A lag, also known as a link offset, is a way to prevent a link from collapsing to zero gap; or, in other words, to enforce an absolute minimum gap. Lags may be used, for example, to more accurately model the curing time for concrete, where using an activity may incorrectly suggest that there is work taking place.
The value of the lag equals the number of days from the predecessor’s finish node and adopts the calendar of the predecessor activity. If a link had a lag of 7 days, and its successor were pulled back as far as it could go, the link would cease collapsing once the gap equaled 7. If a negative value is entered, this allows the start of the successor to push before the finish of the predecessor activity even when logic is on.
A lag displays as a red dot on top of the link, but it is not visible if the lag is too close to an activity node. You can also tell when a link has a lag in place by a red dot placed next to its gap value. This gives a cue that a lag is affecting the link gap value. Both the gap, and the dot next to the gap, however, will automatically be hidden if the gap reaches zero.
NOTE: Negative lags can only be applied to finish-to-start relationships. Upon opening a legacy schedule with negative lags on relationships other than finish-to-start, they will be removed automatically, a message will display, and the links with lags removed will become highlighted on the canvas.
- With the Select Tool chosen in the toolbar, double-click on a desired link, or right-click and choose
- Under Properties, enter a number other than 0 next to Lag.
- To display a red dot next to the gap value, check the box next to Lag Notation.
- Click OK to close the window and apply the lag.
Lag Gap Calculation
Once a lag is defined, it affects the gap calculation. The gap of a link with a lag is calculated as the gap of the predecessor to the successor minus the lag value. In the following example, the original gap between the predecessor and successor is 5 calendar days.
Below, a lag of 3 calendar days is placed.
As you can see, the new gap of 2 days is equal to the original gap of 5 days minus the lag of 3 days.