Leads and Lags

Posted on March 20, 2009 by

1


Applying Leads and Lags is a tool and technique of the fourth edition PMBOK®’s Sequence Activities process; and Adjusting Leads and Lags is a tool and technique of the Control Schedule process. In order to manage a project schedule, a project manager should understand the simple concepts of leads and lags. Leads and lags are sometimes needed in order to more accurately describe the relationship among project activities. For example, the relationship between two project activities may be that one is the predecessor (it needs to be done before the successor activity) and the other activity is the successor (it occurs after the predecessor). If one activity is mixing cake ingredients and the other activity is placing the cake batter in the oven, then the relationship between these two activities is that mixing cake ingredients is the predecessor and placing the cake batter in the oven is the successor.

A lead allows the successor activity to begin more quickly, and a lag means there is a delay before the successor activity may begin. For example, the activity of turning the oven on may take less than 1 minute, and the successor activity of placing the cake batter in the oven may take less than 1 minute. The schedule would be flawed, however, if only a couple minutes were allowed for completing both turning on the oven and placing the cake batter in the oven. This is because there is a lag of the time it takes for the oven to heat to the desired temperature. We may need to add a lag of 15 minutes to account for the oven heating before we can place the cake batter inside the oven. However, even though the oven is not done heating, we could be mixing cake ingredients while it is heating, which gives us some lead time (the ability to start on this activity sooner).