Which ABC TV series will be renewed or cancelled for the 2011-12 season? In large part this depends on a series’ ratings, particularly the 18-49 demographic. The lower a show’s ratings, the more likely it is to be cancelled.

Before we get to the season-to-date averages, here are the winners and losers of the last seven days — based on the coveted 18-49 demographic, as measured against the last original episode.

Week-to-Week Gains: Supernanny (+9.1%), Castle (+4.2%), V (5.9%), Detroit 1-8-7 (+8.3%), Modern Family (+10.9%), and Private Practice (+4.0%).

No Change: Primetime: What Would You Do?, The Bachelor, The Middle, Better with You, Mr. Sunshine (new), and Off the Map.

Week-to-Week Losses: No Ordinary Family (-16.7%), Wipeout (-12.0%), and Grey’s Anatomy (-9.3%).

You can see a complete list of the cancel/renew standings of this season’s shows by checking the ABC cancelled/renewed post.

Below is a list of the regular ABC TV shows and their 2010-11 season average ratings to date. It only incorporates the ratings for original episodes that have aired this season (no repeats). The data is sorted by the 18-49 demographic averages, the group that advertisers will pay the most to reach and, therefore, how the network measures success. The “compared to” column reflects whether the demo average has gone up, down, or stayed the same since last week.

The closer a show is to the bottom of the list, the closer it is to being cancelled (if it hasn’t been cancelled already like The Whole Truth and My Generation). If it’s closer to the top of the list, it’s more likely to be renewed. Shows that are in the middle (“on the bubble”) can be a little tougher to call. Their future typically comes down to other factors like overall costs, contracts, who produces the show, and what new shows are on the horizon.

The list has been separated into two sections. Of the two, scripted programming is the most expensive. News and reality TV shows are less expensive to produce but the networks can’t program just news/reality (though they might like to since they are less of a financial risk).

It should be noted that Friday and Saturday nights are the least watched evenings of television so ratings for shows on those nights are lower. The networks understand this and take it into account. However, regardless of when a show is aired, its production costs remain the same. Ultimately, it must still be profitable enough for the network to keep ordering more episodes.

Raw data: © The Nielsen Company via Media Week.

What do you think? Do any rankings surprise you? Which shows do you think won’t survive to see a new season?

Image courtesy ABC.