There’s been so much speculation in recent weeks about the ill effects the writers’ strike is having on the broadcast networks that one might think ratings are plunging and that ad revenue is sliding along with them.

Not so.

The writers’ strike is so far having virtually no impact on the networks outside of late night, where shows like NBC’s “Tonight Show” went into reruns when the strike began six weeks ago, on Nov. 5.

Ratings are in fact down but not because of the strike. Driving the declines are factors that would be at work if the strike had not been called, from increased DVR recording to normal year-to-year audience erosion.

And so far, broadcast ad revenue hasn't taken a hit.

Over recent days, reports have surfaced that NBC and the CW are having to return money to advertisers because of ratings shortfalls from the strike. The two networks deny those reports, and media buyers dismiss them as well. The CW says it hasn't returned any ad money, while NBC says it has returned small sums but nothing out of the ordinary.

In fact, the networks are doing quite well. The broadcasters are fetching premiums in the scatter market of 25 percent to 40 percent over upfront prices for inventory left over from last summer’s $9.3 billion ad market.

“The primary driver here is that there’s slightly more demand,” says Lee Westerfield, an analyst with BMO Capital Markets, noting, “There’s no good substitute for branding away from the television networks.”

That's been further helped by a drop in inventory resulting from the fall in ratings.

Indeed, in discounting the reports of networks returning money, one major buying agency says inventory is so valuable to their clients that they would turn down cash-back offers in favor of makegoods some time in the future when inventory does become available.

All this in turn fits into a historical pattern that was well in place before the strike, that of rising prices from the networks in response to stable or rising demand against dwindling inventories of rating points.

Networks have been able to raise rates because even at lower audience levels network TV still delivers the largest mass audience available. Like rare stones, rating points only gain in value as fewer are available.

Each of the networks, except Fox, is down from last season in 18-49s. The CW is down 20 percent. NBC is down 11 percent, CBS 10 percent and ABC 5 percent.

It’s almost impossible to predict how much a prolonged writers’ strike will further reduce these ratings. But it will likely be modest and it's not expected to hurt the ad revenues of the networks, which will simply jack up prices on their reduced supply of inventory.

Some major media buying agencies think the networks could see a 5 percent ratings decline in January if the strike continues. Most of the networks are loaded up with reality shows, newsmagazines and midseason scripted shows that weren’t in play the last time a strike took place, back in 1988.

Moreover, the networks’ overall ratings declines aren’t necessarily painting a complete picture of how well the networks are doing.

Some of the networks’ most profitable shows have actually seen ratings rise this year, like NBC’s “Office.” And in the end how profitable individual shows are matters far more than average weekly ratings.

“The full-week ratings we generally track really mask the revenue-weighted ratings on a night-by-night basis,” says Westerfield. “When viewing in-demo strength on nights that are doing well against the nights that are dragging the networks down, it looks like the higher-revenue nights of the week are doing well.”