No wonder the stock market seems so unsettling in 2018. Through Thursday, the average daily change, up or down, was 0.84 percent. That is a big increase. Yet going back to 1928, the average has been 0.75 percent. The current market, which has been depicted as wildly disruptive is, basically, just an average one, Mr. Detrick said.

Recall that in June 2016, after British voters said they wanted to leave the European Union, the stock markets fell into what seemed, at the time, to be a major panic. Less than a week after that vote, though, markets resumed their rise. Since then, the American market, measured by the S.&P. 500, was in positive territory every single day — until the cumulative declines of 2018 pulled the index into the red on Feb. 5 and into a correction on Feb. 8. On Friday, the S.&P. 500 closed at 2619.55.

Even if the current downturn doesn’t deepen, some sectors are already feeling severe pain. Traders who had bet that calm conditions would continue have lost fortunes. In just two days, the assets in two funds that trade in instruments linked to the VIX — officially, the Chicago Board Options Exchange Volatility Index — shrank from a combined total of $3 billion to about $150 million. There are few indications so far that trading losses in those instruments — or in the battered cryptocurrency markets — have damaged the overall financial system, but that is a worry in a serious downturn.

There have been major losses in the broader market, however. Through Thursday, the stocks in the S.&P. 500 had lost $5.2 trillion from the index’s peak on Jan. 26, according to Howard Silverblatt, senior index analyst at S.&P. Dow Jones Indices. That is a staggering amount, but even with those losses, the S.&P. 500 through Thursday had swollen by $3.55 trillion since the 2016 election.

How far will the current market decline go? I wish I knew. One troubling factor is the change in the leadership of the Federal Reserve, with Jerome H. Powell taking over as chairman on Feb. 3, succeeding Janet L. Yellen. In his confirmation hearings, Mr. Powell signaled a continuation of Fed policy, but the markets are testing him and it is not clear how he will respond.

The timing is awkward. The Fed has been tightening monetary policy while the federal government is loosening fiscal policy by cutting taxes, and then adding hundreds of billions of dollars in government spending in a last-minute budget deal on Friday. Furthermore, the Trump administration has been pushing for lighter regulation of financial markets. These shifts during a period of stress raise short-term risks for investors in stocks and bonds.

Bear markets in stocks — defined as downturns, from peak to trough, of at least 20 percent — rarely occur without a recession, and, at the moment, none is visible. That’s one reason for Mr. Detrick’s belief that the market is “probably getting fairly close to its bottom now.”