Steve Sternberg, executive vice president of audience analysis for media agency Magna Global USA, says prime-time ratings among advertisers' key 18-49 viewers could drop by 9% from January to May. Live program ratings are already down 10% this season versus a year ago.

The good news for advertisers is that viewer declines will start out slowly--from a 5% decline in January and February to 8% in March. But after that, viewer erosion will accelerate.

Sternberg says April could see a 12% hit; May, a 13% drop. It is during these months that networks typically air more original programming as they build toward season and series finales.

Part of the equation is the level of reruns already in networks' scheduling plans. Sternberg says repeat episodes have grown steadily since the last writers' strike in 1988, which lasted 22 weeks and cost the industry $500 million.

For instance, in 1988 in the big non-sweep months where repeats occur --December, January, and March--networks ran an average of 14% of its prime-time schedule with reruns. In the last three seasons--2004 to 2005, 2005 to 2006, and 2006 to 2007--that percentage more than doubled to 36% of their schedules. Sternberg notes that February 2007 hit an all-time high: 35% of network prime-time schedules in reruns.

With fewer TV alternatives in 1988, ratings sank 9%. What helps this time around, he notes, is that viewers are accustomed to many reality shows (which can be staffed by non-union writers) and more reruns.

The ratings drop may even be less for some networks, he says--especially if more nights of big-rated original reality shows are added to the schedule, such as Fox's "American Idol" or ABC's "Dancing with the Stars."

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