alanson wrote:
So suppose that Kiprop, unable to match his previous results (again, suppose for the moment that they were run clean), tried to revive his career starting last year by using EPO. Would the IAAF then rescind his previous medal awards? Or would they have to prove contemporaneous violation of the doping rules in those earlier years?
Short answer, no.
A positive test and the sanction are forward looking. Typically, an athlete will have results forward from the date of the positive test disqualified, with the remaining time of the sanction being a ban from future competitions.
Retroactive bans only happen with retests, where the forward looking sanction from the date of the test includes results already recorded.
ABP cases bans are forward looking, with disqualification from the period in question. The case decides the extent of the doping. If an athlete competing since 2010 shows doping on their ABP from September 2015 to October 2017, and the case is decided in May 2018, they will be disqualified from results since Sep 2015, and banned 4 years from the date of the case: May 2018, back competing in 2022.
If an athlete admits to doping to get a lighter sentence, the authorities get to determine when to start the ban. It has been the case that athletes admitted doping in testimony on other cases, such that some results were disqualified, but not all of them since the violation.
If an athlete appeals a case, gets permission to compete, and then is found ban, their ban starts from the date the case opened. It is possible that an athlete is caught in Jan 2016, appeals the case, is allowed to compete, but then appeal is denied in May 2017, then there 4 year ban starts/started in Jan 2016, meaning there results since then are vacated, but their "ban" is only another 2.5 years.
I hope that made it clear, although I may have muddied the waters.