Critical Path Drag

Majority of project managers are familiar with the critical path method (CPM) and think that they know how to apply it to manage their projects.

Many scheduling software can compute and visualise Critical Path, so it is a no-brainer to use it. Usually, a project manager accepts the calculation as the schedule. The Critical path method doesn’t take resources, material and financial supply constraints into the consideration. So, the next steps become to assign resources, baseline the schedule and start to manage project progress.

This is because the project managers have forgotten that the “M” in CPM stands for method. The reality is that the first network is only the tentative schedule, giving just the data to implement the method fully. Once the initial CPM schedule has been computed and a logic diagram produced, the project manager is in a position to use the critical path method to save time and money. If, for instance, we want to shorten the project duration, we now know where to start: the current critical path!

There are three ways of trying to shorten the project:

1. Shorten the duration of activity on the critical path by:

• adding extra resources
• assigning resources with better productivity
• ask resources to work more hours
• change delivery method (a backhoe instead of 10 labours)
• prune scope (gold planning)

2. Start critical activities earlier by:

• applying anti-crashing method

3. Change logic and remove activity from the critical path by:

• applying fast-tracking method

But the question is: Where should we add/remove these resources? Where should we review the scope? What will the effect be of such actions on the schedule and cost?

One of the CPM metrics that could help us find the answers to the above questions was proposed by Stephan A. Devaux and explained in his “Total Project Control” book called Critical Path Drag. In the 1st edition, published in 1999, Stephan used the term DRAG (Devaux’s Removed Activity Guide), but since the 2nd edition, it was written as “Drag”.

In his book, Stephan described the Critical Path Drag metric as:

To some extent this metric could be considered as an opposite to the Total Float metric.

By contrast, drag is:

• only#1 on the critical path; and
• the amount of time by which an activity duration can be shortened#2 before its drag is reduced to zero.

Further decrease in the duration of the activity would not reduce the duration of the project as another path became critical. In other words, it is the amount of time that could potentially be saved on the project by reducing the duration of the activity (or removing an activity completely).

#1,2 The proposed explanation covers the majority of project scenarios, but not all of them. We will review such scenarios below.

Most of the software that computes Critical Path also could calculate Total Float (TF), but just a few can calculate Critical Path Drag, so this metric is still not well known. However, this computation is very important. Without understanding activity drags, well too often, implemented project optimisation measures don’t give the expected effect.

A programming formula may look something like this:
= IF (Critical = “Yes”, Min (duration, min({total float of parallel activities}), 0)

#3 Parallel activities belong to a parallel stream. They are not necessarily planned to be performed at the same time as the analysed activity.

Let’s review the following simple project:

With all “Finish-to-Start” dependencies, the duration of this project is 18 days. Drag for first and last activities is equal to the duration of activities.
The drag for the second critical activity is only 3 days. This means that if the activity will be reduced by 3 days, the project also will be reduced by 3 days. Any further optimisation would not decrease the duration of the project as this activity would not be on the new critical path anymore.

Let’s assume it is possible to add resources to the 10-days activity and complete this activity twice quicker. The new duration of this activity will be 5 days:

The project has a new Critical Path and as we expected duration decreased to 15 days.

Drag for 2nd and 3rd critical activities is 2 days each. However, it doesn’t mean that if both activities are reduced by 2 days, the project will be 4 days shorter! The drag is not cumulative.

Let’s consider a slightly more complex example: a working typical fragment “1 km of Road Construction”. There are only 20 activities in this fragment, but there are much harder to calculate critical path drag manually. Let’s use a scheduling tool, Spider Project, to calculate CP drag for each activity:

1 km of Road Construction.

There are 10 critical activities and all of them have Critical path drag <>0. In this project fragment, the first and the last critical activities have CP drag equal to the duration of the activity. In other cases, drag is less than the activity duration. Also, there are 3 critical activities where drag is less than zero. Let’s analyse how it is possible.
As it was explained in the “Project Anti-Crashing Method” post, in some cases project duration could be reduced by increasing a critical activity duration:

In this example, the CP drag of the activity “C” will be negative. If the duration of this activity is reduced to zero, the duration of the project will be increased from 42 to 50 days. So, CP drag will be minus 8 days.
At the same time, if it is possible to increase the duration of activity “C” by 14 days, the duration of the project will be decreased from 42 to 28 days (!)

It is logical to think that project duration could be reduced by optimising activities on a critical path only. However, there are project scenarios when decreasing the duration of the non-critical activity could reduce the overall duration of the project.
In the previous post, we reviewed a project fragment when activities have different calendars.

Activity “A” only could be performed on weekdays and activity “B” only could be performed on weekends.

Activity “A” has 4 days of the total float, so it is not on the critical path. However, if we reduce the duration of this activity to 5 days, the overall duration also will be reduced from 14 days to 7 days.

So, the CP drag of activity “A” will be >0. How much exactly? Well, it depends..
If it is based on the activity “A” 5-days calendar, CP drag is 5 days. If 7-days calendar, CP drag is 7 days.

• The Critical Path method gives project teams information to optimise their project plans, not the final schedule. The method has optimisation metrics for that.
• Activity Drag is an important Critical Path metric that allows prediction of how the applied effort would shorten project duration
• In some cases Activity Drag could be negative
• When different calendars are applied non-critical activities may have a positive ‘Activity Drag’. Optimisation of these activities would shorten the project duration.

We are going to review other Critical Path optimisation metrics in future posts.