Very!
One thing I would add is that in many ways if you consider 3 and 3.5, 4 and Essentials in potential "groups" as almost a single "edition", I would argue 1E and 2E could also be grouped as a single AD&D edition. Most people freely combined rules from 1E and 2E, much of the text in 2E was nearly copy and pasted from 1E.
In many ways, 2E was simply "1.5E".
Interestingly enough, if you group 1E/2E, as many people played it as a single AD&D, but also group all the "Basic" versions, you end up with three nearly equal groups: Basic, AD&D, and WotC-era.
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