Anetwork diagram has been referred to in the preceding section. It lists each activity required to complete a project giving each a reference and estimated duration usually in weeks. An assumption is made concerning the order in which construction will proceed. A network of connections are then made between activities, stipulating the earliest and latest times each can start relative to some prior activity. The diagram thus comprises many parallel strands of activities interconnected at many points where an activity cannot be started before another activity is wholly or partly complete. The computer traces through this network to find the longest total time taken by some unavoidable sequence of activities. This is the critical path which determines the minimum time to complete the whole project, on the assumed order of doing the work. Modern computer network programs reproduce the analysis findings as bar charts on the screen, with differing colours for critical and non-critical activities, and showing float times also the latter being the spare time available for completion of a given activity before it becomes critical. This presentation makes the results of the analysis easier to understand.
But a difficulty is that the computer program has to be continually updated to include changes in the order of construction and changes in duration times for activities, which can arise from many causes, such as weather, troubles with labour or plant, delays in getting materials, etc. Hence critical path planning is sometimes started, but then abandoned because the program has to be continuously updated and the critical paths revealed are of little practical value since they can alter with every update. Bar charts are preferred for most jobs, since they involve less work, are easy to draw and keep updated.
Contractors sometimes use the critical path method to support a claim for delay; but the same problem applies that any critical path is based on only one particular order in which work is constructed, and other orders may be possible.