Which CW TV series will be renewed or cancelled for the 2011-12 season? In large part this depends on a series’ ratings for the 18-49 demographic. The CW also puts a particular lot of weight on the 18-34 female 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 from February 11th until February 17th — based on the 18-34 female demographic, as measured against the last original episode. As was the case with the other networks, most shows were down.

Week-to-Week Gains: The Vampire Diaries (+5.0%)

No Change: Hellcats

Week-to-Week Losses: Smallville (-25.0%), Supernatural (-38.5%), 90210 (-33.3%), Gossip Girl (-27.8%), One Tree Hill (-7.7%), Nikita (-10.0%)

Wonder if your favorite FOX show’s been renewed or cancelled? Check out our comprehensive list. We’ll keep updating it as new info becomes available.

Below is a list of the regular CW 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 18-34 female demographic averages, the group that the CW focuses on attracting and selling to advertisers. The “compared to” column reflects whether the demo average has gone up, down, or stayed the same.

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 Life Unexpected). 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.

It should be noted that Friday nights are one of the least watched evenings of television so ratings for shows on that night are lower. The network understands this and takes 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? Any surprises? Which shows do you think won’t survive to see a new season?

Image courtesy The CW.