Digital video recorders are a leading reason for the slide in television viewing this year, Nielsen Media Research said Thursday

Many of the top shows -- from ABC's "Grey's Anatomy" to Fox's "American Idol" to CBS' "CSI" -- saw their ratings drop in the spring.

Prompted this month by questions from NBC, Nielsen began an investigation into the factors that could have led to the slide. Nielsen's probe is almost complete, but in the meantime the company has discovered several things.

"DVRs appear to be the largest factor in that," said Pat McDonough, Nielsen senior vp planning policy and analysis.

But there are other factors in Nielsen's early findings, among them the difference between an Olympic year (2006) and a non-Olympic year (2007) as well as a higher number of repeat programs this spring than in previous years.