I am confused with the assumption of linearity to the logit for continuous predictor variables in logistic regression analysis. Do we need to check for the linear relationship while screening for potential predictors using univariable logistic regression analysis?
In my case, I am using the multiple logistic regression analysis to identify factors associated with nutritional status (dichotomous outcome) among the participants. The continuous variables including age, Charlson comorbidity score, Barthel Index score, hand grip strength, GDS score, BMI etc. My first step is to screen for significant variables using simple logistic regression. Do I need to check for the linearity assumption during simple logistic regression analyses for each continuous variables? Or should I just check for it in the final multiple logistic regression model?
Besides, for my understanding, we need to transform the non-linear continuous variable before enter it into the model. Can I categorize the non-linear continuous variable instead of transformation?