Use of Lags

“Slaying the dragon of delay is no sport for the short-winded.”

- Sandra Day O’Connor

In this metric, DCMA looks at how lags are used in the schedule. A lag is a fixed delay added to a relationship between two activities.

Suppose that Task-B is a finish-to-start successor to Task-A. As such, Task-B can start after Task-A finishes. When it’s appropriate, the scheduler can add a lag to their relationship. If the scheduler wants two days to elapse after Task-A finishes before Task-B starts, they would add a 2-day lag to the FS relationship. A lag is essentially a waiting period.

There are real-world examples where adding lags is the correct thing to do. You can apply a second layer of paint to a room after the first layer is applied, but you’ll need to wait a fixed amount of time so the first layer of paint can dry. The scheduling engine, which calculates the critical path, will add that lag when calculating the start date of the successor.

Because there are legitimate reasons to use lags, DCMA is merely looking for a red flag, a sign that lags are (possibly) being misused. They ignore completed activities in this check because the past cannot be changed. They do consider milestones in this metric. If the percentage of activities with a predecessor relationship containing a lag exceeds 10% of the total number of incomplete activities, the metric fails.

If your schedule fails this metric, give each relationship with a lag some scrutiny. Is the lag necessary? Is adding a linking activity between the predecessor and successor the right solution? Is there a true dependency between the two activities? 

Any reduction in lags along the critical path pulls in the end date, so this scrutiny is a good idea anyway.

What about leads?

A lead is essentially a negative lag. While a lag introduces a fixed delay between a predecessor and successor, a lead is an overlap. DCMA checks for leads in their 06A212A check (Out of Sequence Conditions).

Next steps

Investigate the reason behind the use of each lag to ensure they are being used properly. Look for opportunities to reduce lag along the critical path.

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