Originally Posted by

**nebulus**
Interesting. I had tried to do a Box-Cox transformation in SAS prior to posting my question here and it suggested Lambda = -1 (i.e. the reciprocal transformation), which is essentially using developmental rate (1/d) rather than time. I tried that transformation and it did not help heteroskedacity. Since that is not a standard power transformation like you suggested I will use Minitab and see if I get the same results you reached. Thank you very much, this will definitely help me in the future.

Edit* What p-value did you get for the variance test you used? After transforming the data and using Levene's Test I got an F=2.75 and p=0.02 This is much better than any transformation I ever did but still not non-significant. I tried other values proposed in that range and X^0.33 seemed to be best with p-value of 0.0327.