Which CBS TV shows will be cancelled or renewed for the 2011-12 season? In large part this depends on a series’ ratings, particularly the 18-49 demographic. The lower the ratings, the more likely it is that a show will 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: CSI: NY (+11.8%), Criminal Minds (+12.5%), and Blue Bloods (+31.3%).

No Change: The Good Wife, Live to Dance, and The Mentalist.

Week-to-Week Losses: The Defenders (-13.3%), 48 Hours (-47.1%), How I Met Your Mother (-2.6%), Rules of Engagement (-3.0%), Two and a Half Men (-4.3%), Mike & Molly (-5.4%), Hawaii Five-0 (-3.4%), NCIS (-8.7%), NCIS: Los Angeles (-7.9%), $#*! My Dad Says (-3.6%), and CSI (-6.7%).

You can see a complete list of the current status of this season’s shows by checking the CBS cancelled/renewed post.

Below is a list of the regular CBS 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 Medium). 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 to produce. News and reality TV shows are less expensive to make 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. * Nielsen doesn’t release the final Friday and Saturday night ratings until a few days later. Numbers with an “*” may change slightly once the final numbers are released.

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

Image courtesy CBS.