vital index

This satirical revue has established itself as a vital index to the health of the mainstream American musical.

Chapman relates to a "vital index":

Biotic potential is the highest possible vital index of a species; therefore, when the species has its highest birthrate and lowest mortality rate.

Over the last two decades, "Forbidden Broadway" - which manages to see its subjects with both the reverence of an acolyte and the coolness of a diagnostician - has established itself as a vital index to the health of the mainstream American musical.

There are people, mean-spirited people, who sneer at our democratic institutions, saying, "If the speech is such a vital index to greatness, we ought to vote for the speech writers instead of the people who read their words aloud to the wakeful hundreds of the caffeine-besotted."