Not really helping your point about hold backs. The Relative age effect is based on comparing kids within a school year/calendar year, i.e. those born in Sept are going to do better than those born in May. Those born in Jan do better than Dec for hockey, etc. So applying "Relative Age effect" to hold backs (who are born in June, July, August) would conclude that that they would be the worst in their actual age group. Is that how I should read this?