Schedule Analysis Metrics: Recovery Gap

We are continuing to review advanced scheduling metrics. One of the biggest misconceptions in planning & scheduling is that Total Float metric can be positive, zero or negative.

There are many definitions of this metric but mostly, it is agreed the metric shows “a period of time an activity can be delayed without impacting the delivery date”.  Other popular Total Float definitions: 

1. Time by which activity can be delayed or extended without affecting the total project duration or delay finish day.
“Planning, Scheduling, Monitoring and Control”, APM

2. The amount of time that a schedule activity can be delayed or extended from its early start date without delaying the project finish date or violating a schedule constraint.
“Practice Standard for Scheduling”, 3rd edition, PMI

2. The amount of time an activity can be delayed or extended before delay affects the program’s finish date.
“GAO Schedule Assessment Guide”, GAO

 All above definitions said: “CAN BE DELAYED”. It means Total Float can’t be negative as it is impossible to delay an activity for a negative period. It can be zero or positive only.

At the same time, an activity may be behind a target in the context of project deadline(s), but it is a different scheduling metric!
Such activity may or may not have Total Float = zero!

If it has TF = 0, it means that the delay of the activity will result in further project delay. Otherwise, activity delay (within the existing float) will not further delay the project.

It is important to know which set of activities drives an achievable (calculated) delivery date, known as the Forecast Date. The set is defined by activities with TF = 0.

On top of that, it is useful to know what other activities need our attention, and that is where the second metric is needed. Let’s call it the Activity Recovery Gap (RG).

Total Float: Measures how much activity can be delayed before it impacts the project’s completion (forecast) date.

Recovery Gap: Measures how much time is needed to accelerate an activity (or set of activities) to meet the project’s completion (target) date.

The Recovery Gap can be more than the activity duration. In this case, the acceleration needs to be done for other activities as well.

These two metrics complement each other. RG indicates which activities need to be accelerated, and TF indicates where an acceleration effort may give a result.

Additionally, if the project can find a way to accelerate delivery by applying corrective actions to the critical path activities (with TF=0) or changing critical path logic, the expected accelerations will only be archived if it doesn’t negatively impact activities that have a negative Recovery Gap.

For example, reallocating resources may reduce the duration of a critical activity. However, if additional resources are taken from an activity with a negative Recovery Gap, the desired acceleration may not be achieved. While it reduces the duration of analysed critical activity, it also increases the duration of another (critical or near critical) activity, and the overall project duration may not be changed as expected.

Another metric, Critical Path Activity Drag, indicates the possible effect of acceleration.

This metric is essential for project acceleration analysis.

Assume a project has activities with negative Recovery Gaps. It indicates that acceleration is essential. Total Float indicates where acceleration may give a result. Activity Drag measures the maximum effect of accelerations if such is possible. However, Activity Drag doesn’t consider how acceleration is achieved. So, Recovery Gaps of other activities need to be analysed to understand where additional resources can be taken from.

Let’s review a scheduling fragment to understand how these metrics work together.

In this scheduling scenario, Activities A, B & C have TF = 0 and define when the project can be delivered: in 13 days.

The project has a target to be delivered in 10 days. The critical path activities have negative recovery gaps – minus three days each. Additionally, activity D also has a recovery gap – minus one day.

However, the acceleration of activity D will not give an overall acceleration as this activity has a positive Total Float.

The project team needs to find out where accelerations are possible. Assuming doubling resources on activity A can reduce the duration of this activity to three days (50%). However, Drag for this activity (2 days) indicates that even if it can be reduced by three days, the overall effect will not be more than two days, and the project needs to optimise other activities to bring the delivery date back to the target.
Let’s continue the schedule analysis with Activity A. Assuming the project may relocate resources from parallel Activity D to Activity A, and it gives a proportional effect: reduce the duration of Activity A by two days and increase the duration of Activity D by two days as well.

Activity D has Recovery Gap = -1 day. It indicates that taking resources from this activity may have an overall negative effect.

Let’s apply a partial acceleration first: reduce Activity A by one day and increase activity D by one day, as well.

The acceleration is achieved! Overall project duration is reduced by one day.

Now, let’s apply the full acceleration action: reduce Activity A by two days (based on the original scenario) and increase Activity D by two days. The acceleration action has the opposite effect: overall project duration increased back to 13 days!

By reducing the duration of critical path activities, we increased the duration of non-critical activity, changing the project’s critical path. The new critical path is Activities D, B, C.

The Recovery Gap of activity D gave us an earlier indication of such an effect.

When someone refers to Total Float, they actually refer to Total Float Start metric.
Total Float Start indicates how much flexibility there is to push the activity’s start without affecting the finish date of the project.

Another metric, Total Float Finish, indicates how much flexibility there is to push the activity’s finish without impacting the project’s overall completion.

The same principle is applied to the Recovery Gap metric. There are two metrics: Recovery Gap Start and Recovery Gap Finish.

If a schedule has other than Finish-to-Start dependencies both metrics need to be analysed.

Unfortunately, popular scheduling tools, Primavera P6 and Microsoft Project, use the Total Float field to represent both metrics, Total Float and Recovery Gap. If deadline(s) are added to the delivery model, it calculates the Recovery Gap, otherwise Total Float. Without a more profound analysis it is hard to understand which metric is calculated.

It creates a lot of confusion among planners. Some believe that Total Float in these tools is calculated in the context of project forecast, and others think it is in the context of target(s).

MS Project, compared to Primavera, uses a dedicated field (Deadline) to capture an activity target. It opens an opportunity to create VBA macros and calculate both Metrics (TF Start and RG Start only).

You can find this macro in previuos post 

Total Float in MS Project called Total Slack.

Primavera and MS Project don’t calculate activity Drag metric, but another VBA macro can be used to calculate Activity DRAG and Activity Elapse DRAG metrics.

Advanced project delivery tools, Spider Project, can calculate all discussed metrics. In Spider Project Recovery Gap is called ‘Float Negative’.

The Recovery Gap is a crucial metric for schedule analysis, providing a fresh perspective on project progress and aiding planners in optimising the delivery plan.

It highlights which streams in the delivery model require acceleration and which have built-in flexibility.

When used alongside other scheduling metrics, such as Total Float and Activity Drag, the Recovery Gap enhances planners’ ability to assess the impact of potential changes on overall project outcomes.