Baseline Analysis
Schedules deviate from the baseline as a program is executed. Changes in resource availability, late or early key deliveries, unexpected additional work activities, and risks can contribute to deviation. Although they are often perceived as something bad, negative variances provide valuable insight into program risk and its causes. Positive variances can indicate problems as well. For example, early starts may cause issues with out-of-sequence logic and can disrupt the scheduling of future resources.
Understanding the types of activities that have started earlier or later than planned is vital as well. For instance, positive variances may not be desirable if only relatively easily accomplished activities are completed early while critical activities are delayed. Variances empower management to decide how best to handle risks. Schedule deviations from the baseline plan give management at all levels information about whether corrections will bring the program back on track or completion dates need updating.
A schedule variance does not necessarily mean program delay; it means that work was not completed as planned. Negative schedule variances should be investigated to see if the effort is on the critical path. If it is, then the whole program will be delayed. In addition, activities that vary significantly from their baseline may create a new critical path or near-critical path.
Carefully monitoring the schedule allows for quickly determining when forecasted completion dates differ from the baseline dates. In this respect, progress can be evaluated for whether it has met planned targets. Activities may be resequenced or resources realigned. It is also important to determine whether schedule variances are affecting successive work activities. For example, a schedule variance may compress remaining activities’ durations or cause “stacking” of activities toward the end of the program, to the point at which it is no longer realistic to predict success.
A schedule Gantt chart can be used to show baseline dates, forecast dates, and actual progress for each activity. Consider the activities in the foundation and underground work phase of the house construction project in figure 48. Red activities are forecasted to be critical and blue activities will have 4 days of total float on their paths. The gray bars below the red and blue activity bars represent the baseline start and finish dates. The original plan is to begin staking on September 15 and to complete pouring footings and pads by September 24. Before any progress is made on the project, baseline dates and forecast dates are the same.
Figure 48: Baselined Activities
Figure 49 gives the current schedule for the same plan, now statused through September 19. The green vertical line represents the current status date, and the gray horizontal bars represent progress on each activity.
Figure 49: Updated Status Compared with Baseline
As seen in the current schedule, the “layout and stake property and excavation,” “dig foundation and basement,” and “lay out and form footings and pier pads” activities started and finished on time. However, not only did the “excavate for and install underground sewer” activity start 3 days late but also its effort is expected to take 32 hours longer than originally planned. The 3-day late start and the extra effort consume the available 4 days of total float on the path, and the activities “excavate for and install underground sewer” and “inspect underground sewer” are now critical. The activities “install footing and pier rebar” and “inspect footing and pier rebar” activities are no longer critical, because the delay in the underground sewer work created a day of total float for those activities. The start variance of “excavate for and install underground sewer” is 3 days and the finish variance is expected to be 5 days. The effect of the underground sewer delay can clearly be seen in the Gantt chart. Visually, the activity bars are far removed from their original baselined positions in the timeline.
In this situation, the general contractor has several options to get the construction schedule back on track:
Adjust the working calendar. Installation could take place over the weekend. Alternatively, the plumbers could work overtime to finish installing the sewer by the morning of September 23. Both options assume that resources are available for overtime work on short notice and that the project can afford the additional labor rate associated with overtime. It also assumes there will be no breach of local law such as restrictions on construction noise.
Increase resources. Adding additional workers to the sewer installation activity would reduce the activity’s duration by 1 day. Again, this assumes that additional resources are available on short notice. The additional labor cost is unavoidable because of the additional work required, but this option leaves the labor cost at the normal rate.
Perform activities concurrently. Overlapping work is typically an option but not in this case. The underground sewer work must be finished before it can be inspected.
Do nothing and allow pouring of the footing and pads a day late. This assumes that the negative float of 1 day will be addressed at some point beyond the foundation and underground work phase, perhaps by adding resources during framing or finishing. However, this option affects many subsequent resource assignments such as carpenters, plumbers, electricians, HVAC specialists, and bricklayers. Each assignment will have to be rescheduled if the delay is carried beyond foundation and underground work.
Figure 50 shows the effect of the general contractor’s plan to get the program back on track by adding extra resources to the installion activity. This reduces the installation activity by 1 day and places the “pour footings and pads” activity back on its originally planned date.
Figure 50: Updated Status and Proposed Plan Compared with Baseline
The threshold for reporting variances varies by program size, complexity, and risk. All stakeholders should agree on the threshold and it should be formally defined in a governing document. In particular, guidance should take into account the threshold for the number of days the activity is delayed as well as the available float. If a variance exceeds the threshold, it should be reported to management along with a detailed description that includes the cause and recommended corrective actions. Note that because level-of-effort activities support work activities, they never show a variance.
Various schedule measures should be analyzed to identify and better understand the effect of schedule variances. Some examples of measures for comparing the current schedule to the baseline include the number of activities that
have started early, on time, and late;
have finished early, on time, and late;
should have started but have not;
should have finished but have not;
should not have started but have;
should not have finished but have.
It is important to note that the validity of variances is directly related to the reliability of the schedule. If the schedule is not well constructed, comprehensive, or credible, any variances resulting from comparing actual status to the baseline schedule will be questionable.