Joined: Jan 2010
Posts: 9,834
Likes: 1,811
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
I commented on the SI term as a hoax to elicit some explanation from the OP, but thinking about it I may have been right.
I seem to recall from a seminar at Interbike many years ago an in-house quantitative term used to describe the performance of shifting performance/robustness that related to how much of an adjustment range would still give acceptable performance.
That term just may have been the "selectivity index".
But it seems that the OP is going to let us toss this one around for a while.