Critical path analysis is a planning technique for scheduling future activities. After the project actually begins, activities start and become "in progress". Eventually activities finish. At any given time, some activities are finished, others are in progress and others have yet to begin. Finished activities may require different times than originally estimated, and estimates for in-progress activities may also require adjustment. Once these are reflected on the data, planning for future activities can and should be changed. The add-in offers several options through the Update button at the top of the worksheet.

We start with time 0 of the pump example. The scheduled start and finish times are the early start schedule. The current time is shown in the green cell above the delay column, S9. Here it is 0.

We can track the progress of the project in real time by simply changing the current time in cell S9. The Percent Finished column adjusts to indicate the proportion of the activities that a complete at time 8, and a red bar advances in the Time row of the Gantt chart. The scheduled start and finish used for the Gantt chart and the percentage finished assume that the activities start at the ealiest start time and require the mean time to complete.

Update Button

The program provides and automated update procedure implemented through the update button. The button is used at the end of each time interval, assumed to be days for the example, and provides a series of dialogs that describe the events that may have happened during the day. We start with the current time as 0 as given in the top figure of this page.

To update the project to account for advancing time, click the Update button. The Update dialog appears as below. The current time is given and a field for a new time is provided. The default value for the new time is 1 greater than the current time, but any positive number may be used depending on how often the project is reviewed. The Do not change time checkbox is provided so that the display may be changed while keeping the same time.

Buttons at the bottom left determine which activities are to be displayed. These are illustrated later on this page.

The buttons at the bottom right determine which activities are to be reviewed during the update process. When the None review button clicked, activities start and stop at the times given in the Scheduled Start and Finished columns without interaction by the user. This is the easiest case for the users, but it assumes the unlikely result that activities are completed at their expected durations. This is satisfactory with deterministic times, but not practical for random times. With the "Elapsed >= Min" button clicked, an in-progress activity is reviewed when its elapsed time is at least as great as the minimum time given in the data. When the All in Progress button is clicked every activity currently in progress is reviewed. We illustrate below the second case. The review decisions must be made carefully because they change data on the worksheet that are sometimes difficult to reverse. Of course in practical instances it is not possible to go backward in time.

With the "Elapsed <= Min" option, the review process considers activities that are about to begin and those that are in progress and asks the user what has happened during the interval from the current time to the new time. For the example, three activities are scheduled to begin at time 0, A, B and C. The first activity considered is A. The program presents the following dialog.

We assume that it did begin, so we click the Yes button. The program then asks exactly when it began.

We enter 0.5 as an example. Next the program asks about activity B.

In this case we answer no. The program then asks how long it will be delayed.

Since there is a large slack for B, we delay it 10 hours. The review continues for activity C and we say that C begins at time 0. In this way the project data is updated as below. Notice that the length of the critical path has increased to 57.5 because A was started late.

The current time is now 1. A gray area on the Gantt chart shows that one day has passed. The Percent Finished column now has nonzero entries showing the percentage finished of each activity. Activities with 100% are complete. These are colored dark orange. In-progress activities have percentages strictly between 0% and 100%. These are colored light orange. The remainder of the activities have not yet started and are yellow. The black bar for activity B shows the delay of 10 hours. Activity B only requires 0.25 hours, so its time does not show on the Gantt chart.

Review of the data will show that the two activities in progress, A and C, have minimum durations of 9 and 20. respectively. The next activity due to start is B, at time 10, so nothing is due to happen from hours 1 through 9. Using the update button, we advance to time 10. At time 10, activity A has an elapsed time of 9.5 hours that is larger than the minimum of 9. Since it could possibly be finished, the program presents a dialog below. Clicking the activity finished button will indicate the activity is finished. Otherwise the estimates for the activity time may be changed. Since the activity is in progress, more information about its completion time may now be available. We assume the activity did not finish, so click OK.

Activity B is due to begin at time 10. The program asks if it did. We answer that it has.

Since B is very short, it may also finish during the interval. We answer Yes.

We specify the finish time as 10.5.

At time 11, the partially completed project is shown below. Notice the minimum activity times are adjusted as the activity progresses. The activity time for A was originally 9, but since it has not finished by time 11, the minimum must be at least 10.5 (the activity started at 0.5). Similarly, since B has finished with at time of 0.5, the minimum, most likely and maximum times for B have all been set to the duration observed, 0.5.

The update and review process continues until the project is complete. The three time estimates for an activity become the same as activities are completed, so the variance of the project duration decreases as the current time increases.

The process considers activities in precedence order, that is, an activity is reviewed before any of its successors. This is necessary because decisions regarding the delay or time estimate for an activity will affect the scheduled start times of its successors.

It is always possible to update data directly on the worksheet rather than through the review dialog. For most changes the results on the worksheet dynamically recompute. For changes in the project structure, defined by set of activities and the precedence relations, the model must be rebuilt by clicking the Solve button. Click the Schedule button to rebuild that worksheet after a change in the project structure.

The review process will increase an activity's delay when the activity starts after its scheduled start time. There is no opportunity, however, to decrease a planned delay during the review. This is particularly important for delays on the critical path as these affect the project completion time. After an update operation, the manager should play close attention to the delay column for the critical activities and reduce the delays where possible and advisable.

There are three alternatives for displaying the activities: In Progress, Critical and All. The last option shows all the activities as illustrated above.

The Critical option shows only the critical activities. This is useful because the project completion time is sensitive to these activities. With this display the analyst can focus on the important estimates. The figure below shows the results after 11 hours.

The program constructs the display by hiding some of the rows of the worksheet. The information is not lost, however. The rows will reappear when a different display option is chosen or if the user selects all the hidden rows and chooses the Format/Rows/Unhide command from the Excel menu bar. Hidden columns can be displayed with the Format/Columns/Unhide command.

The final display option shows only the activities that are currently in progress. These activities are important because the impact of current events such as weather and resource availability are most apparent while the activities are in progress.