FOR the second time in less than 18 months, the market has taken a steep plunge and the patience of buy-and-hold investors is being put to the test.

But if investors learned anything from the previous bout of acute volatility  when the Standard & Poor’s 500-stock index plummeted to a low in March 2009 before soaring 80 percent during the next 13 months  it’s that it often pays to remain in the market, even in frightening times.

That doesn’t mean investors must stand pat with their existing portfolios, especially if they’re not well diversified. While the sharp, 12 percent slide in equity prices since April 23 has yet to become an official bear market (defined as a decline of 20 percent or more), “the chances of this correction morphing into a new bear are definitely rising,” said Sam Stovall, chief investment strategist at S.& P.

Even after a strong performance last week, in which the S.& P 500 rose 4.8 percent, the underlying economic optimism that spurred the stunning rally last year seems to be subsiding. “I’m saying to myself that the economy is still getting better, but the healing is going to take a little longer than we thought three months ago,” said David H. Ellison, chief investment officer for the FBR equity funds. “So I’ve become more defensive and have more cash in my portfolio than three or four months ago.”