What is the average CPM (cost per thousand) for a daytime spot or network commercial?

Refer to reference book TV Dimensions  for these statistics.

Which are more effective: 15 or 30 second commercials?

In the Warc database, click on "Topics" Index on top of the screen. Select "Media Strategy" Then "Media Planning" then Ad Lengths & formats".

Does commercial clutter on radio affect recall?

While the Radio Advertising Bureau and Warc databases will both contain information on this topic, the best source to start with is Business Source Complete. Use the terms "radio", "clutter" , "advertising" for targeted results.

Broadcast Media - Books

Broadcast Media - Databases and Websites

Viewership, Ratings and Reach

Key Journals