A lag is the amount of time that must pass after the completion of an enforced predecessor until the dependent task can begin (Positive Lag), or the amount of time that a dependent task could start before the predecessor starts (Negative Lag).
The Planned, Projected and Estimated dates of the successor tasks are calculated taking into account the lag and Planned, Projected and Estimated Start (Completion) dates of the predecessor tasks.
You can indicate lag types on tasks when you define their predecessor relationships.
- Navigate to a task for which you want to define the predecessor and the Lag Type.
- Select the Predecessors tab.
Depending on your configuration, the Predecessors tab might be listed under the More drop-down menu.
- Start typing the name of the predecessor task, then select it when it appears in the list.
- Select the Dependency Type.
For more information about predecessor Dependency Types, see "Understanding Dependency Types."
- Specify a Lag amount using a numeric value. You can specify negative numbers to indicate a negative lag.
- Select from the following options to identify the type of lag you would like to indicate for your predecessor:
- Calendar Days
- Day of the Week
- Day of the Week (Non-Zero)
For more information about these Lag Types and how they are calculated, see "Understanding Lag Types."
- Click Save.
- Navigate to a task list, and select the Standard View from the View drop-down menu.
- Click inside the Predecessors column corresponding to the task for which you want to specify a predecessor and a lag amount.
- Enter the following with no spaces:
- the number of the task you wish to indicate as the predecessor of the task selected
- the abbreviation for the dependency type you want to indicate between the tasks
For more information about the abbreviations for Dependency Types, see "Understanding Dependency Types."
- either a + for a positive lag or a - for a negative lag
- the amount of the lag
- the abbreviation for the Lag Type you want to use.
For more information about the abbreviations for Lag Types, see "Understanding Lag Types."
- Click Enter on your keyboard to save the changes to your task.
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.
The following table illustrates the Lag Types and how to indicate the amount of time for each one:
The delay between two tasks linked by dependency is measured in working days. This is the default Lag Type.
Calendar Days (c)
The delay between two tasks is measured in calendar days, including holidays and weekends.
For example, if there is a finish-start 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 according to the schedule.
Percent (p or pe)
The tag delay is expressed as a percentage of the estimated time to complete the predecessor task.
For 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)
The delay between two tasks is measured by indicating the days of the week for the week that contains the Planned Completion Date of the predecessor.
For this Lag Type, each day of the week is associated with a number:
If you want to indicate that the Planned Start Date of the successor should fall on a Tuesday of the current week, and the Tuesday is prior to the Planned Completion Date of the predecessor, you would code your successor with the following formula:
NOTE If the Tuesday passed for the week of the Planned Completion Date of the predecessor, then the Planned Start Date of the successor task is the first available working day of that week.
If you want to indicate that the lag should fall on a Saturday of the current week, and the Saturday is after the Planned Completion Date of the predecessor, you would code your successor with the following formula:
If Saturday is a non-working day, the next available day after Saturday (to indicate positive lag) is selected as the Planned Start Date of the successor.
To indicate past or future weeks, you can add a number in front of the day number for the lag type. For example, to indicate the Monday of 10 weeks ago, you can use this formula for your successor:
4fs-102w, where 10 indicates 10 weeks ago, and 2 is the number assigned to Monday.
Day of Week Non Zero (k)
The delay between two tasks is measured identically to the Day of the Week lag type, except if the predecessor's time ends on the same day of the week specified. The lag time is then calculated to the adjacent week (+/-).
In this case, the lag time can never be 0.
You can use a negative Lag to indicate the need or ability for the task to begin prior to the predecessor task ending.
Consider the following rules when using negative lags:
- Negative Lag cannot force the Start/Finish dates of a task to be before or after the Planned Start/Finish dates of the project. These dates are specified in the the Schedule From field on the project.
In this case, consider the following:
- Schedule the project From Completion Date.
- The last task on the project should use the Must Finish on Task Constraint. It is recommended 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.
In this case, consider the following:
- Set a Finish-Finish predecessor relationship between tasks.
- The Duration of the successor task should equal or exceed the intended number of lag days between tasks.