I think if the project manager is proactively managing the project, he should never investigate on why a variance happened in his project, he should be able to know on the spot. Having said that, here are some techniques to find the root cause of a variance:
- If it's a schedule/scope variance, then he should ask (himself) the following questions:
- Was there any change request that was OK'd but was not reflected in the project schedule? E.g. resources worked on this change but it is not accounted for in the project schedule.
- Were there some incomplete/missing requirements? How were they handled in the project schedule? Was the project schedule updated appropriately?
- Were there any tasks that took longer than expected? It is very easy to see this by looking at the Gantt Charts. (this one only applies to schedule variance)
- If it's strictly a cost variance (because a schedule/scope variance can also affect costs), then the following questions should be asked:
- Were there any hikes in the salaries of one or more employees?
- Were there any hikes in the material used (this is for construction projects)
- Did we buy something for the project that was not accounted for in the project budget?
- Have we switched vendors?