The portion of the intervals which are unique to one interval class, out of all possible intervals.
For example, the major scale in 12-ET has a stability of 20/21, because there are 2 occurrences of the 600 cents tritone, one for interval class 3 and one for 4. -- (in Scala, do SHOW INTERVALS or SHOW/TOTAL INTERVALS to see this) -- There are 6*7 intervals so the stability is (42-2)/42.
...Stability measures how easy it is for a listener to determine which interval class an interval belongs to. Efficiency measures how difficult it is for her to determine what scale position a pitch represents -- in other words, how long it takes the listener with "perfect pitch" to figure out what key the average melody is in.
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