I'm not sure how useful this idea is, but it is interesting, and we'll see how it all plays out. Applause is an analytics firm that purports to measure app quality and user satisfaction, by scanning the App Store for rankings, sales and reviews (there are a few other companies that do this already, like App Annie and Distimo). But Applause's innovation is that it boils an app's performance down into one 0-100 number, called the Applause Score. From all of the information available for that app, Applause ranks it into one number, which you can then (supposedly) use to compare app quality and popularity.
And Applause has used these numbers to launch the Applause Index, which tracks the scores of 60 high-profile apps, combining them into one number that Applause claims will let you see "peaks and valleys in the overall apps economy." In other words, Applause has tried to boil the day-to-day popularity of both the Android and iOS marketplaces into one number, similar to the way the Dow Jones Industrial Average works in the stock market.
Is it useful? We'll have to see -- it's possible that, over time, this number could show trends in the app stores, or even reflect actual consumer attitudes. But that will probably take some practice and research -- at this point, Applause has only begun to track these defined "scores," so there are undoubtedly some quirks in the system (and it's not clear to me how these 60 apps were chosen to represent the Index, even after browsing through the footnotes). But it is an interesting idea, and who knows? Over time, this number could show us something about the App Store we didn't see before.