Hard to quantify. What criteria do you use to define 'drop out'? Not fly for 6 weeks? 6 months? 6 years?
A better way might be to look at the number of SPLs, PPLs, CPLs & ATPLs that are issued/renewed, then compare that to the number of Class 1 & 2 medicals that are also current.
Since very nearly all SPL holders have the intent of gaining a PPL or CPL then the change in numbers between SPL and the higher licences could be informative. Ditto the comparison between all licences & medicals.