Critical path method

The Critical Path Method (also called activity -arrow- representation; critical path method english CPM) represents the process arrow - network plans, a special network technology. It was 1956/57, the American chemical company DuPont de Nemours in cooperation with the ADV specialists Remington Rand Corp.. developed and implemented on a UNIVAC I, to systematically plan large investment projects and maintenance work at chemical plants and monitor.

Basics

The Critical Path Method uses the form of representation of the process arrow. In CPM schedules the operations as arrows, the events as nodes and the relationships are again displayed as arrows. Prerequisite for meaningful work with a network can, is that all tasks in the project are each set with each individual term in the right relationship.

Creation

The following CPM rules must be observed:

Example of a CPM network diagram

Mathematical representation

If the needed information about the duration, dates, and relationships ( see term network planning ) are present, the calculation of the critical path can be performed independently of the graph. Calculated the times for the individual processes and the resulting buffer times. Any event or process have in the timing of an earliest (possible) and latest ( enabled ) position.

The calculation is performed in two steps:

1 First, a forward scheduling is successively eliminated in the starting from the earliest scheduled start time of the project for all operations into the future. This results in the earliest start and end times of all operations.

2 In the backward scheduling procedure is reversed; from the time of the planned project end is now expected in the direction of the present. This results in the latest start and end times of each operation.

Consists of an operation between the result of the pre-and backward calculation a difference, then there is a buffer time. A process that has a (positive ) Backup time can be shifted by this buffer time between the earliest start date and its latest completion date, without affecting the duration of the project is adversely affected. The one path through the network from the start to the destination node, in which all events have equal earliest and latest event times, ie, if the sum of all buffer times zero is called the critical path or critical path. A delay in one of the events of the critical path that is always a delay in the completion date for the episode.

Advantages and disadvantages of the network techniques generally

Advantages:

  • Compulsion to exact thinking through life of project
  • Clear representation of the dependencies
  • Possibility of minimizing the project duration
  • Higher security in delivery time
  • Significant emphasis of project constraints
  • Early recognition of possible delays and estimating consequences

Cons:

  • Uncertainty in timing
  • Existence of different ( subjective) perceptions of the project cycle
  • Buffer times are quickly consumed by the swelling of the individual activities

Advantages and disadvantages of process arrow network diagrams ( CPM network analysis )

Advantages:

  • Simple calculation rules and therefore easy programming

Cons:

  • Modeling difficult, since few opportunities exist to easily describe complex dependencies between different tasks. See in particular the rules 4 to 7 Because of this disadvantage, now hardly more process arrow networks, but only process node network plans used in practice.
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