FLEX is a metric of activity duration flexibility, introduced by Rafael Davila. Flex shows whether using fewer resources (or reducing resource workload) is possible without delaying other activities.

Activity Start FLEX shows the time difference between the earliest activity start and the planned activity start in the current schedule without violating any constraints and without delaying activity finish.

Activity Finish FLEX shows the time difference between the latest activity finish and the planned activity finish in the current schedule without violating any constraint and without delaying activity start.

Both metrics associated with FLEX mean increasing activity duration by keeping activity start or finish intact.

Start FLEX = Activity Early Start – Max {Early Finish + lag of all Activity Start Predecessors}

Let’s review the next scheduling fragment. It has four activities and Finish milestone:

All activities are on a critical path, and it takes 13 days to deliver this project. Activity Durations depend on resource assignment and were calculated based on 100% workload. 3 workers are available to perform the work, but they are mostly on the bench.

FLEX can be used for optimizing resource allocation. If increasing activity duration does not lead to any changes, we may decide to use fewer resources on activities with positive FLEXes to optimize resource requirements and project budget.

Should we baseline this project and commence execution? Not yet. We need to check if project duration can be reduced first. How can we find out if an acceleration is possible? Let’s calculate Activity Drag, Start and Finish FLEX!
I will use the project delivery tool, Spider Project, and specify in the setting that DRAG and FLEX need to be calculated:

The schedule is calculated with the next metrics: Total Float, Drag, Start and Finish Flex.

From the calculation, we can see that Activity C has some duration flexibility! If we reduce the allocated workforce, we can commence this activity 5 days earlier (Start FLEX) and/or delay it for 3 days later (Finish FLEX) without delaying the successors.

Also, this activity has a Negative Drag! By increasing the duration of this activity to 3 days, we can reduce the duration of the project. Let’s recalculate the schedule again but this time let’s tool automatically adjust durations and workforce.

As expected, the duration of the project is reduced to 10 days (23%)

The logic in the schedule is exactly the same as before. The acceleration was achieved by reducing the workforce of Activity C from 100% to 38%. It increases activity duration from 3 to 8 days. Critical Activity D can now commence 5 days earlier (as it has Start-Start dependency) and even has 2 days of Total float. Overall project duration is reduced from 13 tp 10 days. This schedule acceleration method is called Anti-Crashing.

It is counterintuitive to accelerate project delivery by reducing resource allocation. It is hard to identify such opportunities without calculating FLEX and DRAG metrics.

For effective project delivery, it is important to understand how resources could reallocated and what effect it makes to project delivery dates. Activity FLEX metrics give such visibility!

In simple words, FLEXes show how an activity duration can be changed without delaying other activities. It allows the use of freed-up resources for other activities and even, in some cases, reduces project duration.

Alex Lyaschenko

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