Walking the Critical Path

30 June, 2014

For tight-rope walkers, the critical path is literally a life and death matter. For project managers, the critical path also focuses the mind, but rarely results in such catastrophic consequences. However, failure to manage activities on the critical path will see the schedule run past its due end date, which is not a good result for any project manager.

By definition, the critical path through a schedule is the sequence of project activities that take the longest to get from the start of the project to the end. Any delays on this path will result in the overall project being late. The other paths through the schedule enjoy a measure of slack, or float. That means a delay in these activities might occur without pushing out the overall project duration.

To calculate the critical path you need a network diagram and duration estimates (as we saw in the last article). You also need to calculate the start and finish dates, which involves making two passes through the diagram. The forward pass – working from start to finish – calculates the early start and finish dates for each activity. So, for instance, the first activity will start on day 0. If it has an estimated duration of X days, it will finish on day X. Day X will now be the early start date for the next activity on the path. However, we might find activities along the way that have two or more predecessor activities. In this case, the early start date is the largest of the predecessors’ early finish dates. The early finish date of the last activity on the diagram represents the expected project completion date.

Now, we need to complete a backward pass. Take the early finish date of the last activity on the diagram and make this the late finish date of the last activity. Now subtract the estimated duration to determine the late start date. The late finish of the predecessor activity is now the same as the late start date of its successor activity. However, if an activity has two or more successors, the late finish is the smallest of the successors’ late start dates.

Knowing the early and late start and finish dates means we can now calculate the float for each activity. This is easily done by subtracting the early start from the late start value. This makes sense because if you are supposed to start an activity on the 1st of the month (early start), but could start on the 3rd of the month (late start) without delaying the project, then those two days represent the leeway, or float, we have available for that particular activity. Not only does float allow the activity to start late, it also allows the duration to take longer than estimated. So we might start on the 1st of the month as planned, but take two days longer to complete than estimated. This is not a problem if the late finish date is at least two days later than the early finish.

However, it is the path where all the activities have the same early and late start dates that is of interest. Here, no delay is possible and any under-estimation will be punished by a slip in the schedule. It is vital that this path through the project runs smoothly. There is a temptation for the project manager to assign the best resources to this path and manage it closely. But there is always the danger that slippages on other paths transform them into critical ones as well, so the wise project manager maintains an overall perspective on the entire schedule.

Similarly, close analysis of the critical path can show potential parallel activities. If the project manager is successful in obtaining extra resources, the critical path can be dramatically shortened. But the benefit is not likely to be as great as expected – because another path could take over the critical mantle. No matter how much you optimize the schedule there always will be a critical path.

There is a final concept that might appear on schedules: that of negative float. The critical path will have zero float and the non-critical paths will have positive float (extra days that can be used to absorb delays in execution. However, if a hard deadline is placed on the schedule, negative float can appear. This means that the late start dates will be earlier than the early start! In its simplest terms, this means that our estimated schedule will not meet its deadline. Negative float is not a good thing to see on a schedule and needs to be removed. If crashing (adding more resources) or fast tracking (starting activities before their predecessors are fully complete) do not solve the problem, you will need to consider sacrificing scope or (as a last resort) quality.

Velopi’s project management training courses cover project time management. If project management is intriguing for you, you might consider one of our project management certification courses that are held in Dublin, Cork, Limerick and Galway. Find out more by visiting our training page or by contacting us directly.

By Velopi Seamus Collins


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