Is PERT better than CPM

Network plan technology

References are missing in this article.

1 Introduction

Ever since mankind understood its existence, it began to plan instinctively. Noah planned his ark, Iago his ruse against Othello and Alexander the great his war with Persia. Even then, the resources were distributed consciously and unconsciously and the “merciless” factor of time was taken into account.

In the course of time, planning became more and more detailed and, above all, more complicated. One planned in the most varied of ways, using the most impossible planning routes. There was chaos in the understanding between the “strategists” and the disagreement of opinion about the same processes in different cultures or even individual companies was amazing. An easily understandable, open planning system with the possibility of taking into account all the most important details and factors was urgently needed.

A plan became a project and the planning became a feasible and cross-lingual network plan. Since then, the network plan technology has been refined and optimized again and again. The graphic representations changed their shapes. There were new requirements as well as new revolutionary ideas for their solution. The rush of appointments and the time buffers became more important. Today, modern planning methods require complex software to be implemented in order to keep up with company requirements. But in order to better perceive the importance and the development steps of the contemporary planning chains, one needs undoubtedly the knowledge about the cornerstone of the rough planning - the network plan technique.


The term Program evaluation and review technique (PERT) or English Program Evaluation and Review Technique refers to a network plan technique.

It was developed in 1958 as part of the Polaris project. The particular difficulty of the Polaris project was that both research and development and the production of components, which had never been manufactured before, had to be outsourced to suppliers. Neither costs nor time requirements could be estimated even approximately exactly. The deadlines could only be based on probability. Each supplier was therefore asked to estimate the time required. It is estimated that PERT could complete the Polaris rocket two years, 45% earlier.

3 PERT and CPM

PERT is very similar to the critical path method developed at almost the same time (critical path method), but uses the average duration of processes as the basis for the calculation in network planning instead of the most common duration. CPM is used for projects that can be standardized, the processes of which are largely known from experience in other projects and there is comparatively little uncertainty regarding the time estimate. PERT is used for projects with a high level of uncertainty and little experience.

4 PERT specialty

The time in which a process can be carried out is not estimated as a scalar quantity with PERT. Rather, a probability distribution is assumed, with PERT based on the beta distribution. As a rule, the minimum or optimistic estimated duration is used to estimate the effort of a process dmin, the most common (to the best of our knowledge) duration dnorm, and the maximum or pessimistic estimated duration dmax needed. The beta distribution then gives the mean duration to be used for the process [math] d_ {medium} = \ frac {d_ {min} + 4 * d_ {norm} + d_ {max}} {6} [/ math] By definition, there is a 50% probability that the process will be completed within this time. In addition, buffer times must be specifically planned for possible overruns.

With PERT it is also possible to specify the structure "not unambiguously" (stochastically). The network plan is evaluated with the help of decision nodes, with the following node types. On the entry page: AND, OR, EXCLUSIVES-OR; on the exit side: deterministic and stochastic).

5 An example from practice

Business informatics students at the Augsburg University of Applied Sciences are planning the next barbecue party. Since this should take place during the pre-examination period, you don't want to lose a single precious mitute that can be used for learning. A production plan is therefore drawn up with a subsequent PERT network plan.

Production plan

PERT network plan


In order to be finished in the allotted time, consideration is given to how bottlenecks can be avoided on the "critical path" (numbers: 1, 2, 3, 4, 5). The longest block is "heating coal" and the duration of the other blocks can be badly influenced. So you could try to solve the bottleneck at number 2 by heating the coal to the right temperature faster by fans and thus shortening the time. Experience has shown that most students will not show up until 6:45 p.m., which can mean that there will not be enough students to prepare. So that this shortage does not delay the bottleneck, the student who is responsible for setting up the tent set can only start setting up at 18:27 (because there is buffer time) and in the time from 18:06 to 18:26 for fanning can be used. In this way a bottleneck can be avoided.

Practical: it is often not worthwhile to draw up production and network plans for such small projects. In the time it took to create the plans shown above, the tasks outlined therein could be done and started eating!