By creating a schedule model, we are able to use the Critical Path Method (CPM) to help us determine when our project can be completed and which tasks are the most likely to cause a problem in meeting our schedule. Using the information from the schedule model, dates and slack are calculated for each task and milestone using CPM.
CPM is a three step process:
- Forward Pass – begins at the start of the project and calculates the ‘Early’ dates.
- Backward Pass – begins at the project’s end and calculates the ‘Late’ dates.
- Slack Calculations – calculates the Total Slack which represents the flexibility, in our schedule.
Below is the standard format in a schedule model for displaying dates and slack. The Early Start and Early Finish dates are shown on top of the task box. The Late Start and Late Finish dates are shown below the task box with the Total Slack value between them.
If scheduling in increments of days:
- Start dates are at the beginning of the day.
- Finish dates are at the end of the day.
Although Microsoft Project performs the forward pass, backward pass, and Total Slack calculations for us, it is critical to understand the details of how those calculations are performed. If we don’t understand how Microsoft Project is calculating information, we will not be efficient or effective in analyzing changes and troubleshooting paths that are pushing out our end date.
Therefore, in this lesson you will be covering some examples of detailed calculations for the forward pass, the backward pass, and Total Slack values.