A recent episode in the stock market offers useful lessons for investors. Let’s start by reviewing the facts:

• Last Friday, the S&P 500 suffered its worst day in months. And the Dow had its worst day in more than a year. The proximate cause: news about Omicron, a new coronavirus variant. Overnight, investors seemed to rewind their playbook to the depths of the recession last spring. Airline stocks dropped precipitously. Oil dropped 13%. Meanwhile, bond prices rose, along with the stocks of vaccine makers, such as Moderna, which gained 20%.
• On Monday, though, most everything seemed to reverse. The market staged a rebound, with the S&P gaining and bonds dropping back. Why the sudden reversal? Reporters at The Wall Street Journal tried to piece it together. They quoted the president, who said Omicron was “not a cause for panic.” And they quoted a Wall Street analyst, who offered this explanation: “Friday was a panic selloff. Traders have had time to sit back and breathe a bit.”
• The market took another steep dive on Tuesday. Why the reversal? Here’s some of the commentary the Journal offered this time: “We really don’t know in regards to the new variant what we’re dealing with,” said one Wall Street strategist. And the CEO of a fund company suggested this explanation: “People are trying to protect what they have.”
• On Wednesday, the market rebounded again in the morning. The proximate cause this time: the release of some positive employment data. But by the afternoon it gave up those gains and ended up in negative territory. What happened? A report that someone in the U.S. had been infected with the Omicron variant.
• Finally, on Thursday, the market bounced back yet again, despite news that three more people in the U.S.—all vaccinated—had been infected with the Omicron variant.

What conclusions can you draw from this snapshot of five days in the stock market?

In the world of investment theory, there is a school of thought that believes in “the wisdom of crowds”—the idea that groups of people, in aggregate, are smarter and will make better decisions than any one individual. A related concept, the efficient markets hypothesis (EMH), takes this a step further.

EMH argues that investors, collectively, are always monitoring the news and are quick on their feet. As a result, when new information becomes available, not only do they understand it, but they also jump into action. EMH postulates that these quick-thinking investors will buy or sell stocks in response to the new news. This causes the prices of those investments to quickly adjust to the new information. In this way, they are said to keep the market efficient.

Intuitively, this makes sense. People do generally make better decisions when they collaborate on a question or problem. By extension, then, it seems like we ought to put faith in the wisdom of crowds. There are certainly cases in which the market has exhibited remarkable efficiency. One notable example occurred about 35 years ago.

On January 28, 1986, the Space Shuttle Challenger exploded shortly after liftoff. While there were four publicly-traded companies that were contractors to the Shuttle program, a government panel found that just one was ultimately responsible: Morton Thiokol. It took the panel five months to reach that conclusion. But if you look at how the stock market reacted on the day of the crash, you’ll see the market seemed to figure this out much more quickly—almost immediately, in fact. The shares of all four companies dropped on the day of the crash within just 13 minutes of the news hitting wire services. But the drop in Thiokol shares was far more pronounced. Its stock ended the day down 13%, while the stocks of the other three companies lost just 2% or 3%.

Supporters of the efficient markets hypothesis often cite this case to support their view. In my opinion, though, this is more the exception than the rule. If you contrast the Challenger case with the five-day microcosm described above, you’ll see two significant differences:

First, the facts are rarely as clear as they were in the Challenger case. That was a single tragic episode, and there were just four possible culprits. Even among them, Thiokol was a reasonable guess because concerns had been raised before—by Thiokol itself—about the safety of its rocket boosters in cold weather conditions. It was an unseasonably cold 36 degrees at Cape Canaveral at the time of the launch.

By contrast, Covid-19 is a moving target. It is literally evolving. As we’ve seen recently, it’s hard for anyone to know when there will be further variants. And when there are, no one knows what they’ll look like or what their impact will be.

One theory floated this week was the idea that Omicron will be more contagious but less lethal than Delta. If that were the case, the thinking goes, Omicron might crowd out Delta. Then more people would become infected, but with a more benign virus. If that were the result, Omicron might actually be a welcome development. But that’s just a theory. No one knows for sure if anything like this will occur.

In contrast to the complexity and lack of clarity surrounding Covid, the Challenger case was more like a controlled scientific experiment, with a very straightforward set of facts. But that happens very rarely in the stock market. It’s why, in my opinion, the Challenger case is the one that EMH enthusiasts always seem to cite. It’s one of the only cases that perfectly supports their hypothesis.

The other difference between the Challenger case and the stock market’s normal function is that stocks are usually subject to a mix of countervailing forces. Over the past week, for example, the Omicron news—which was itself inconclusive—was clouded by several other factors. Notable among them: Federal Reserve chair Jerome Powell was in front of Congress providing testimony on Tuesday and Wednesday. Powell weighed in on the potential impact of Omicron. So did the World Health Organization and the CEO of Moderna, among others. Mixed in with all that was new economic data, like the employment report that came out Wednesday.

The result: As the saying goes, things were clear as mud. That’s why we saw the stock market seesawing the way it did. And that’s why the investors that the Journal interviewed seemed to be grasping at straws in trying to explain the stock market’s movements.

Bottom line: The market always reacts to news. And sometimes its lightning fast reactions make it seem efficient. But whether the market’s reactions ultimately make sense is a different question. In very rare cases perhaps, the market is truly efficient. But most of the time, information is incomplete, inconclusive or clouded by other factors. As a result, it’s hard to argue that it’s truly efficient on a day-to-day basis.

Where does this leave you as an individual investor? I’d turn, as I often do, to Benjamin Graham, the father of modern investment analysis. His view: In the short term, the stock market is like a “voting machine.” It’s a popularity contest and not necessarily logical. But in the long term, Graham said, the market is a “weighing machine.” In other words, if you can make it through the day-to-day noise, the market is, in fact, logical over time, and generally a good investment.

The key, then, is to try hard to tune out the noise. As Warren Buffett once said, “If you can enjoy Saturdays and Sundays without looking at stock prices, give it a try on weekdays.” It’s a bit of an overstatement, but maybe more true than not.