PRJ: Incorrect Critical Path Calculation Using Delay (88322)


The information in this article applies to:

  • Microsoft Project for Windows 1.0
  • Microsoft Project for Windows 3.0
  • Microsoft Project for Windows 4.0
  • Microsoft Project for the Macintosh 1.1
  • Microsoft Project for the Macintosh 3.0
  • Microsoft Project for the Macintosh 4.0
  • Microsoft Project for Windows 95 4.1
This article was previously published under Q88322

SYMPTOMS

Microsoft Project does not take Delay values into account when calculating the Late Finish for a given task. This can result in incorrect critical path calculations when Delay is used.

STATUS

Microsoft has confirmed this to be a problem in the versions of Microsoft Project listed above.

MORE INFORMATION

Adding Delay to a task is a convenient way to alter the start of a task without actually having to place a constraint on it. For any given task, the following formula holds true:

Scheduled Start = Early Start + Delay

The Late Finish of a task (task x) is calculated from the Late Start (assuming a FS relationship) of its successor task (task x+1):

(1) Late Finish(task x) = Late Start(task x+1)

This is the formula used in standard project management methods of calculating critical path, and it is also used by Microsoft Project. These standard methods, however, are based solely on task durations and predecessor-successor relationships; they do not incorporate the concepts of "constraints" or "delay." To take Delay into account, the formula for Late Finish should be:

(2) Late Finish(task x) = Late Start(task x+1) - Delay(task x+1)

Formula (1) results in a Late Finish for task x that is greater than it ought to be when task x+1 has a Delay > 0. This may produce a Total Slack > 0 for task x, incorrectly making it a noncritical task.
Modification Type:MajorLast Reviewed:11/25/2003
Keywords:kbbug kbpending kbusage KB88322
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