A few weeks ago, with my tongue partly in my cheek, I wrote a piece suggesting that some might want to steer clear of the stock market, as it's been slumping lately. My point was that a stock slump is often a good time to buy.

I heard from a reader, Jason Tanner, who took issue. He asked, "Are you really saying you couldn't think of a type of person who should not buy now? I'll give you two quick examples ... "

And you know what? He made several good points backed up with solid examples.

If you're about to retire and you'll need all your money soon, you should not have everything in the stock market -- a slump could force you to work a few extra years. (Although you might want to work a little longer anyway.) If you'll need your money soon, for a down payment perhaps, you also shouldn't have it all in stocks.

Even after a slump, the market can slump again, and sometimes for several years. For instance, several stocks I discussed in that article -- including Goldman Sachs (NYSE: GS), Merck (NYSE: MRK), and Boeing (NYSE: BA) -- have fallen further. Stock investing should be for the long run, ideally 10 years or more.

I've made these points repeatedly in many articles. Rereading that article I found this:

Of course, [what you'll earn in bonds] is less than you're likely to get with stocks over long periods. According to the research of business professor Jeremy Siegel, stocks have outperformed bonds 74% of the time over all five-year periods between 1871 and 2001. Over all 10-year periods in the same span, that figure rises to 82%. Meanwhile, stocks outperformed bonds 95% of the time over all 20-year periods, and 99% of the time over all 30-year periods!

As you deliberate between bonds and stocks and other options (and remember, you can invest across an array of vehicles), keep such statistics in mind.

Remember also that even if you're retiring today, you may live another 25 years -- so much of your money might still fare well in stocks.

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Longtime Fool contributor Selena Maranjian doesn't own shares of the companies mentioned in this article. Try our investing services free for 30 days. The Motley Fool is Fools writing for Fools.