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[[File:Friedmans mars ozone model.png|frame|right|Variable interaction in a MARS model]]
This expression models air pollution (the ozone level) as a function of the temperature and a few other variables. Note that the last term in the formula (on the last line) incorporates an interaction between <math>\mathrm{wind} </math> and <math>\mathrm{vis}</math>.
The figure on the right plots the predicted <math>\mathrm{ozone}</math> as <math>\mathrm{wind}</math> and <math>\mathrm{vis}</math> vary, with the other variables fixed at their median values. The figure shows that wind does not affect the ozone level unless visibility is low. We see that MARS can build quite flexible regression surfaces by combining hinge functions.
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