Schedule prediction: From a retained to an overridden logic

Submitted by Mario Vanhoucke on Tue, 01/17/2012 - 17:14

One of the primary tasks during project control is periodically updating the baseline schedule to reflect the actual progress of the work done and to present a realistic forecast of the remaining work. A project in progress can be monitored using Earned Value Management (EVM). In this technique, the schedule predictions using the performance metrics available in EVM completely rely on estimates about the percentage completion of the activities and give a helicopter view on the performance of the project at the current status day (see “Earned Value Management: Forecasting project outcome”).

A more time-consuming and detailed alternative is to update the baseline Gantt chart at the activity level. In this so-called tracking Gantt chart, the schedule prediction is made by taking the actual and estimated remaining durations of each activity into account and by updating the Gantt chart. In doing so, the tracking Gantt chart gives a prediction of the future schedule based on the inputs of actual and remaining durations/costs of activities. In this article, periodic updates of a project in progress using the tracking Gantt chart in order to obtain predictions of timing of the remaining work is illustrated using three options, as follows:

Overridden logic: When work is performed out-of-sequence, some of the precedence relations of the remaining work are no longer respected
Retained logic: All remaining work is assumed to follow the logic of the original baseline precedence relations
Percentage overridden/retained logic: When work is performed out-of-sequence, the logic of the precedence relations is only partially violated as an option in-between the overridden and retained logic

Figure 1 displays an illustrative baseline schedule Gantt chart with two activities. Activity 1 has an estimated duration of 15 days while its successor activity 2 has an estimated duration of 20 days. The precedence relation between the two activities is assumed to be a finish-start relation with a minimal time-lag equal to zero. The earliest start (ES) and earliest finish (EF) times of each activity are indicated, resulting in a total project planned duration of 35 days. For more information on the use of precedence relations, see “Activity links: How to add precedence relations between activities?”. Detailed information on the earliest and latest start calculations can be found in “Scheduling projects: How to determine the critical path using activity slack calculations?”.

Figure 1: An example earliest start baseline schedule?

Retained or overridden logic

Since work is often performed out-of-sequence, the original logic captured by the precedence relations between activities as specified in the baseline schedule is often violated. This situation can cause unrealistic deviations between the baseline scheduling logic and the project tracking Gantt chart, and often leads to unnecessary adaptations and modifications to the baseline schedule.

Generally, there are two options available in software tools to handle out-of-sequence progress during the tracking phase, as follows:

Retained logic assumes that the original precedence relations are still valid, even when activity overlaps during progress have taken place. This logic respects all precedence relations of the remaining work, but often leads to unrealistically long project duration forecasts.
Overridden logic assumes that an activity that started with a certain overlap will violate the original precedence relation logic completely. This logic assumes that the remaining work of an activity in progress can be done without being affected by its incomplete predecessor activities, but it often leads to unrealistically short project duration forecasts. This logic is also known as out-of-sequence progress.

These two extreme logics are illustrated in figure 2 where it is assumed that the current status day is equal to day 8. Activity 1 has been in progress for 8 days, and the project manager estimates that it will take another 12 days to finish this activity (i.e. the remaining duration (RD) is equal to 12 days). Even though activity 1 has not finished yet, activity 2 has already started and has been in progress for 2 days. Consequently, the original precedence relation between both activities has been violated. The remaining duration is estimated at 18 days. More information on setting actual and remaining durations can be found in the “Updating the schedule: Setting actual and remaining cost values of activities in progress” article.

Since the relations between the two activities have not been respected, the overridden logic assumes that this will not be the case in the future, and hence, the remaining work of activity 2 can start immediately at the status day, leading to an earliest finish of 26 days. The retained logic keeps respecting the precedence relations incorporated by the baseline scheduling logic (figure 1) and assumes that the remaining work of activity 2 can only start after the expected finish of activity 1. This logic leads to an expected project duration equal to 38 days.

Figure 2: Overridden (left) and retained (right) logic for the remaining work at status day = 8

Figure 2 shows that the expected delay of activity 1 will be recovered by the out-of-sequence work of activity 2, leading to a shorter project duration than stipulated by the baseline schedule (26 < 35). In the retained logic, the delay in activity 1 will cause a delay in the expected project duration (38 > 35), despite the out-of-sequence work of activity 2.

Percentage overridden/retained logic

Since both the overridden and the retained logic display a schedule forecast taking both the actual activity durations (the work done) and the remaining activity durations (the work yet to be done) into account, a third option is available that constructs a schedule forecast for the remaining work in between the overridden and retained logic.

Motivated by the observation that the tracking Gantt chart is nothing more than a schedule forecast, the option to shift between the two extreme logics should allow the user to adjust the forecast according to his/her own wishes.

The percentage overridden/retained logic displays a tracking Gantt chart as a percentage in between the overridden (0%) and retained (100%) logic, as follows:

0%: In the overridden logic, the remaining work of activity 2 can be started immediately (i.e. 0 days after the current status day).
100%: In the retained logic, the remaining work of activity 2 can be started when its predecessor is finished, i.e. 20 - 8 = 12 days after the current status day.
x%: The remaining work of activity 2 can be started in-between the start defined by the overridden logic and the start defined by the retained logic. More precisely, x% of the retained logic start minus the current status day (x% * 12 days) is added to the status day to define the start of the remaining work.

The percentage between the overridden and retained logic obviously affects the total project duration estimate. In figure 3, the percentage overridden/retained logic is set at 33% (left) and 66% (right), leading to an expected project duration equal to 30 and 34, respectively. As an example, a percentage of 33% leads to an intermediate tracking Gantt chart where activity 2 will start 0.33 * 12 days = 4 days after the current status day.

Figure 3: Percentage retained/overridden logic equal to 33% (left) or 66% (right) at status day = 8

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Schedule prediction: From a retained to an overridden logic