For the Fixed Time option, only a single data column is provided for the activity time. This is the case for the CPM model.

The PERT model assumes that activity times are random variables. For the Random Time option, three data columns and four computed columns are used as illustrated for the example. Cells for data are white with maroon borders and cells with formulas are yellow. The contents of yellow cells are created by the add-in and should not be changed.

The traditional PERT model describes a probability distribution for an activity time with three parameters. This is convenient for those unfamiliar with more complex representations of probability distributions. The parameters are the minimum time (a in the formulas below), the most likely time (m) and the maximum time (b). Although these parameters are appropriate for the triangular distribution and Beta distribution, the moments computed below are not accurate for either form. The values of a, m and b are placed in columns H, I and J.

The analysis uses the Mean, Standard Deviation and Variance values of the activity time distributions. These values are computed with the emperical formulas below:

The computations are preformed in columns K, L and M.

The analysis uses only the mean value for activity time estimates, so this value is repeated in column N.

Precedence and Resource Data

An important aspect of scheduling is that an activity cannot begin before all its predecessor activities are complete. Columns O through R are available to enter this data for the example. The figure below hides columns H through N so we can easily see the predecessors along with the activity descriptions. Predecessors are indicated by name. Thus we see a predecessor for activity D (Cure Slab) is activity A (Pour Slab). The names in the predecessor range must be exactly the same as those in the name column. After the problem is solved, the number in the yellow cell O25 will hold the number of predecessor entries.

Columns are provided to enter the resource requirements for each activity. For the example, we have a single column to hold the crew sizes for the activities. The name of the resource is entered just above the column (S9). This name is used to identify the resource on the Schedule page of the workbook.

To find the Critical Path for this problem, click the Solve button at the top of the worksheet. The critical path is discussed on the next page.

Change

Clicking this Change button at the top of the page allows additions or deletions from the set of activities. The dialog for adding activities is below. Clicking OK with this data will add a new row between the first and second activities. The indices of the activities change, but the names are not affected. Activities cannot be added before the first activity or after the last activity.

The dialog for deleting activities is similar. When an activity is deleted all references to it are deleted from the Precedence range. The first or last activities cannot be deleted.

The third option is for changing the number of variable activities. We describe this on a later page.