The Dow Jones Industrial Average set a startling record that shows just how flawed it truly is

The past few days in the stock market have been so wild—a plunge on Monday, a sharp pivot upward on Tuesday, a rise with lots of oscillations on Wednesday—that a record set by the Dow Jones Industrial Average on last week’s final day of trading has been largely overlooked. That’s unfortunate, because there’s a lot to be learned from that record about how financial markets work.

I’m referring to the record loss inflicted on the Dow last Thursday by the three-digit share price drop of UnitedHealth Group (NYSE...