Advertising buyers trying to figure out within which podcasts to advertise now have a free research and planning tool from podcast ad network Podtrac.

The tool compiles information on 4,000 podcasts and allows ad planners to filter them and drill into the data on them to find the right show for them. The audience data in the system was compiled by Podtrac based on the results of a survey it did of 70,000 podcast audience members.

Because of the single focus and continuous thread running through most podcasts, the audience data for a particular show is not likely to vary widely from show to show from the data already assembled.

The lack of statistics and measurement relating to the audience of podcasts, either video or audio, is one factor that's made some advertisers hesitant to explore the format deeply. Podtrac's new system hopes to overcome that reluctance.

The company places two ads in the shows in its network: a short one at the beginning, and a 30-second spot at the end.