The stock market’s Sept. 15 sell-off was only the 10th time since 1978 the S&P 500 closed down more than 4.5 percent. The previous time was Sept. 17, 2001, which was the first day the U.S. market reopened after the Sept. 11 terrorist attacks.

In one way, the rarity of these events underlines their impact — these moves don’t typically come out of nowhere. Nonetheless, a one-day move is a one-day move, and the context in which it occurs is critical, if also much more difficult to assess.