I have several different regressions for various sites within an area estimating Y from X.

I would like to combine all these sites in to one region with one regression equation.

Is there a way statistically show that this is possible to certain level of confidence.

I know this will increase my error from site to site, but would like to some how quantify that increase in error.

Thanks for your help, it's been years since I've taken a stats course.

P.S. I am using Excel and SigmaStat software, if you know of a quick function to use.

Thanks

Danny Klimetz