Critical path drag

Critical path drag is a project management metric ^[1]developed by Stephen Devaux as part of the Total Project Control (TPC) approach to schedule analysis and compression ^[2] in the critical path method of scheduling. It is the quantified amount of time that an activity or constraint on the critical path is adding to the project duration.

In networks where all dependencies are finish-to-start (FS) relationships (i.e., where a predecessor must finish before a successor starts), the drag of a critical path activity is equal to whichever is less: its remaining duration or (if there is one or more parallel activity) the total float of the parallel activity that has the least total float.^[3]

*Activity-on-node diagram* showing critical path schedule, along with total float and critical path drag computations

In this diagram, Activities A, B, C, D, and E comprise the critical path, while Activities F, G, and H are off the critical path with floats of 15 days, 5 days, and 20 days respectively. Whereas activities that are off the critical path have float and are therefore not delaying completion of the project, those on the critical path have critical path drag, i.e., they delay project completion.

Activities A and E have nothing in parallel and therefore have drags of 10 days and 20 days respectively.
Activities B and C are both parallel to F (float of 15) and H (float of 20). B has a duration of 20 and drag of 15 (equal to F's float), while C has a duration of only 5 days and thus drag of only 5.
Activity D, with a duration of 10 days, is parallel to G (float of 5) and H (float of 20) and therefore its drag is equal to 5, the float of G.

In network schedules that include start-to-start (SS), finish-to-finish (FF) and start-to-finish (SF) relationships and lags, drag computation can be quite complex, often requiring either the decomposition of critical path activities into their components so as to create all relationships as finish-to-start, or the use of scheduling software that computes critical path drag with complex dependencies.

Critical path drag is often combined with an estimate of the increased cost and/or reduced expected value of the project due to each unit of the critical path's duration. This allows such cost to be attributed to individual critical path activities through their respective drag amounts (i.e., the activity's "drag cost"). If the cost of each unit of time in the diagram above is $10,000, the drag cost of E would be $200,000, B would be $150,000, A would be $100,000, and C and D $50,000 each.

This in turn can allow a project manager to justify those additional resources that will reduce the drag and drag cost of specific critical path activities where the cost of such resources would be less than the value generated by reduction in drag. For example, if the addition of $50,000 worth of resources would reduce the duration of B to ten days, the project would take only 55 days, B's drag would be reduced to five days, and its drag cost would be reduced to $50,000.

Sources

^ Devaux, Stephen A. Total Project Control: A Manager's Guide to Integrated Project Planning, Measuring, and Tracking. John Wiley & Sons, pp. 138 - 146, 1999. ISBN 0-471-32859-6. {{cite book}}: Cite has empty unknown parameter: |1= (help)
^ William Duncan and Stephen Devaux "Scheduling Is a Drag" Projects@Work on-line magazine
^ Stephen A. Devaux "The Drag Efficient: The Missing Quantification of Time on the Critical Path" Defense AT&L magazine of the Defense Acquisition University.

Sources

Further reading