NEW YORK – Television heads into its biggest week with the hangover from a 100-day writers strike persisting.

Viewership is down, although it’s hard to tell how much the strike is to blame. This week’s “upfront” presentations by broadcasters outlining their fall schedules, which annually precedes a multibillion dollar ad buying binge, promises to be much different than before.

“The strike had a number of impacts,” said Alan Wurtzel, NBC Universal research chief, “but as with everything it’s never very clear or direct or black and white.”

ABC, CBS, Fox and NBC had nearly 9 percent fewer viewers in April and May so far than during the same period a year ago, according to Nielsen Media Research.

Yet viewership declines are sadly typical for the big networks. Take the same period a year earlier, and the drop was more than 5 percent over 2006. People didn’t watch less TV while the strike was on, they just watched cable more, said Steve Sternberg, an analyst for Magna Global.

Shows with ongoing stories seemed to lose the most momentum from the strike; ABC’s “Grey’s Anatomy” on May 1 had its smallest audience since moving to Thursday night. Decisions by NBC to keep “Heroes” for next fall and Fox to delay “24” until next season may prove prescient, unless people forget about the characters altogether.

Comedies were hurt least by the strike. CBS was so buoyed by the performance of their Monday night comedies that the network is considering adding comedies on another night.

CBS’ rack of procedural dramas had done relatively well, at least until a week ago: “CSI: Crime Scene Investigation” had its second least-watched episode for a Thursday original, and “CSI: Miami” hit a series low for an original.

“There’s no question that it could have been a lot worse,” said David Poltrack, CBS’ top researcher. CBS’ strategy was to make as many new episodes of existing shows as possible until the season ends later this month so people got back in the habit of watching again.

The explosive growth of digital video recorders, now available in 25 million homes, means more people are setting their own schedules.

They could also be bored. Broadcast viewing was already off 7 percent during the last three months of 2007, before the strike’s impact was felt. Several weeks of reruns during the midwinter, when TV viewership is at its highest, really hurt. But the networks were already hurting.

The strike also constricted the networks’ process of developing new material.

Networks made fewer pilots of prospective new shows this year, in part because the strike meant less time to prepare them. In some cases, network executives are making decisions on shows based only on scripts or brief “presentations” of what the series might look like, instead of a full episode, said Brad Adgate, who monitors series development for Horizon Media.

That’s not entirely unwelcome in the business, particularly when the economy is bad. Pilots can cost millions of dollars to produce, and the shows may never make it on the air. Even the shows that do make it on the air are much more likely to fail than succeed.

It doesn’t take an MBA to identify this as an area to save money.

This could be a wave of the future – unless, of course, the series developed without pilots fail miserably. Then there would be pressure to go back to the old way.

Pinched development also gives a real advantage to ideas and creators with proven track records, said Jeffrey Stepakoff, author of “Billion Dollar Kiss: The Kiss That Saved Dawson’s Creek and Other Adventures in TV Writing.”

Familiar names like Joss Whedon (“Dollhouse” on Fox), Brian Grazer (“Lie to Me” on Fox), Jerry Bruckheimer (“Eleventh Hour” on CBS) and David E. Kelley (“Life on Mars” on ABC) have projects with good chances of making it on the air next season.

Networks are also pursuing an unusually large number of adaptations of series that have succeeded overseas, Adgate said. “Life on Mars,” with Kelley remaking a BBC series, hits both buttons.

“It’s not smart to develop by throwing darts on a wall,” said Stepakoff, who’s written for several prime-time series over the past decade. “Similarly, it’s not good to develop with just A-list writers. Some of the greatest television in history, even in the modern age, came from totally unexpected sources.”

“Desperate Housewives,” which Marc Cherry wrote totally on his own and shopped around, is the most prominent recent example.

Not surprising for television, some of the ideas have a whiff of familiarity. Cedric the Entertainer is developing a comedy for ABC about a suddenly rich family moving to Beverly Hills (“then one day he was shooting for some food, and up through the ground come a bubbling crude”).

The strike likely accelerated changes in how the networks present their schedules to advertisers.

Fox is staying traditional, but the glitzy upfront presentations of the past are gone. It was only a few years ago that CBS brought the Who to Carnegie Hall to perform privately for advertisers. This year ABC and CBS both plan more sober, abbreviated sales pitches. NBC announced its schedule a month ago, and will invite guests to an NBC Universal pep rally.

The increasing tendency of networks to order early a new season of episodes of some favorite shows means much of the mystery has already been removed from such announcements. There are some shows on the bubble, however, like “Boston Legal,” “‘Til Death” and “The New Adventures of Old Christine.”

When the announcements are over, it will be up to advertisers to speak with their wallets, to say what programs they find promising and want to place commercials on.

That will be the most important measure to date of the strike’s impact.