Previously, we discussed using cumes to measure public TV audiences. Ratings are another way public TV stations assess audience. While cumes are non-duplicative measures--a household is counted only once during a measurement period--ratings are gathered each time a household watches public TV. This helps stations judge viewer loyalty--valuable information in an industry that asks its audience for financial support. Many observers say ratings are a more efficient measurement than household cumes.

Ratings help stations judge viewer loyalty.

Ratings are the percentage of homes or persons viewing a program or daypart. So if KOPB in Portland, Oregon, gets a four rating for Mystery, this means four percent of the 868,000 households in the designated market area (DMA) -- or 34,720 households -- tuned into the program.

Ratings allow comparisons between different sweeps and different markets. A four rating in Albuquerque means the same as a four rating in Los Angeles in terms of audience performance. This doesn't mean both markets reach the same number of homes, since the markets are different sizes. But these differences are controlled in ratings. The denominator, the number of TV homes in a market, changes as households increase or decrease, but a four rating always means four percent of the households are viewing a station during time X or Y.

Ratings traditionally are based on the household, but person ratings also exist. A market's rating for 18-to-49 year-old women is based on the number of women in that age group viewing a program divided by the number of those women in the market's TV homes.