Originally Posted by
Stephen Wright
I'm no expert in such matters and I don't know about the ROI per se, but Regan's entire method is based on ratings:
"Ken Regan's algorithm is based on the player's rating - the idea being that a 1700 playing 2500 moves over a prolonged period is cheating, whereas a 2500 playing 2500 moves isn't.
The strength of your opponents isn't relevant, because playing 2500 moves is the same regardless of your opponent. But your own rating is fundamentally relevant, because it's the bar you're being measured against."
[This taken from the English Chess Forum, where there have been two long threads (50+ pages) on fair play.]
Thus having an accurate input rating is important, which can be an issue when players have differing national and FIDE ratings, for example, or the underrated improving junior.