Critical Path Analysis
This is designed to be simply a brief introduction to Critical Path Analysis and anyone wishing to learn the technique will need to undertake a training course in Project Management. The complete technique is probably not suited to all projects but all project managers will find that the first part "Create the Network" would be a good starting point for Gantt Charts, Stages Diagrams, or Milestone Plans.
Critical Path Analysis is the most powerful of the project management time planning tools and is particularly suited for engineering projects. However it is not suited to all projects. Where it is suited, it is very powerful indeed.
In the 1950's when the US military were developing the Polaris missiles and submarines, they found them faced with a huge number of activities being carried out by a large number of subcontractors, all needing scheduling and time management. The Rand Corporation, a US Think Tank, was asked to advise and they created Critical Path Analysis. The technique was quickly taken up by organisations involved in large projects such as shipbuilding aircraft, space missions, petrochemicals and construction. These days, almost all such projects use critical path analysis and all project management software is based around it.
The approach is in three parts:
- Create the Network
- Calculate the Timings
- Update and Monitor Progress
1. Create the Network
Break the overall task down into discrete activities and then create a logical network showing the relationships between the activities in terms of what follows or precedes what. The diagram below shows such a network for the project "Write a Letter".
Notice that some activities must be done in sequence whereas others can be done in parallel. Thus, activities like "Write Letter" cannot be started until the two activities that feed into it, "Get Paper" and "Get Pen", are complete. This network is not the only possibility but it gives the greatest opportunities for minimising the total time taken. It does assume that you have access to extra resources - one person to get the pen whilst another gets the paper..
This first part is best done with a piece of flip-chart paper and a wall or desk and Post Its.
- list the activities with each Post It being an activity
- then just move them around the paper until you feel that you have the logic right.
- draw the arrows linking the activities in that logic.
2. Calculate the Timings
Make estimates of the time required for each activity and calculate the total time required for the project and the critical path - the path of activities which must happen to estimate if the project is to be completed on time. The diagram below shows this for Spaghetti Carbonara.
This diagram tells anyone that knows critical path analysis that:
- the project will take 21 minutes to complete
- the critical path lies on the activities - "Get Ingredients - Cook Spaghetti - Drain Spaghetti - and Mix Sauce and Spaghetti and Serve".
- Any delay in these activities will increase the total project time (as well as spoiling the meal).
We will not attempt to explain the arithmetic - that really does require a training course. For those that have been on the training we have included the CPA Rules below.
3. Update and Monitor Progress
This is where critical path analysis is really important in areas such as construction. Having formed the network and made the estimates and initial calculations, the progress of the activities can be monitored and the whole network updated as work progresses, to ensure that the project completion is not delayed. Computer programs such as Microsoft Project automate this updating progress.
Critical Path Analysis Rules
To remind those who have been trained, the numbers are as below.
- Break the Task down into component sub-tasks/activities
- Draw the Network - the Logic linking the tasks.
- Which activity comes first?
- What must follow?
- What must precede?
- Do the Forward Pass.
- Put time "durations" to each activity.
- Make the earliest start time to zero
- Progress through, working out Earliest Start Times and Earliest Finish Times.
- (Earliest Start of one activity = Earliest finish of activity preceding)
- (Earliest Finish = Earliest Start + Duration)
- (Look for highest Earliest Finish time if two activities feed into one)
- Continue to the end to find out how long project takes.
- Do Backward Pass.
- Make last Latest =project duration
- Make each Latest Finish Time = last calculated Latest Start Time
Progress through, working out Latest Finish Times and Latest Start Times.
- (Latest Finish of one activity = Latest Start finish of activity it feeds into)
- (Latest Start = Latest Finish - Duration)
- (Look for lowest Latest Finish time if the activity feeds into two others)
- You should get back to zero as the first Latest Start.
- Find the Critical Path.
- There will be one path where there is no Slack; the Earliest Start Time = the Latest Start Time. (note: there can be more than one path)
- There is no Slack Time on this path - if any activity is delayed then the whole project is delayed.