Assume a simple clinical study with N=200. Half of the participants are men and half of the participants are women. The hemoglobin of the participants ranges between 80 and 150. There's also several other variables.
I would like to split the data into training and tests for a classification task, in a way that the gender and hemoglobin levels would be balanced in each set.
It is easy to pick 50 male/female to each set, but having simultaneously similar hemoglobin levels is difficult. I guess ideally I should check that the mean and s.d. are on about the same range. How should I go about doing this?
If this has been considered in an article, refs. would be great!
EDIT: to clarify, I want to exclude the possibility of gender or hemoglobin inbalance from affecting the classifier result. It should only depend on the other data I have.