As a starting point, he considered the buy-and-hold approach, which requires staying in the markets, come what may, even during terrible decades. That simple strategy, which is what I try to follow, produced an 11.5 percent annualized return for the S&P 500 from the end of 1987 through the end of last year, Mr. Stovall found.

Then he illustrated some of the temptations and pitfalls of varying from this approach. First, he looked at the potential rewards of an impossible feat — timing the market perfectly. Mr. Stovall calculated what an investor’s annualized return would have been if she had been prescient enough to have missed only the market’s worst days in that period, from Dec. 31, 1987, through Dec. 31, 2025, and to have stayed in the market every other day:

• Missed the 10 worst days: 14.1 percent, annualized.

• Missed the 20 worst days: 15.1 percent, annualized.

• Missed the 30 worst days: 16.5 percent, annualized.

• Missed the 40 worst days: 17.9 percent, annualized.

All of these returns would have been wonderful. The problem, of course, is that you wouldn’t have known in advance which days to avoid. If you had abandoned the market at the wrong times, you would have missed some eye-popping gains. As I’ve observed in other columns, the best days for stocks are often clustered near the worst ones, and you can’t skip the nightmares without passing up some awesome returns.

Mr. Stovall illustrated this conundrum by computing what would have happened to your investment in the S&P 500 over that same period if you had missed only the best days:

• Missed the 20 best days: 7.3 percent, annualized.

• Missed the 30 best days: 6.0 percent, annualized.

• Missed the 40 best days: 4.8 percent, annualized.

Being absent from the market on the strong days isn’t what any investor would want, but that’s likely to happen if you stay out of the market at any time.

That said, there’s no argument about this: You would have been better off with no stock holdings at all from Dec. 31, 1999, until March 9, 2009, which marked the end of the bear market associated with the financial crisis. I ran the numbers on FactSet. For that entire period, the cumulative return for the S&P 500 was minus 46 percent, including dividends. By contrast, over the same stretch, the Bloomberg U.S. Aggregate Bond Index, a widely followed benchmark for investment grade bonds, rose 72.3 percent.

Sticking with the stock market in that decade was to experience aggravation and heartache. Holding bonds has been painful lately, but they were a balm then. That’s worth remembering now — not as a reason to flee the stock market but as a reminder to hedge your bets.