Project Management ABC: P for PERT – Program Evaluation and Review Technique

PERT in focus: Strategies for realistic time planning

What is PERT?

The Program Evaluation and Review Technique (PERT) was developed in the 1950s by the United States Navy to manage the Polaris program – the development of a ballistic missile defence system. The goal was to create a tool that would account for uncertainties in scheduling and still provide a realistic basis for project control.
At its core, PERT is a network-based planning process that depicts project activities in relation to each other and considers the estimated durations for each activity. Unlike traditional Gantt charts, which show linear sequences, PERT focuses on the logical relationships and shows the critical path of a project. This critical path is essential as it determines the minimum duration of the project.

The basics of PERT

PERT is based on the assumption that the duration of individual project activities is not precisely known and is therefore specified as a probability distribution. Three time estimates come into play here:

1. Optimistic time (“a”): The shortest duration possible under ideal conditions.
2. Pessimistic time (“b”): The longest duration that can occur under unfavorable conditions.
3. Most probable time (“m”): The most realistic estimate under normal conditions.

The expected time (“te”) for each activity is calculated from these three values by applying the following formula:

te = (a + 4m + b) / 6

This weighted average takes into account that the most probable time (m) has a greater weighting than the two extreme values (a and b).

Creation of a PERT diagram

A PERT chart is created in several steps:

1. Identify tasks: All necessary activities are defined and broken down into their individual components.
   Example: A team plans the organization of a conference. The main tasks include: Selecting the venue, inviting participants, preparing materials and conducting a dress rehearsal.
2. Define sequences: The logical sequence and dependencies of the activities are determined. This involves clarifying which tasks are interdependent and which can be carried out in parallel.
   Example: The venue must be determined before the participants are invited and the materials are prepared. The dress rehearsal takes place after the materials have been prepared and all other tasks must be completed before the conference.
3. Make time estimates: The three time estimates (a, m, b) and the expected duration (te) are determined for each activity.
   Example:
   

   Task	a (optimistic)	m (probable)	b (pessimistic)	te (expected)
   Select venue	2 days	4 days	6 days	4 days
   Send invitation	1 day	2 days	3 days	2 days
   Prepare materials	3 days	5 days	7 days	5 days
   Perform dress rehearsal	1 day	2 days	3 days	2 days

4. Create a network diagram: The activities are displayed graphically in the form of a network. Nodes represent the events (milestones), while arrows symbolize the activities and dependencies. Dashed arrows often stand for a dependency without resources, i.e. without an activity having to be carried out.
   Example: The network diagram shows the sequence of tasks and dependencies.
5. Determine critical path: The critical path is identified by calculating the paths with the longest total duration. This path is crucial, as delays in these activities delay the entire project.
   Example: The critical path in this case is: Select venue → Prepare materials → Dress rehearsal → Conference. The expected minimum duration of the project is therefore 11 days.

Advantages of the PERT method

The use of PERT offers numerous advantages:

• Realistic time planning: By taking uncertainties into account in planning, PERT delivers more realistic time forecasts than traditional methods.
• Focus on critical activities: The critical path clearly shows which tasks require special attention in order to avoid project delays.
• Flexibility: The method is ideal for projects with a high degree of uncertainty or many unknown variables, as it takes various scenarios into account.
• Transparency: The visual representation of the project structure makes it easy for all stakeholders to recognize which tasks are interdependent and how changes will affect them.
• Early problem identification: Potential bottlenecks or delays can be identified at an early stage so that countermeasures can be initiated.

Challenges and limitations of PERT

Despite its strengths, PERT also has some weaknesses that project managers should be aware of:

• Time-consuming creation: The detailed recording of activities and time estimates requires a great deal of planning effort, especially for large and complex projects.
• Subjectivity: The accuracy of time estimates depends on the experience and judgment of the people involved.
• Unsuitable for simple projects: For small or less complex projects, the benefits of PERT may not justify the expense.
• No consideration of costs: PERT focuses exclusively on time planning and does not include financial aspects. For more comprehensive project planning, it should therefore be combined with other methods such as cost-value analysis.

Conclusion

The Program Evaluation and Review Technique (PERT) is a valuable tool in project management, especially for complex projects with uncertain time estimates. It offers a structured approach to planning and controlling projects and helps to identify and minimize risks at an early stage. At the same time, however, the method requires a high level of planning effort and well-founded time estimates. However, by combining PERT with other methods, project managers can ensure holistic planning and lead projects to success more efficiently.

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