Insomniacal Fan
Starter
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The NBA doesn't seem to have a delineated time frame, other than perhaps the NBA/ABA merger in 1976.
The Shot clock Era and the Three point era seem to be the major delineations in the NBA, where it’s irrelevant to compare statistics from before to statistics after.
I think championships do make sense to be part of a franchise’s permanent legacy, since that’s measuring how an organization performs vs its cohort, and that is a valid comparison regardless of rules changes.