Change measure formula with drilldown

[syedshah]

I have a report, which needs some drill down. I have simply three fields in the report. date, project, and duration. One project has many phases, and some times, two phases can run in parallel, so i need to calculate the duration by max(end time of last phase)- min(start time of first phase). However, when i drill down from projects to phases, i want to use another formula for the duration. Can some one please help me how to change the measure formula when we drilldown from one level to another level. I am using Cognos 8.4

-BR
Arif