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Critical Path AnalysisCritical Path Analysis is an extremely effective method of analysing a complex project. It helps you to calculate the minimum length of time in which the project can be completed, and which activities should be prioritised to complete by that date.
Where a job has to be completed on time, critical path analysis helps you to focus on the essential activities to which attention and resources should be devoted. It gives an effective basis for the scheduling and monitoring of progress.


Sequential and parallel activitiesThe essential concept behind Critical Path Analysis is that some plan activities are dependent on other activities being completed first. For example, you should not start building a bridge unless you have designed it first!
These dependent activities need to be completed in a sequence, with each activity being more-or-less completed before the next activity can begin. Dependent activities are also called 'sequential' activities.

Other activities are not dependent on completion of any other tasks, or may be done at any time before or after a particular stage is reached. These are non-dependent or 'parallel' tasks.


Method
The process of carrying out a critical path analysis is shown below:
List all activities in plan
Show the earliest start date, estimated duration and whether the tasks are parallel or sequential. If the tasks are sequential, show what they depend on.
This will show a task list like them one below (simplified custom written computer installation):

NB: Start week shows purely when resources become available. Whether a
task is parallel or sequential depends largely on context.

1. High level analysis
start week 1, 5 days, sequential
2. Selection of hardware platform
start week 1, 1 day, sequential,
dependent on (1)
3. Installation and commissioning of hardware
start week 3, 2 weeks, parallel
dependent on (2), any time after
4. Detailed analysis of core modules
start week 1, 2 weeks, sequential
dependent on (1)
5. Detailed analysis of supporting utilities
start week 1, 2 weeks, sequential
dependent on (4)
6. Programming of core modules
start week 4, 3 weeks, sequential
dependent on (4)
7. Programming of supporting modules
start week 4, 3 weeks, sequential
dependent on (5)
8. Quality assurance of core modules
start week 5, 1 week, sequential
dependent on (6)
9. Quality assurance of supporting modules
start week 5, 1 week, sequential
dependent on (7)
10.Core module training
start week 7, 1 day, parallel
dependent on (6), any time after
11.Development of accounting reporting
start week 6, 1 week, parallel
dependent on (5), any time after
12.Development of management reporting
start week 6, 1 week, parallel
dependent on (5), any time after
13.Development of management analysis
start week 6, 2 weeks
dependent on (5), any time after
14.Detailed training
start week 7, 1 week, sequential
dependent on (1-13)
15.Documentation
start week 4, 2 weeks, parallel



Head up graph paper with the days or weeks through to task completion


Plot the tasks on the graph paper
Start on the earliest start dates, and mark on the duration. Show the tasks as arrows, and the ends of tasks with dots. Above the tasks arrows, mark the time taken to complete the task. Do not worry about task scheduling yet - all we are doing is setting up the first draft of the analysis.
Once you have plotted the tasks, plot in lines to show dependencies.

This will produce an untidy draft analysis like the one below:



Schedule Activities
Take the draft analysis, and use it to schedule the actions in the plan, in such a way that sequential actions are carried out in the required sequence. Parallel actions should be scheduled so that they do not interfere with sequential actions on the critical path, if possible.
While scheduling, bear in mind the resources you have available, and allow some slack time in the schedule for hold-ups, over-runs, failures in delivery, etc.