A18-49+ takes a show's average rating for a season, divides it by the "league average" for that season, and multiplies by 100. That means a show with a 100 A18-49+ did exactly the "league average" for entertainment programming. The bigger, the better.
For more on the number itself, you can read the intro to the number. I also attached number values to some of the common labels we throw around. Here's the key:
150-199: Big Hit
I've also devised some specialized labels to be incorporated this summer:
55-69: Solid for Friday
70+: Hit for Friday
25-34: Marginal for CW
35-44: Solid for CW
45+: Hit for CW
20-24: Solid for CW for Friday
25+: Hit for CW for Friday
Also coming this summer, I'll fully incorporate four new seasons into the A18-49+ fold, the recently completed 2012-13 season along with three older ones. To preview this, I explained the new numbers and did a recap of each of the three old seasons.
And here are the old Spotlight posts using A18-49+, all from spring-summer 2012 and updated with the full 2011-12 numbers:
Quantifying the Return of the Sitcom - I used A18-49+ to chronicle the steps of a five-year process that took us from a world in which nine broadcast dramas outrated the top comedy (in 2006-07) to a world in which five comedies outrate the top drama (in 2011-12).
The Talent Tumble and Other Unscripted Trends - Using A18-49+, I examined some of the low-priority programming that has amazingly stayed almost exactly the same over the years. I also looked at some of the big dips for the big competition reality programs this season.
All About New Shows from 2006-2012 - I took a look at the new shows from each of the last six seasons and evaluated how each class of new shows affected the primetime landscape as a whole.
The Depreciating Rerun - I compared reruns' declines over the last six seasons with originals' declines and also looked at some of the subtle ways the networks are shying away from scheduling repeats.
The Separate Realms: Sports & Specials - I illustrated how the year-to-year trends for sports and specials seem to be operating on different playing fields than those of regular entertainment series.
Network Shifts, 2006-12 - I examined the evolution of the big five networks' entertainment averages over the last six seasons.
I've also put A18-49+ numbers by season on most of the War of 18-49 pages, which should help give some insight into your individual shows' evolution in the relative landscape.