Notes on verification

Twenty selected and spring 1960 training start are well corroborated by multiple independent sources. The 'almost 3,500' initial pool figure is not independently confirmed elsewhere; other sources cite varying numbers (154, 206, ~3,000) likely reflecting different funnel stages. [cascade flags: wikipedia_dropped_other_sources_exist | tier=silver indep_score=0.867 clusters=3 claim_tier=notable | framework convergence=partial (upstreams: Burgess & Hall, 'The First Soviet Cosmonaut Team: Their Lives and Legacies', NASA official history, declassified Soviet archival documents (Archive of the President of the Russian Federation)) — publishers independent, evidence may not be | kind=empirical eligibility=high]