Help for sample size calculation

in my prospective cohort study I will assess the serum level of a vitamin in a group of patients
the normal reference serum range is 10-30 in normal population

there is no control group

then i will correlate the serum level with later occurrence of outcome (yes or no)

So I need to know what is the extra information you need for sample size calculation


TS Contributor
I think you should first design the study with a bit more detail. E.g. do you want to do a logistic regression or would a simple two sample test be enough ?