PERT CPM Ref Material

2.1 Introduction to CPM / PERT Techniques

  • CPM (Critical Path Method) and PERT (Project Evaluation and Review Technique) are techniques in network analysis developed around two streams—industrial and military.

  • CPM was discovered by M.R. Walker (E.I. Du Pont de Nemours & Co.) and J.E. Kelly (Remington Rand) around 1957, designed for the UNIVAC-I computer, and first applied in construction projects in 1958.

  • PERT was developed in 1958 for the POLARIS missile program by the U.S. Navy, with calculations suited for the IBM Naval Ordinance Research Computer at Dahlgren, Virginia.

2.2 Applications of CPM / PERT

  • Both methods are network-oriented and focus on time scheduling for projects.

  • CPM utilizes deterministic estimates for activity durations, ideal for repetitive jobs in production management where time predictions are reliable.

  • PERT employs probabilistic time estimates, suitable for project management of non-repetitive jobs, like research and development, where activity duration is uncertain.

  • Common applications include construction, maintenance, space flight, cost control, and prototype design.

2.3 Basic Steps in PERT / CPM

  1. Planning: The project is segmented into smaller parts with clearly defined relationships and responsibilities.

  2. Scheduling: Develop a time chart outlining start and finish times for each activity, identifying critical paths requiring special attention.

  3. Allocation of Resources: Resources such as labor and finance are allocated systematically to meet project objectives.

  4. Controlling: Use progress reports and updates to ensure project control using critical paths and resource management.

2.4 The Framework for PERT and CPM

  • Key steps in both methods include:

    • Defining the project and tasks.

    • Developing relationships among activities.

    • Drawing a network diagram connecting activities.

    • Assigning time/cost estimates to each task.

    • Computing the critical path through the network.

    • Using the network for planning, scheduling, and controlling.

2.5 Network Diagram Representation

  • Activity: A resource-utilizing operation represented by arrows indicating direction.

  • Various types of activities include predecessor, successor, concurrent, and dummy activities.

  • Event: Marks the completion of activities, represented by circles (nodes).

    • Categories of events: merge, burst, and simultaneous (merge and burst).

2.6 Rules for Drawing Network Diagrams

  • One arrow per activity, unique end events for activities, and ensure proper precedence are key rules.

  • Avoid overlapping arrows and represent duration without changing arrow length.

  • The network should have a single entry point and an end event.

2.7 Common Errors in Drawing Networks

  1. Dangling: Unfinished activities in the network indicate dangling errors.

  2. Looping or Cycling: Creating endless loops in the network shows a cycling error.

  3. Redundancy: Unnecessary dummy activities reflect redundancy errors.

2.8 Advantages and Disadvantages of PERT/CPM

Advantages:

  • Clearly defines dependencies and facilitates critical path identification.

  • Aids in recognizing slack time and optimizing project duration by better overlapping of activities.

Disadvantages:

  • Requires clearly defined, independent activities.

  • Complexity can lead to unwieldy charts impractical for management use.

2.9 Critical Path in Network Analysis

Basic Scheduling Computations:

  • Notations include( (i, j) ) for activities, and( E_i ),( L_j ) for earliest and latest times respectively.

Determination Steps:

  1. Earliest Times: Forward Pass computation to determine activity starting and finishing times.

  2. Latest Times: Backward Pass computation for latest allowable activity times.

  3. Floats Calculation: Total float, free float, independent float, and event slack times are calculated based on earliest and latest times.

  4. Critical Path Determination: Identifying events and activities with zero slack to define the project's critical path.