Post hoc adjustment in clinical trial I would like to make an adjustment on a baseline variable (which has a small imbalance).This is a post hoc adjustment (I recognize, however, the limits of this adjustment which has not been pre-specified). It is a randomized clinical trial. I am wondering if analysis of variance is more appropriate here than linear regression. What factor should be taken into account to decide which type of analysis to use?
However I have some missing value for the baseline variable that I shoud adjust for. I hypothesize missing data randomly.The variable to be adjusted for is a continuous variable. Is it a good idea to categorize the variable and transform into a non-responding category, the individuals for whom we have NA ? or should we do an imputation by the mean and leave the variable in numeric?

It is not permissible to adjust for observed imbalances.  This causes a bias and inflates the standard error for treatment.  Covariate adjustment needs to be pre-specified.  Failure to pre-specify covariate adjustment as the primary analysis is a failure to understand how to maximize statistical power for an RCT.  Details are here.
NEVER categorize a continuous variable.
