Tübingen/Harvard study. Prof. Pielke Jr. summarized that article in a 2018 paper in Sports Med as follows (DOI 10.1007/s40279-017-0792-1):
Source for "very few are caught": see AIU's web page, currently they catch 1 - 2 athletes per month, while literally thousands dope.
With those numbers, and these effects of blood doping (from the Karamasheva CAS hearing), it is virtually unimaginable that any winner can be clean.
Also, Henrik I. made a showing with 3 suspicious test results on the IAAF likely doping list, which is also a lot shorter than the list of actual dopers.
Why are very few caught? Listen to Pound, he knows.
Dick Pound, ex WADA President:
"you can miss two tests simply by not answering the door if you're on something."
"There is no general appetite to undertake the effort and expense of a successful effort to deliver doping-free sport."
"There's this psychological aspect about it: nobody wants to catch anybody. There's no incentive. Countries are embarrassed if their nationals are caught. And sports are embarrassed if someone from their sport is caught."