This is a very elementary problem; but unfortunately, my strengths do not like in statistics:

I am trying to describe plant growth over time using accumulated temperature (dd) as my independent variable and the plants 'growth stage' (gs) as my dependent variable.

The shape of the data is sigmoid and I have three different sets of data for different geographic areas.

I decided to use a third order polynomial to describe the data (which it does well in all three cases). Example below:

GS = - 0.969 + 0.02909 DD + 0.000062 DD**2 - 0.000000 DD**3

Now; what I really want to know, is if the relationship described in one place is similar to another place: i.e. can I describe plant growth in a geographic area using a polynomial model that was derived in a different location.

I thought the best way to go about doing this was by using the standard errors associated with the coefficients of one model and checking to see if the other two models coefficients fell within these error limits? Am I going about this in the wrong way? Minitab doesn't seem to provide the standard errors when using a 'fitted line plot' to fit the polynomial.

Many thanks for any help..