First Derivative

Hi,

I have two columns of data (P, t) and am trying to get the derivative dP/dt defined (P3-P1)/(t3-t1), that is skipping points inbetween to achieve two objectives, one to reduce the amount of data i have and two to achieve smoothing of my dP/dt vs t curve. I assume writing a VB code would be easy to do it, but I dont have any experience doing the same. Could someone help me?

Performing derivative as dP/dt = (P2-P1)/(t2-t1) was easy. I basically used a macro to insert a new row after every data point (copied from this forum). Then just setup a formula in this new row of the subsequent column to do (P2-P1)/(t2-t1). I have attached a file showing how I did it. But doing the same when I want P3-P1)/(t3-t1) is proving a big pain!!!

Someone please help me..

Thanks
Karthik

P.S: Don't bother about the mismatch in data, I just wanted to show how I did it..