If you missed the rally, then you just made the most classic mistake in investing

cme chicago futures trader
cme chicago futures trader

AP Images It's been a brutal few months for stock market investors.

We've seen a couple of violent sell-offs. The first came when the S&P 500 quickly tumbled 9.8% from its then all-time high of 2,019 on Sept. 19 to as low as 1,820 on Oct. 15. The second came when the S&P plunged 5.1% from 2,079 ion Dec. 5 to 1,972 on Dec. 16. From there, the S&P surged 6% to 2,092 by Dec. 29. And then we fell 5.3% to 1,980 Feb. 2.

And once again, we've surged right back to new all-time highs.

Because of the way our brains work, some of us probably worried about the possibility that any of these sell-offs could turn into an outright market crash. Our instinct was to dump stocks. Surely, many investors sold and told themselves they would "wait out the volatility" on the sidelines. A confident few likely even shorted the market.

However, history shows this is the most classic mistake investors make. So, kudos to those who held on to their long positions.

"Corrections are part and parcel of the investment process, they come and go, and it is imperative to take a deep breath and realize that what is most important for building wealth is not 'timing' the market but rather 'time in' the market," David Rosenberg said on Oct. 14.

"Time in" the market is crucial, especially when things get scary for investors. There's tons of data on this. We talk about it all of the time. Even the folks who sold the sell-off probably know about it. But let's revisit some of the data anyway.

Missing A Few Good Days Will Destroy Your Long-Term Returns

When volatility picks up, it's tempting to trade in and out of the market with the hope you'll protect your wealth. Unfortunately, this increases the risk you'll miss some of the best days in the market. And that can be very costly.

JPMorgan Asset Management illustrated how much an investor's returns collapsed when they missed a few of the best days in the market. They found that if an investor stayed fully invested in the S&P 500 from 1993 to 2013, they would've had a 9.2% annualized return.

However, if trading resulted in missing just the ten best days during that same period, then those annualized returns would collapse to 5.4%.

cotd stock market missed returns
cotd stock market missed returns

JP Morgan Asset Management Missing these days do so much damage because those missed gains aren't able to compound during the rest of the investment holding period.

"Plan to stay invested," they recommend. "Trying to time the market is extremely difficult to do consistently. Market lows often result in emotional decision making. Investing for the long-term while managing volatility can result in a better outcome."