Project Planning and Control Using Oracle Primavera P6 - Version 8.2 EPPM Web
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.
Understanding Constraints in Primavera P6
Constraints are applied to Activities when relationships do not provide the required result and are often a result of External Dependencies.
Typical applications of a constraint are to constrain an activity to a date for:
- The availability of a site to commence work.
- The supply of information by a client.
- The required finish date of a project.
Constraints are often entered against Milestone activities to represent contract dates and may be directly related to contract items using Notebook Topics.
Constraints are covered in detail in the Constraints chapter.
There are four types of dependencies available in Primavera P6:
- Finish-to-Start (FS) (also known as conventional)
- Start-to-Start (SS)
- Start-to-Finish (SF)
- Finish-to-Finish (FF)
Two other terms you must understand are:
- Predecessor, an activity that controls the start or finish of another immediate subsequent activity.
- Successor, an activity where the start or finish depends on the start or finish of another immediately preceding activity.
The following pictures show how the dependencies appear graphically in the Bar Chart and Activity Network (also known as PERT, Network Diagram and Relationship Diagram Views):
The FS (or conventional) dependency looks like this:
Task Type and Effort Driven Options in Microsoft Project
If you are not sure which option to use then I recommend Non Effort driven as a default. This option prevents changes to Task Durations and/or existing Resource assignments as Resources are added or removed from a task.
* Select Fixed Units for activities when the Units per time period must stay constant as either the Duration or Work is changed. For example, a crew of 1 Excavator and 3 Trucks must stay constant as the Duration or Work is changed.
* Select Fixed Duration for activities when the duration must not change as either resource Units per time period or Work is changed.
* Select Fixed Work if you wish the Work to stay constant as the duration is changed. The Task will be made Effort driven automatically with the Effort driven option grayed out. For example, a programmer assigned full-time for a week will have 40 hours’ work.
When the duration is doubled to a week, the programmer will work 50% of the time over 2 weeks but still work 40 hours. If you assign another person to help then the total Work will remain at 40 hours and the Duration not change.
The default for new Tasks is set in the Tools, Options…, Schedule tab. Click on the icon and new projects will have these as their default options.
Related MS Project Article