For investors thinking that Thursday’s big plunge is a sign that the stock market has bottomed, Mark Hulbert has a warning.

“Market declines rarely end with days like Thursday’s 513-point drop for the Dow,” Hulbert writes on MarketWatch.com. “So even if you think that we’re just suffering a mere correction within an ongoing bull market, you still should be prepared for lower prices in coming sessions.”

Hulbert says that after analyzing past bear market bottoms, he found that “the days on which those bear markets actually registered their final lows typically were rather uneventful — nothing like what we saw on Thursday.” He cites past bottoms, like the March 2009 low and the low that followed the 1987 crash, as examples. (What’s not clear is whether he found any difference between bear market bottoms and bottoms during corrections.)

“Chances are that the final low of the decline we’re experiencing will not be recognized as such until well after the fact,” he says. “It’s most unlikely that, on that day itself, so many traders will be doing what they did on Thursday — falling over themselves announcing that the bottom has been seen.”