Stephen Devaux proposed some control and optimisation metrics in the Total Project Control book.

One of them, Critical Path Drag, was explained previously, read more in this post: Critical path drag. CP Drag helps to identify activities that directly contribute to project duration. The metric measures the potential to reduce project duration. However, activities with positive or negative CP Drag are good candidates for optimization, but they do not guarantee that the desired optimization is possible.

Resource elasticity

There are many factors that impact activity duration. Critical path method challenge activity duration estimation. One of them is resource elasticity.

Resource elasticity

The ability to change activity duration by changing resource allocation.

Stephen recommends an additional CPM metric, Doubled Resource Estimated Duration (DRED), to measure resource elasticity. DRED is a second duration estimate for establishing a task’s resource elasticity.

It is established by asking the question:

“If Activity X is estimated to take (20?) days with current resource estimates, how long would it take if we doubled (or could double) the resources?”

Some reasonable answers are:

  • 1D (Rare)
  • 10D (Said to be “perfectly resource elastic” — same total resource use, double the daily resources for half the time)
  • 12D (Very common situation — some “bang for the buck”, but not perfect).
  • 17D (Some value, but not huge — requires rigorous analysis of the true cost to be confident in implementing such a change).
  • 20D (Not resource elastic at all).
  • 30D (Negatively resource elastic — the resources will get in each other’s way, like in working in a cockpit).

Stephen recommends capturing the DREDs for each activity in a secondary Duration field and performing true cost analysis on them. Cost analysis includes CP Drag, Drag Cost and true cost computation.

PROS and CONS

DRED metric has some advantages and downsides.

PROS:

Simplicity
The question ‘If we can double resource allocation, what gain can we get?’ is straightforward, and the answer gives a good sense of resource elasticity.

Easy to implement.
Creating an additional field to capture additional activity duration is not difficult and can be done with most planning tools

Informative
It helps to identify activities with the highest resource elasticity.

 

CONS:

Lost effort
It requires upfront identifying DRED for all activities, but most of them (>90%?) will never be used. It is a lost effort.

Resource quantity
Resource quantity is not the only option to reduce activity duration. Other options may include changing the resource calendar, changing resource calendars, removing a material supply bottleneck, assigning resources with better productivity, reducing activity scope, changing process, etc.

Optimistic duration
DRED may not be the optimistic activity duration, and that is what we actually want to know. Activity duration is usually a spectrum between optimistic and pessimistic durations. DRED is somewhere on the curve but we don’t know where.

Monte Carlo Analysis
It can’t be used for Monte Carlo Simulation Analysis. MCS requires optimistic, expected and pessimistic estimations.

Triple estimation
What if it is possible to triple allocation? Should we also have the TRED metric? What if a resource with half capacity is available? Could such a resource be useful?

Resource availability
DRED is CP Drag’s complementary metric, but both metrics are not sufficient for optimisation. Practical optimisation depends on resource availability.

Freed-up resources
For schedule optimisation, it is important to understand if an activity can be performed with fewer resources, as freed-up resources can be used to accelerate CP Drag activities. DRED doesn’t give us this vital information.

Alternative approach

An alternative to DRED is Optimistic Activity Duration, or Crash Durations (original CPM method) Original critical path method and beyond.
While Activity DREDs is simpler, Optimistic Activity Duration is more accurate and usable.

There are four options to identify Optimistic Activity Duration:

Subjective
Collect optimistic durations from SMEs.

Statistical
Use collected statistical data.

Use primary uncertainties
Calculate Optimistic activity Durations based on primary uncertainties (volume of work, resource productivity, calendars) as 3-point estimations and technological constraints (max possible assignment quantity, min required workload, etc.).

Statistical primary uncertainties
The same as above but use statistical primary uncertainties.

Resource Supply limits

Optimistic activity duration has theoretical and practical values. 

It is important to know the theoretical duration, which is based on technological limitations only, and the feasible duration, which also considers resource supply constraints.

Summary

The main idea of DRED is to develop a proxy to complement CP Drag by analysing activity durations with double resource assignment. DRED is easy to understand and use but it needs to be applied carefully. An optimistic activity duration may be different from DRED.

An alternative approach is to identify 3-point activity estimate. It requires more effort but gives more accurate data for analysis and optimisation.

Alex Lyaschenko

PMO | Portfolio Planning & Delivery | PMP | P3O Practitioner | AgilePM Practitioner | Six Sigma