Hi, I am trying to compare the slopes and y-intercepts of the power curves between two plant species to see if they follow similar trends (photosynthesis y-axis, stomatal conductance x-axis). I can fit linear regression lines to the data and run the stats using an ANCOVA, but the specific parameters require I fit a 2 Parameter Power Curve to the data. Is running an ANCOVA with the abline function the best bet here, or does R have a specific function to fit and compare power curves? Here is my code thus far, and I was able to compare the linear regressions lines with this code, but the data requires a power curve fit. Any thoughts? Any help is much appreciated, Steve ###compares slope mod1<-aov(photosynthesis~conductance*species,data=data) summary(mod1) ###compares y-intercept mod2<-aov(photosynthesis ~ conductance +species,data=data) summary(mod2) anova(mod1,mod2) BA<-subset(data,species=="B.attenuata") BM<-subset(data,species=="B.menziesii") reg1<-lm(photosynthesis ~ conductance,data=BA) summary(reg1) reg2<-lm(photosynthesis ~ conductance,data=BM) summary(reg2) plot(photosynthesis ~ conductance, data=data, type='n') points(BA$conductance,BA$photosynthesis, pch=20) points(BM$conductance,BM$photosynthesis, pch=10) abline(reg1, lty=1) abline(reg2, lty=2) reg.todo<-lm(photosynthesis~species/conductance-1, data=data) summary(reg.todo) [[alternative HTML version deleted]]