To assess nonlinearity in the logit, you can use the Box-Tidwell transformation. This involves adding the term, (X)ln(X), to the equation. If the coefficient for this variables is statistically significant, there is evidence of nonlinearity in the relationship between logit(Y) and X. However, this procedure isn't sensitive to small departures from linearity. In addition, this procedure won't tell you the precise form of the nonlinearity.
Scott Millis
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