In a recent thread about likely starters for the KU/Holy Cross game started by @benshawks08, some discussion about per minute productivity occurred that caused me to recognize at least part of why I tend to interpret who should be playing and who should be sitting somewhat differently than quite a few board rats. It caused me to try to distill the issue of analyzing per minute productivity.
Per minute based productivity stats are fine for analysis, but they have to be indexed for the differences in starting and rotating in. The starter, if he were a cornerstone player, has to start against all kinds of opponents. He has to go against the guys ready for the NBA and the green wood; against the 1ADs, 2ADs, 3ADs, 4ADs and 5ADs; against the best of whatever the opponent has to start. The guy rotating for the starter is often going up against the opponent's lesser players, or going up fresh against the opponent's tired starter. Against a long stack, like Duke, UK, and UNC, you can probably compare starter per minute productivity somewhat with rotating back up per minute productivity, because those long stacks are so deep, your back up is playing against a pretty good player. But against a mid major, or against an unranked major, there is probably no unadjusted comparison worth making, because the unranked major and the mid major are usually very thin after the first five, maybe first six guys.
The other adjustment that has to be made to get to a common denominator in comparing per minute production stats among starters and relievers is the situational nature of many relievers. When they are sent in, they are often sent in with a very narrowly scoped purpose. Self doesn't want Diallo going in and being a first, second, or third scoring option right now. He wants him rebounding and trying to block and alter everything in sight. It makes his per minute scoring deceptively less than he is probably capable of. It is the same at many positions. The sub has a much narrower job description. It inflates some of his per minute productivity numbers and deflates others.
What most board rats fail to take into account when arguing for more time for back ups based on their per minute productivity numbers are: quality of opponents encountered by back ups; and the differently scoped roles of back ups.