The ReTweetability Index measures and ranks Twitter users based on the infectious power of their tweets. This number accounts for number of followers and Tweets overall, so it is a true indicator of infectiousness.
There is some math involved, but in simple terms, users with a high ReTweetability Index have a greater percentage of their content spread by a greater percentage of their followers, implying that when they do tweet, it’s worth reading.
Some might say their content is just that good. Whatever the reason, these users have proven to be highly ReTweetable, and this metric ranks them in a way that accounts for disparities among follower counts and Tweet totals.
And, it does take quite a while to measure everyone’s ReTweetability score, so the index has a delay.