Understanding Lags and Leads in Primavera P6
A Lag is a duration that is applied to a dependency to make the successor start or finish earlier or later.
- A successor activity will start later when a positive Lag is assigned. Therefore, an activity requiring a 3-day delay between the finish of one activity and start of another will require a positive lag of 3 days.
- Conversely, a lag may be negative when a new activity may be started before the predecessor activity is finished. This is called a Lead or Negative Lag.
- Leads and Lags may be applied to any relationship type.
Here are some important points to understand about Lags:
- The lag duration is calculated on the lag as in Microsoft Project and other Primavera products. A lag is not assigned to one or both of the Predecessor and Successor activities as in Asta Powerproject.
- Lags may be assigned one of four calendars from the Calendar for Scheduling Relationship lag drop down box in the General Schedule Options form. This form is opened by selecting Tools, Schedule… and clicking on the tab. The four Lag Calendar options are:
- Predecessor Activity Calendar,
- Successor Activity Calendar,
- 24-Hour Calendar, and
- Project Default Calendar.
Lags are calculated by Primavera P3 and SureTrak software using the Predecessor Calendar. Microsoft Project 2003 to 2010 uses the Successor Calendar or may have an Elapsed Duration Lag. Earlier versions of Microsoft Project used the Project Calendar.
Asta Powerproject assigns the lag to either or both the predecessor or successor thus allowing either the Predecessor or Successor Calendar to be selected for each relationship.
You must be careful when using a lag to allow for delays such as curing concrete when the Lag Calendar is not a seven-day calendar. Because this type of activity lapses nonwork days, the activity could finish before Primavera calculated finish date.
You must be extremely careful when opening multiple projects when the Lag Calendar option is different for each project. This is because all the project options are changed permanently to be the same as the Default Project and therefore some of your projects may not calculate the same way as they did before opening the projects together.
Please read the Multiple Project Scheduling chapter for more details on this topic.