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Lag Types

Overview

A lag is the amount of time that must pass after the completion of an enforced predecessor until the dependent task can begin.

For example, if you use a Finish-Start dependency type, and you want the dependent task to start two days after the predecessor task has completed, you should set the lag time to two days by putting a two in the lag field. You could also use a negative number, such as minus four days, to indicate that the dependent task start four days before the predecessor task ends.

  • "Lag Types"
  • "Negative Lag Types"

Lag Types

An example of a task that would require a lag time might be sawing trees into lumber. If the freshly-cut wood must dry for a time before it can be cut, then there would be a lag time between cutting the trees and sawing them into lumber.

Value

Description

Days (d)

Delay between two tasks inked by dependency is measured in working days. 

Calendar Days (c)

The delay between two tasks is measured in calendar days, including holidays and weekends. 

Example: If there is a start-finish dependency with a 2 calendar day lag and the predecessor task finishes on Friday, the dependent task will start on Sunday even though its marked as a non-working day for the user. 

Percent (p or pe)

The tag time is expressed as a percentage fo the estimated time to complete the predecessor task. 

Example: if there is a finish -start dependency with at 20% lag between on a 10 day predecessor task, the system will calculate how many days is 20%of the predecessors task duration and use that as the lag, in this case  it would be 2 days after the task's completion. 

Day of Week (w) 

Lag time is computed by finding the earliest day of the week that satisfies the day of the week specified. The lag must be (+/-) 1-7 corresponding to Sunday to Saturday. Values of 10 can be added to represent 1 week after next Monday (12) or 3 weeks after next Friday (36). If the week is a non-working day (holiday/weekend) the start date will be the earliest day available. 

Day of week non zero ( k)

Identical to Day of the week, except if the predecessors time ends on the same day of the week specified the lag time is computed to 1 week. This lag time will never result in a 0 lag time. 

Negative Lag Types

Users can incorporate negative lag types to indicate a need or ability for the task to begin prior to the predecessor task ending. 

Consider the following rules when using negative lag types:

  • Negative Lag cannot force a task's Start/Finish dates to be before or after the Project's Planned Start/Finish dates.  These dates are based on the the project settings (scheduled from).  
    • ​Consider scheduling the project from Completion Date. 
    • The last task on the project should use the Must Finish on task constraint. It isrecommended to give the task a large enough duration in order to account for all tasks on the project. The remaining tasks work well with the As Soon As Possible constraint. 
  • Using a Finish-Start predecessor relationship with tasks might produce an error message.
    • Consider setting a Finish-Finish predecessor relationship between tasks.
    • Duration should equal or exceed the intended number of lag days between tasks. 
  • When using Calendar Days lag type, use 'C' in the statement. Example 20ff-60c

Considerations

Dependent tasks planned, projected and estimated start (finish) dates are set considering the lag and planned, projected and estimated start (finish) dates of the predecessor tasks.