I have used R to form a 95% prediction interval for the number of endemic species on an island.
My lower bound is negative – is that mathematically sound?
In the linear model used in the prediction interval, the data used are: Area Surface area of island, hectares DiscSC Distance from Santa Cruz, kilometres Elevation Elevation of higher point in metres and it is coded as such:
selected.model <- lm(ES ~ Area + Elevation + DistSC + I(Elevation^2) + (Elevation:DistSC) + (Area:Elevation))
and stepwise regression was performed to find this "best" model
I'm not exactly sure how a prediction interval works. I just want to make sure it is OK. Obviously a negative number of species is incorrect, but I know it takes into account the uncertainty of the mean as well as data scatter.