This is the type of advice I have seen.

**“How to diagnose**: nonlinearity is usually most evident in a plot of

**observed versus predicted**

**values**or a plot of

**residuals versus predicted values**, which are a part of standard regression output. The points should be symmetrically distributed around a diagonal line in the former plot or around horizontal line in the latter plot, with a roughly constant variance.”

I am not sure what this means in practice. In the following from analysis I just ran it does seem to me to be roughly symmetric around the horizontal line, but I don’t think the variance is constant. Does it mean it is non-Linear?

I generated the partial residual plot (sas calls this the partial regression residual plot) which is recommended by some for non-linearity and see no obvious pattern of such (although I have found few details of what to look for in these).

I have about 4700 data points if that matters