Wondered if anyone could help. I've fit the below model in R using GLS from the nlme package.
m2_p1q2 <- gls(log(ratio) ~ level + levellag + trend + time, method = "ML", correlation = corARMA(p = 1, q = 2, form = ~ time), data = a1)
The plotted regression shows a curve and I wondered whether this might be viewed as a problem with fit?
Any advice appreciated....