When the Split in-progress tasks option is enabled in the Tools, Options…, Schedule form, a task will Split automatically when a task commences before its predecessor finishes.
Microsoft Project is inconsistent when the Split in-progress tasks option is used with tasks that are assigned with one of the following options:
- An Actual Start and 0% Complete — these tasks are not split, or
- An Actual Start and % Complete set between 1% and 99% Complete — these tasks are split.
The two examples below are from the same schedule, both with the Split in-progress tasks option checked, one with 0% and one with 1%. You will notice the task assigned 0% has an earlier Finish date than the task assigned 1% Complete, which has split.
Note: You therefore need to pay careful attention to any warning messages Microsoft Project presents.
Excerpt from 99 Tips and Traps for Microsoft Office Project