Concept: What level of performance is required to recover the project’s target date?
The To Complete Schedule Performance Index (TSPI) answers the question.
Practice: TSPI is a prescriptive metric. It prescribes the future performance level needed to achieve the desired duration.
TSPI is a ratio between two differences: the difference between planned duration and earned schedule and the difference between total duration and actual time.
In the context of the plan, total duration is given by the Planned Duration, and the formula is (PD – ES) / (PD – AT). In the context of estimates, total duration is given by the Estimated Duration, and the formula is (PD – ES) / (ED – AT).
Planned Duration is derived directly from the baseline schedule. It is the difference between the baseline finish date and baseline start date. Estimated Duration can be based on statistical analysis of past performance or on analysis of risks, opportunities, and external factors.
TSPI can be used to assess the feasibility of the planned or estimated duration. For this purpose, TSPI is compared to a specific threshold value of 1.10.
If TSPI is steadily below the threshold or falling away from it, recovery is not called for. As TSPI climbs toward the threshold, the need for action rises. Once the threshold is breached, the duration can be regarded as unrecoverable.
TSPI in Action
TSPI can be represented in a chart. Add the 1.10 threshold to make trends easy to see.
In the figure below, the vertical axis represents the index value. It starts at 0 and increases to the maximum observed value or to the threshold value, whichever is greater.
The horizontal axis is set at the estimated duration. Note that the estimated completion date can be used in place of numerical periods—simply add duration estimates to the starting date.
The recoverability threshold is represented as a straight line at 1.10. The line extends from the first period to the last period.
TSPI is then represented as a series of values between zero (0) and the maximum value. The line expressing TSPI extends from the first period in which value is earned to the actual time.
Note that the maximum and minimum values of the index can be adjusted up or down to improve the appearance of the chart.
TSPI is steadily below the threshold or falling away from it, recovery is not called for. As TSPI climbs toward the threshold, the need for action rises. Once the threshold is breached, the duration can be regarded as unrecoverable.
Figure 1
In Figure 1, recovery is not indicated over the first three months, as TSPI is below the threshold and is essentially flat.
Over the next three months, the TSPI rises steadily toward the threshold value. That’s a call to action: recover or else.
In this example, action is taken. Problems are identified. Root-cause analysis is done. Remediation is planned and initiated. While those steps are underway, TSPI hovers just below the threshold value. During that period, it is unclear whether or not remediation is successful. But, at least, the situation is not worsening.
Finally, the last three months show that remediation is effective, and recovery is in hand.
From this point forward, TSPI can be monitored to ensure an on-time finish.
Trend Analysis
From one time period to the next, there is natural variance in schedule performance. If that variance happens to cross a threshold value, it can signal a need for action. Although you can take action immediately, you run the risk of an unnecessary escalation. Too many of them and the call to action loses its strength.
Trend analysis is one way to avoid unnecessary escalations. Rather than respond immediately to an individual threshold breach, you determine if the breach is part of a trend and then determine if action is called for.
Trend analysis is especially important for TSPI. With TSPI, waiting until the 1.10 threshold is breached makes it too late to respond effectively. You need to take action long before that point is reached.
Trends comprise three or more consecutive measurements. For TSPI, if the consecutive measurements converge on the threshold value of 1.10, take action. If the trend is away from the 1.10 threshold, you can stand down.
Apart from the direction of a trend, there is another property that can be used to invoke action: the magnitude of the trend. The magnitude is the percentage change in a measurement from one period to the next.
A single jump toward 1.10 can signal the need for action. How much of jump qualifies? The rule-of-thumb at ProjectFlightDeck is a change of 20% or more.
In practice, we use the direction of the trend more often than a single change to trigger action.
Finally, TSPI sometimes hovers just under the 1.10 threshold. Proximity to the threshold suggests the need for escalation, but it also leaves open the possibility of an on-time finish.
Isuch cases, other recovery measurements can help decide the matter: the window of opportunity for recovery, probability of recovery, and performance level required in each recovery period. All of these metrics will be addressed in future posts.
With support from other earned schedule measurements, the call to action can be made before the threshold of 1.10 is breached.
Here are the Pros and Cons of TSPI:
Pro:
- Quantitative metrics resist cognitive bias and strategic misrepresentation.
- The threshold value (1.1) is supported theoretically and empirically.
- Visuals of TSPI vs. threshold value are useful for analysis, communication, decision making.
Con:
- Not an early warning—a threshold breach signals unrecoverability—late for correction. |
