CS605 - Software Engineering II - Lecture Handout 24

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Scheduling

Once we have the task network, we are now ready to prepare a schedule for the project. For this we use two techniques known as:

    • Program evaluation and review techniques (PERT)
    • Critical Path Method (CPM)

    These are quantitative tools that allow the software planner to determine the critical path – the chain of tasks that determines the duration of the project and establish most likely time estimates for individual tasks by applying statistical models. They also help the planner to calculate boundary times that define a time window for a particular task.

    The boundary time defines the following parameters for a project:

      • The earliest time that a task can begin when all preceding tasks are completed in the shortest possible time
      • The latest time for task initiation before the minimum project completion time is delayed
      • The earliest finish
      • The latest finish
      • The total float – the amount of surplus time or leeway allowed in scheduling tasks so that the network critical path is maintained on schedule
      In order to use the PERT and CPM, the following is required:
        • A decomposition of product function
        • A selection of appropriate process model and task set
        • Decomposition of tasks – also known as the work breakdown structure (WBS)
        • Estimation of effort
        • Interdependencies

        Timeline Chart

        To develop the schedule for a project, time required for each activity in the Task Network is estimated. This analysis and decomposition leads to the development of a Timeline or Gantt Chart for the project which portrays the schedule for the project. As an example, let us assume that Concept Scoping (the first task in the above list) is further subdivided into the following sub-tasks with the associated estimated time requirements:

        1. Identification of needs and benefits (3 days)
        2. Definition of desired output/control/input (7 days)
        3. Definition of the function/behaviour (6 days)
        4. Isolation of software elements (1 day)
        5. Researching availability of existing software (2 days)
        6. Definition technical feasibility (4 days)
        7. Making quick estimate of size (1 day)
        8. Creating scope definition (2 days)

        We also assume that the following task network for this was developed.

        Timeline Chart

        This is now converted in the following schedule in the form of a Gantt Chart. Note that, the concept of boundary time allows us to schedule Task Numbers 1.1.4 and 1.1.5 anywhere along Task Number 1.1.3. The actual time is determined by the project manager is based upon the availability of resources and other constraints. Each task is further subdivided in sub-tasks in the same manner until the schedule for the complete project is determined.

        Timeline Chart 1

        Tracking a Schedule