What unit is t measured in?

And is it simple or compound interest? (I assume from the post title it's compound inteerst, in which case that should have been stated).

It would help ifallof the question was posted rather than just the bits you think are important.

You will probably need to do an exponential regression to get the growth factor, and compare the value each time period with what the compound interest formula says it should be (treating i as the unknown in each case).