graphic PERT (PERT: initial of Program (or Project ) Technical Assessement and Review , litt. “technical of evaluation and examination of programs” or “projects” and word game with the English adjective “ PERT ”, meaning “malicious”, “mutineer”) makes it possible to visualize the dependence of the tasks and to carry out their scheduling. A graph of dependences is used. For each task, one indicates a start date and of end as soon as possible and at the latest. The diagram makes it possible to determine the critical path which conditions the minimum duration of the project.

This tool provides a method making it possible to optimize and plan the Ordonnancement of tasks. It is used in the Project management. Its goal is to find the best organization possible so that a project is finished as soon as possible, and to identify the critical tasks, i.e. the tasks which should not suffer any delay under penalty of delaying the whole of the project.

This method of organization is undoubtedly one of most demanding in rigor but also one of most powerful (see “Actigramme” of OST (Scientific management); it is thanks to such methods that it was possible to build in 2 years only one machine as gigantic as the deck chair “Queen Mary II” (see historical below).

For example, the following tasks are to be carried out during the preparation of a meal:

  • a: to prepare the menu (30 min)

  • b: to buy the ingredients (90 min)
  • C: to prepare the aperitif (30 min)
  • D: to clean the table (10 min)
  • E: to put the table (10 min)
  • F: to prepare the ingredients (30 min)
  • G: to cook the dishes (60 min)
  • H: to serve the meal (10 min)

Some of these tasks cannot start before some others are carried out, while there exist tasks which can be carried out in parallel. Graph PERT is composed of stages and tasks.

In method PERT, one calculates two values for each stage:

  • the date as soon as possible : it is about the date on which the task could be started as soon as possible, by taking account of the time necessary with the execution of the preceding tasks.

  • the date at the latest : it is about the date on which a task must be started at all costs if one does not want to delay the whole of the project.

One can thus represent the relations between the tasks and the stages by the following diagram:

To determine the date as soon as possible of a task, it is necessary to traverse the diagram from left to right and to calculate the time of longest ways leading of the beginning of the project to this task.

To determine the date at the latest of a task, it is necessary to traverse the diagram from right to left, and to withdraw date at the latest of the following task the duration of the task which one calculates the date at the latest. If there are several under-ways, one carries out same calculation for each one and one chooses the smallest date.

The difference between the date as soon as possible and the date at the latest in a task is called the total margin .

The tasks having a date as soon as possible equal to their date at the latest belong to the critical path , i.e. the way on which no task must have delay not to delay the whole of the project.

The free margin is the maximum time of delay which one can bring to the startup of a task, without for all this the following tasks are affected. It is equal to the difference between:

  • the smallest date as soon as possible of the following tasks

  • the date as soon as possible of the task which one calculates the margin to which one adds its duration

History

Technique of establishment and updating of the programs, created in 1957 for US Navy.

See too

  • comparative Evaluation of the free management solutions of projects

See too

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