I think something like this has been posted before, but I found it interesting enough to reiterate. Courtesy of Money.
::If you would have invested $10K in the S&P 500 in 1982 and let it ride you would have had $93K by the end of 2001. If you would have tried to maximize gains and missed
The best 10 days you'd have only $56K
The best 30 days you'd have only $28.1K
The best 50 days you'd have only $15.78K
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I know the arguement can be made that you could have maximized gains by being out of the market the worst 10, 30, 50 days, etc. but the point is the difference over the long run that missing only a few great days makes. So the question I've been asking myself is, "Is it really worth it in the long run to be out of the market for a .25% drop only to miss a 2% follow thru day the next day?"