I have a data set i am pre-processing. However in my categorical columns (3 of them) i have "??" in it's place. They constitute 50% of the data. In fact 3 columns have this. My question is how should i impute this? Do i replace ? with unknown or do i replace it with NaN.
Since my end goal is a classification problem i will be applying various algos on it such as NB classifier, random forest, decision tree, K neighbours. And sometimes it does not matter whether you impute or not since some algos do it for you