Sticking with a good long-term investment is better than trading in an out as it goes up or down. The problem is timing. Investments can make big gains and losses in a short period of time, but it's impossible to predict such short-term movements. The challenge isn't unique to investors trying to pick the "right time" to buy a particular stock. Studies show that it's tough to time the broad market, too.

If you've done the homework necessary to find a good stock or fund, just stick with it. Timing an investment's highs and lows is nearly impossible to do.

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