That’s a common perception. The listenership appears evenly split: 29 percent describe themselves as “conservative or very conservative,” with 30 percent described as moderates, and 31 percent described as “liberal or very liberal.”

Those numbers are far less lopsided than the equivalent categories for the TV networks. It’s a fascinating topic.

Fair enough. Let’s take a closer look. Ratings are measured two ways: cume and AQH. Cume measures the total audience that listens at least five minutes at a time (time to admit an error on my part: the 26 million I cited is an outdated figure). The correct figure for public radio’s cume as reported in March 2009 is 33 million people. So that means 33 million Americans tune into a public radio station for at least five minutes at a time.

That cume is even more impressive given the continual fragmentation of media markets; i.e., competition from the Internet, television, and satellite radio.

AQH, on the other hand, is a way to measure audience loyalty — that is, how long someone is listening. That’s measured in fifteen-minute increments. It’s a little trickier, and varies from station to station. You’d have to cosult your local station to find out.

As to your other point, about the service area: There are 903 stations in this country that carry public radio programming. It sounds like a lot, but you have to consider that some are 100,000-watt powerhouses such as KQED while some are low-power local stations that could be as small as 100 watts. Some states have great coverage, others are sparse. So we can’t assume the number public radio stations is equal to the areas the network can reach.

I hope this helps shed some light on what the numbers mean.