To shorten the project duration, we must modify one or more activities on the critical path, either by eliminating them, reducing their duration, or doing portions of them in parallel with other activities. But which critical path activities will give us the most bang for our buck?  Depending on the dependencies of our activity schedule, a six-week activity may be adding only one day to our project, while a five-day activity might be delaying our completion by its full five days.  This is precisely what DRAG quantifies.

If an activity is not on the critical path, traditional CPM software quantifies its slack (float) the amount by which it is removed from the critical path.  However, project management software does not calculate this much more critical (literally!) and hard-to-compute number: a critical path activity’s DRAG.

Yet without this information, a project manager’s attempts to reduce the duration of complex schedules become little more than a hit-or-miss proposition.

What is DRAG Cost?

Time, said Benjamin Franklin, is money!  And this is never more true than on a project. In a sense, DRAG Cost could be referred to as Franklin’s Number. It is the amount of money that an activity within a project’s critical path is costing, simply by delaying the harvesting of the project’s value at its completion.  The amount can be millions of dollars per week, if an extended project duration results in a missed market window, dissatisfied customer, late delivery penalty, or delay in implementing a mission-critical or productivity-enhancing software package.

No one ever listens when I say I need more resources! is the project manager’s eternal lament.  But now, by tying the cost of delay to the DRAGs of individual activities, the impact of not having more resources for that activity becomes tangible and monetized!

If reducing project duration is worth $100,000 per week, and an activity has DRAG of three weeks, surely no one would argue against a $20,000-per-week resource that reduces the DRAG to zero?

How is DRAG assigned?

DRAG is assigned in three ways:

1. To critical path activities.
2. To finish lags (finish-to-start and finish-to-finish) on the critical path.
3. To start lags (start-to-start and start-to-finish) on the critical path only where the lag value is greater than the duration of the predecessor activity in the relationship.

Normally, DRAG is not assigned to start lags (start-to-start and start-to-finish).  The reason is that start lags typically reflect work associated with, and encompassed within, the predecessor activity.  For example, if PLASTER WALLS is 5 days long, and is a start-to-start 2 (SS2) predecessor of PAINT WALLS,  compression of the time for plastering would also be expected to compress the lag.  If PLASTER WALLS is eliminated, or compressed to zero duration, PAINT WALLS would be expected to start with no delay, the work represented by the lag having been eliminated also.

However, in cases where the predecessor is of less duration than the lag, the DRAG is assessed against the predecessor until its duration is exhausted, and any remaining DRAG is then assessed against the lag in the relationship. For example, if Activity A has a duration of 5 days, but is an SS7 predecessor of Activity B, A might be assessed as having Activity DRAG of 5 days, while the lag between A and B would have Extra Start Lag DRAG of 2 days.

Conversely, finish lags (both FS and FF) are really work/delays that occur after the predecessor has finished.  They can be represented by activities instead of lag values, and therefore they have DRAG independent of either their predecessors or successors.

For more on DRAG, DRAG Cost, and other TPC metrics, see Total Project Control: A Manager’s Guide to Integrated Project Planning, Measuring, and Tracking by Stephen A. Devaux (John Wiley & Sons, 1999)