The best way to get a grip on this is to read one of the SPIVA reports, a quarterly report from Standard & Poor's that measures the performance of managed funds against major indices. Unlike most reporting, this one adjusts for survivor bias. Here's a link to the most recent report: http://www2.standardandpoors.com/spf/pdf/index/SPIVA_2007_q1.pdf . As you'll see from the report, active management doesn't come out very well and they are only measuring a 5 year period. The longer the time period, the greater the odds that the index fund will beat managed funds.
Over the last 15 years, for instance, Morningstar shows that both the Vanguard Total Market Index fund and the Vanguard 500 Index fund were in the 34th percentile for large cap blend funds--- and their data doesn't adjust for survivor bias.
This report verifies research I have been tracking since the early 70s. This research, done by a multitude of different parties, has consistently concluded that about 70 percent of all active managers fail to beat their appointed index benchmark. This doesn't mean the 70 percent figure is consistent, it actually moves up and down over long cycles. But the general range is from about the 50th percentile to the 90th.
In rising markets the index funds have an advantage over managed funds because they have little or no cash so they get 100 percent of a rising market. In declining markets the index funds have a disadvantage for the same reason--- while active funds hold more cash and are slightly less vulnerable to declines, index funds feel the full drop.
Scott