I am currently trying to undertake a linear regression analysis in SPSS and I have come across some difficulties. My data does not show linearity and therefore is violated. I have transformed both predictor and outcome variables numerous ways but this does not change the outcome of linearity. I have read the next step could be to do a nonlinear regression analysis. Is this correct? If so, how it this performed in SPSS? Anyone know of any good websites that explain this?
Depending on what you mean by linear (as asked by @Macro), you could do a polynomial regression. I'm not familiar with SPSS, but you could create a variable called x2 that is simply x squared, then include that term in a multiple regression model.
A quick search showed that in SPSS you don't even have to create an additional variable, but could check an option to have SPSS model a quadratic term: http://www.ucdenver.edu/academics/colleges/nursing/Documents/PDF/NonlinearExampleHowTo.pdf