^Following on to that idea . . .
Say you include singles league results in NTRP. In Mid-Atlantic we have singles leagues. Let's say that in Middle States, they do not.
The Mid-Atlantic players who play singles will likely have more singles results in their NTRP calculation than the Middle States players.
Say I (a player who is stronger in singles than doubles) play winter singles, adult and fall singles. Because of these two additional opportunities to play singles, I might have 15 singles league matches, 3 adult singles matches and 5 adult doubles matches.
Say we have someone in Middle States who is also stronger in singles. Because there are no singles leagues there, she might have 3 adult singles matches and 5 adult doubles matches.
If I am the same skill level as the Middle States player, we could nevertheless wind up with different NTRPs. This is because my weak doubles results are diluted by my strong singles play much more than my Middle States counterpart.
Therefore, if you "standardize" by including singles league results despite the fact that many areas do not have singles leagues, you would not be standardizing at all. You would be doing the opposite of standardizing.
Which is what often happens in life when you try to standardize things that are not in fact the same.
-- Random Error Generator, Version 4.0
-- Master Moonballer