Hi, I have started to learn local regression models so as to identify statistically significant peaks in urban areas, such as population densities and congestion. I successfully ran /locfit/ and got several information on the fit. Now I got stuck. This is a very silly question, but isn't the first derivative of the fitted curve zero or close to zero? I got some very high numbers on that. The commands I put are: x<-Longitude y<-Latitude model.local<-locfit(log(POPDENSITY)~lp(x,y, nn=0.55)) span was determined by spgwr adaptive bandwidth. Any help appreciated. Thank you very much. Taka PhD student Indiana University, Geography