Concept: The Schedule Performance Index for time (SPIt) is the ratio between the time earned on a project (i.e., the Earned Schedule, ES) and the time actually consumed (the Actual Time, AT).
Practice: The SPIt is an ES metric commonly used to monitor schedule performance. Given that the project is already tracking the amount of schedule earned, the SPIt is easy to calculate: simply divide ES by AT.
The SPIt measures schedule performance efficiency. It indicates whether or not time is being earned according to plan.
If the SPIt is either above or below 1.0, time is not being used according to plan. If the SPIt is above 1.0, time is being used more efficiently than planned. If the SPIt is below 1.0, time is being used less efficiently than planned.
The SPIt is widely applicable. Because it's a ratio, the SPIt applies equally to projects of different sizes. It also applies when diverse project management approaches are in use. For instance, SPIt works just as well on Agile projects as it does on plan-driven projects. That makes SPIt a useful measure across diverse projects.
Unlike other metrics, the SPIt is accurate across a project’s whole lifecycle. If the project is late, the SPIt reflects the shortfall, rather than ending necessarily at a perfect 1.0.
The SPIt can be used in conjunction with threshold values to assess how well (or poorly) time is being used. That, in turn, indicates whether or not action is required.
Thresholds values should be set at the beginning of the project or program. They can be unique to that project or program or be based on a standard set either by a governance document or contract.
One commonly used threshold is variance from plan by +/-10%.
Negative variance, i.e., SPIt less than 0.9, garners the most attention, perhaps because it occurs frequently. But, schedule performance above 1.10 can equally well indicate a problem. For instance, excessive allowance for uncertainty can inflate the apparent performance level, as value is earned at a faster pace than planned.
Regardless of the threshold breached, if it is crossed, it warrants attention. The focus intensifies in the presence of a trend.
Three measurements headed in the same (bad) direction constitute a trend. If the movement combines with a threshold breach, the priority of action increases.
In any case, action steps include: problem identification, root-cause analysis, remediation planning, and remediation implementation.
Note: One limitation on the use of SPIt: it does not predict future performance. While an SPIt trend line suggests how the project will perform in the future, it does so based on impression and experience, rather than on quantitative analysis. We have found that other ES metrics, function better than SPIt for predicting future performance.
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