During the Great Purge, a lot of military men were removed (executed, imprisoned, exiled, expelled from the party). I have heard in countless documentaries that those on the list were "perceived as politically unreliable".
So I would like to know, how many of them served as officers in Tsarist times? (Which would make them former Imperials.) That should be an objective criteria investigatable.
I did a brief investigation into the 3 Marshalls who were purged: Tukhachevsky, Blyukher, and Yegorov.
Tukhachevsky indeed served in the Imperial Russian Army, went to two military schools, and made the rank of Second Lieutenant. I don't know if that's an officer or maybe NCO.
Blyukher also served the Imperials and made it to the rank of Corporal. I'm not sure if this is considered an officer, but I would think it's at least an NCO.
Yegorov was also an "army officer", according to Wikipedia, in the Imperial Russian Army until 1905.
So the Marshals, which I believe were the highest ranks in the Red Army at that time, are 3 for 3. Hopefully someone knows of a study into this so some percentage can be had for the whole batch.