Issue using GDP and GDP squared in regression analysis

[Sabrina Deegan]

I am currently completing a study on the EKC hypothesis. I am using a time series data set and have taken logs of all the variables. For one part of the study I am calculating the turning point, however to do this, I need to calculate the coefficient of GDP and GDP squared. Every time I use the regress command with my variables on Stata, it omits GDP due to collinearity, therefore, not allowing me to calculate the turning point. Is there a way around this?

If I have not given enough information please let me know and thank you in advance for any help.

[Rich Goldstein]

"center" gdp before squaring it; while traditionally centering means subtracting the overall mean, you can also subtract any other number in the range of the data so if some value is substantively meaningful in your case you might want to use that

possibly relevant - your text is not clear on why you need the turning point but if your hypothesis is that the slope changes at some particular point, you might be better off using splines (e.g., linear splines) with a knot where you hypothesize a change; see

h makespline

[Sabrina Deegan]

"center" gdp before squaring it; while traditionally centering means subtracting the overall mean, you can also subtract any other number in the range of the data so if some value is substantively meaningful in your case you might want to use that

possibly relevant - your text is not clear on why you need the turning point but if your hypothesis is that the slope changes at some particular point, you might be better off using splines (e.g., linear splines) with a knot where you hypothesize a change; see

h makespline

Thank you so much for your help! centering the gdp solved my problem.