Calculating the drag of critical tasks when rescheduling
You can calculate the drag of critical tasks when rescheduling. Critical path drag (Devaux's Removed Activity Gauge) can be thought of as:
- The amount of time that a task or constraint on the critical path is adding to the total project duration.
- The maximum amount of time that the duration of a task can be shortened before the task is no longer on the critical path or before its duration becomes zero.
- A way of identifying the tasks that should be addressed first in an attempt to shorten the duration of a project - ie those with the greatest amount of drag.
To calculate the drag of critical tasks when rescheduling, select the Critical path drag check box, on either the Reschedule tab of the Options dialog (used to set default reschedule options) or the Reschedule dialog (used to set options at the time of a reschedule).
If you choose to calculate critical path drag, it will take longer to reschedule a project; for this reason, drag is not calculated whenever Risk Analysis or the resource leveller reschedules the project.
You can display the drag of critical tasks in the spreadsheet, using the Critical path drag field. Drag is also displayed in a field on the Task tab of the Bar and Task Properties dialog and on the corresponding tab of the properties view.
You can filter for tasks with particular amounts of critical path drag, using the Float and Criticality page of the Filter Wizard. This is a useful way of identifying tasks with a large amount of drag, which are those that should be addressed first in an attempt to shorten the duration of a project.
How critical path drag is calculated
Only tasks that are on the critical path have drag. For each critical task, critical path drag is calculated by identifying the finish date of the network of tasks and calculating the amount by which the finish date would differ if the critical task was turned into a milestone.
If a task on the critical path has no other tasks in parallel, its drag will equal its duration; if a task on the critical path has one or more other tasks in parallel, its drag will equal whichever is less: either the total float of whichever parallel task has the least total float, or its own duration.
In the example below:
- Task 1 is on the critical path; it has no tasks in parallel, so its drag equals its duration.
- Task 2 is not on the critical path, so it has no drag.
- Task 3 is on the critical path and is in parallel with Task 2; the total float of Task 2 is less than the duration of Task 3, so the drag of Task 3 equals the total float of Task 2: 1d.
In the example below:
- Tasks 1 and 5 are on the critical path; they have no tasks in parallel, so the drag of each task equals their duration.
- Task 2 is on the critical path and is in parallel with Task 4; the total float of Task 4 is less than the duration of Task 2, so the drag of Task 2 equals the total float of Task 4: 8d.
- Task 3 is on the critical path and is in parallel with Task 4; the duration of Task 3 is less than the total float of Task 4, so the drag of Task 3 equals its duration.
- Task 4 is not on the critical path, so it has no drag.
In the example below:
- Tasks 1 and 5 are on the critical path; they have no tasks in parallel, so the drag of each task equals their duration.
- Task 2 is on the critical path and is in parallel with Tasks 6 and 8; the total float of Task 8 is less than the total float of Task 6 and the duration of Task 2, so the drag of Task 2 equals the total float of Task 8: 8d.
- Task 3 is on the critical path and is in parallel with Tasks 7, 9 and 10; the total float of Task 9 is less than the total float of Tasks 7 and 10 and the duration of Task 3, so the drag of Task 3 equals the total float of Task 9: 3d.
- Task 4 is on the critical path and is in parallel with Tasks 7 and 10; the duration of Task 4 is less than the total float of Tasks 7 and 10, so the drag of Task 4 equals its duration.
- Tasks 6, 7, 8, 9 and 10 are not on the critical path, so they have no drag.
Calculating the drag cost of critical tasks
If you calculate the drag of critical tasks, you can also calculate their drag cost - the amount by which the drag of a task on the critical path is reducing the project's expected value. Drag cost is a very important metric in project management, as it enables you to optimise project investment by justifying the cost of resources used to reduce the drag of critical tasks.
The drag cost is calculated by multiplying the critical path drag for each task by a value that represents either the cost each day that the project is delayed, or the benefit that can be gained by each day of early delivery.
You can use a formula to calculate the drag cost of critical tasks. For example, in the following formula, a drag cost per day of £1,000 is included in the formula:
