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Dependency Lag & Lead

You can enter Lead/Lag time to represent an overlap or delay for the successor of a task depen­dency. For example, for tasks linked with a finish-to-start task dependency, you can designate lag days between the completion of the predecessor task and the beginning of the successor task. If you have two tasks: "Paint Walls" and "Hang Pictures,” you may need a delay between the completion of "Paint Walls" and the beginning of "Hang Pictures" to allow the paint time to dry.

Lead-time is an overlap of two tasks. The successor task begins before the predecessor task com­pletes. This applies for all dependency types. Lead-time is entered as negative lag, such as -1day.  For example, for tasks that are linked with a finish-to-start dependency such as "Lay floorboards" and "Sand floorboards," you can use lead time to begin "Sand floorboards" when "Lay floorboards" is 3/4 complete.

For positive lag values - if the existing successor task’s dates result in the ”natural” lag being equal to or greater than the lag specified in the dependency, the dates of the successor task will not be adjusted by the dependency, because the lag requirement will have been met or exceeded.

For negative lag values (lead) - Successor task dates can be pushed as far back as the negative lag says - regardless of their original start date - if they have an ASAP constraint. If the successor task has a Start No Earlier Than (SNET) constraint, its dates will never be adjusted by a dependency to be earlier than the task’s ”original” dates (dates that are set on create, edit, or via an upward date cas­cade because of a child task date). In this case the date will adjust as early as, but not earlier than the ”original” task date. The examples below illustrate both of these constraints and lead time scenarios.

Lag or lead may be entered in days, weeks, hours or minutes.

Dependencies and Constraints

Flexible constraints work with dependencies to schedule the task to occur as soon or as late as possible. Some constraints give you the ability to specify that the task needs to start or finish before a chosen date. These dependencies still can affect the task schedule. Inflexible constraints take priority over dependencies restricting the task to start or finish on a specified date. See About Constraints for more information and a list of all constraint types.

The default Constraint Type is As Soon As Possible (ASAP).  

Example 1 - no constraints

The example uses the following start/finish dates for predecessor and successor tasks, and uses no constraints (uses default of As Soon As Possible).

  • Project schedule from date = 7/15/04

  • Predecessor dates = 7/15/04 to 8/25/04, ASAP

  • Successor dates = 7/15/04 to 9/15/04, ASAP

Using the above date, the successor start date is calculated for the following lag values:

  • Lag = 0 days, successor start date = 8/26/04

  • Lag = 5 days, successor start date = 9/2/04 (8/25/04 + 1 + 5 work days)

  • Lag = -5 days, successor start date = 8/19/04 (8/25/04 + 1 - 5 work days)

  • Lag = -20 days, successor start date = 7/29/04 (8/25/04 + 1 - 20 work days)  

Example 2 - dependencies and "Start No Earlier Than" constraints

  • Project schedule from date = 7/15

  • Predecessor dates = 7/15/04 to 8/25/04, SNET

  • Successor dates = 8/15/04 to 9/15/04, SNET

Using the above date, the successor start date is calculated for the following lag values:

  • Lag = 0 days, successor start date = 8/26/04

  • Lag = 5 days, successor start date = 9/2/04 (8/25/04 + 1 + 5 work days)

  • Lag = -5 days,- successor start date = 8/19/04 (8/25/04 + 1 - 5 work days)

  • Lag = -20 days, successor start date = 8/15/04 (constraint date is honored and the conflict icon will be displayed)