Television viewers get peeved at the networks this time of year for being too quick to cancel new shows. But the networks know something viewers don’t, and that's which shows simply aren't going to make it.

And so it is three weeks into this new season. More than half the rookie series seem doomed, and another handful are on shaky ground.

Topping the list of disasters is Fox’s canceled reality show “Nashville” and ABC’s much-maligned Tuesday sitcom block: “Cavemen” and “Carpoolers.”

ABC’s sitcoms premiered to higher ratings than most media pundits had predicted, but many viewers didn’t return for the shows’ second episodes.

“Cavemen’s” 18-49 overnight rating on Tuesday tumbled 24 percent from the prior week, to a 2.5. It ranked No. 3, far behind “NCIS” on CBS and “Bones” on Fox. “Carpoolers” tumbled 18 percent, to a 2.8 rating.

Fox’s “K-Ville” has slumped since premiering to solid ratings. The CW’s unscripted Sunday shows “CW Now” and “Online Nation” are outright bombs.

NBC’s new Monday drama “Journeyman” is struggling. Its rating has been slipping since its premiere and it's trailing CBS’s “CSI: Miami” and ABC’s “Bachelor.”

A critics’ favorite, the CW’s “Aliens in America,” is not doing well, either. It lost 20 percent of its “Everybody Hates Chris” lead-in two weeks ago and 27 percent this week, despite holding steady in 18-49s with a 0.8 rating. The CW tends to give shows time to find an audience, so it might not be curtains just yet for “Aliens.”

Networks will begin axing shows soon, the comedies first, then the dramas, once the ratings begin coming out for DVR viewing of the new season.

The fate of several other shows is difficult to predict.

“This is enough time to get a handle on which shows are doing poorly, but it’s not enough time to get a handle on which shows are hits or are doing well,” says Steve Sternberg, executive vice president of audience analysis at Magna, who's authored a report on new shows most at risk.

“A lot of times you need to look at the third or fourth episode to see where it will stabilize.”

Among the shows whose fates seem less certain is ABC’s “Private Practice,” the spinoff of “Grey’s Anatomy.” ABC’s “Dirty Sexy Money” and “Big Shots” aren’t doing great but they’re not obvious bombs. The network’s “Pushing Daisies” premiered to solid ratings last week but will need a few more weeks to prove itself.

NBC’s “Bionic Woman” had the highest premiere rating of any new show but slipped in its second outing. Ratings for its third episode will be out today.

Much will be learned about these shows in the next few weeks, which is also true of CBS’s “Kid Nation,” “Cane” and “Moonlight” and NBC’s “Life” and “Chuck.”

Critics and media buyers like “Chuck,” but its 18-49 rating fell 13 percent from its first week to its second. It was down again in its third outing, ranking No. 4 in its time slot.

A few shows are doing well.

CBS’s Monday sitcom “Big Bang Theory” has stable ratings and is improving on its lead-in. Fox’s comedy “Back to You” with Kelsey Grammer has been losing a few ratings tenths each week but appears to be settling into a modest hit.

“'Back to You' is doing well partly because there haven’t been any good comedies for a while,” says Sternberg. “But we have to see where it is in week four and week five.”

Fox’s “Kitchen Nightmares” picked up steam in its third episode after stumbling in its second.

And the CW’s “Gossip Girl,” which has been picked up for the full season, and “Reaper” are doing well among the network’s young viewers.

“If you look at the shows that were successful last year, and we’ll probably see something similar this year, they were not like any other shows on television,” says Sternberg.