. Flowcharts and structure charts can be used.

This step, you carry out what is planned from the previous step. In a program, this is the coding stage.

This step covers the testing of the system

Here, we monitor the implementation of a system in use, while making recommendations for future improvements/modifications if necessary.

This step investigates the individual stages in a system. This includes estimation the length of time per step, cost and requirements, etc.

Project Evaluation and Review Technique (PERT) diagrams are an important part of systems analysis, especially in the Plan and Control step. PERT diagrams can be used for any system. Not just computer programs.

This is an example of a PERT diagram:

Data Given:

Task	Pre-requisite	Time
A	-	5
B	-	2
C	A	4
D	C	7
E	B	9
F	D,E	6
G	B	1
H	G	3

Diagram:

Each task is given a letter. With each task there is a pre-requisite and a time span. The pre-requisite is the task(s) that need to be completed before the current task can start. The time span is the amount of time that the task will take to complete. The diagram goes from left to right.

Each circle is called a node or a milestone. Each line is a task with a corresponding number representing the time to complete the task.

CPM, or Critical Path Method, is the method where the minimum time required to complete the system is calculated. This is accomplished by looking at all the opaths from one end of the system to the other end (e.g. From 1 to 7). Then, the path with the longest amount of time from start to finish is chosen. This is the CPM.

An important concept is "slack" time. In industrial system this concept is important because it allows the analyst and the managers to effectivly juggle work load without delaying the whole porocess. Slack time is the amount of time a portion of the process (milestone) can wait before starting without slowing down the whole process.

The slack time is calculated by substraction the latest start time from the earliest start time. For example the slack time for node 3 is 5. As you can see, the pre-requisite for node 6 (or Task F), is D and E. The miimum time required to get to node 6 is 16. Going back to node 3, as you can see, the earliest time you can start node 3 is 2. Therefore, the earliest start time is 2. Now, looking back, you know the minimum time required to get to node 6 is 16. You have already used 2 time units to get to node 3. You know that process E requires 9 time units. Therefore, to make the limit of 16, you can start node 3 at 7 time units. If you use the formula of:

Latest Start Time - Earliest Start Time = Slack Time
  you get,
7 - 2 = 5

DEFINITIONS

CASE

• Summarizing initial requirements
• Developing flow diagrams
• Preparing documentation
• Controlling software version
• Developing program code

Source: http://www.zdwebopedia.com/TERM/C/CASE.html

Program Evaluation and Review Technique

Source: http://wombat.doc.ic.ac.uk/foldoc/foldoc.cgi?query=PERT