I have made a GAM model using "mgcv" package with the family = inverse.guassian(link = identity) and I am really happy with the fit. After plotting the smooth terms using gratia:draw(GAM, residuals = TRUE) I am really confused by the y-axis. What does "effect" mean?
Any help would be very appreciated!
Thank you
Technically this should read "Partial effect" (and I'll be fixing this shortly). This is the smooth effect of the covariate on the response conditional upon the other estimated terms.
Most smooths in {mgcv} are subject to a sum-to-zero identifiability constraint (so we can include an intercept in the model, which is especially useful when we have factor parametric terms in the model also), so they are centred about 0. The 0 line then means the overall mean (on the link scale) of the response (or the reference levels if factor parametric terms are involved in the model); negative values on the axis indicate where the effect of the covariate reduces the response below the average value, and positive values on the axis indicate those covariate values where the response is increased above the average. All conditional upon the other estimated model terms.