Countless factors affect stock prices on a daily basis. Some are very broad like global events and macroeconomic trends. Others are more narrow: company-specific news or changes to analyst price targets. But all of those things affect investor sentiment to some degree, making it impossible to predict which direction a stock (or even the broad market) will move in the short term.

You may hear stories about day traders who made a fortune overnight. Well, some lucky people have also become millionaires by playing the lottery, but that doesn't mean you should invest your money in lottery tickets. Several studies have shown the vast majority of day traders actually lose money, and the ones who manage to turn a profit often make less than minimum wage.

Put simply, the best way to make money in the stock market is a long-term investment strategy. For instance, the S&P 500 has produced a positive return 100% of the time over any 20-year window between 1919 and 2021, according to Crestmont Research. That means patient investors who held an S&P 500 index fund for at least two consecutive decades (at any point over the last century) always made money.

Here is one way to benefit from that information.

A simple way to make money in the stock market

The Vanguard S&P 500 ETF (NYSEMKT: IVOO) is a passively managed fund that tracks the performance of the S&P 500, which includes 500 of the largest U.S. companies. That may be less exciting than buying individual stocks, but there are several advantages to this strategy investors should consider.

First, the Vanguard S&P 500 ETF offers instant diversification across all 11 market sectors, and investors get exposure to some of the most valuable brands in the world. For instance, the top 20 holdings include industry-leading names like Apple, Inc. (NASDAQ: AAPL), Microsoft Corporation (NASDAQ: MSFT), Amazon.com, Inc.(NASDAQ: AMZN), The Home Depot, Inc.(NYSE: HD), Mastercard Incorporated(NYSE:MA), Visa Inc. (NYSE: V), UnitedHealth Group (NYSE: UNH), Johnson & Johnson (NYSE: JNJ), Tesla Corp Ltd (NASDAQ: TSLA), Alphabet Inc. (NASDAQ: GOOG) (NASDAQ: GOOGL), and ExxonMobil Corporation (NYSE: XOM).

Second, the Vanguard S&P 500 ETF is cheap and time-efficient. It bears an expense ratio of 0.03%, meaning investors would pay only $1.50 per year in fees on a $5,000 portfolio. Additionally, it requires very little work, because there is no need to research specific companies or stay up to date on financial results. Investors can simply buy the ETF and forget about it.

In short, while it may be boring, buying an S&P 500 index fund is a simple, inexpensive, and time-tested path to making money in the stock market. That's why Warren Buffett has often advocated for this investment strategy.

Third, the Vanguard S&P 500 has generated a total return of 206% over the last decade, which is equivalent to an annualized return of 11.8%. At that pace, $100 invested on a weekly basis would grow into a $1 million portfolio in 28 years, and it would grow into a $2 million portfolio in 34 years.

How I manage my portfolio

An S&P 500 index fund does not have to be your only investment. Personally, I keep a certain percentage of my portfolio in the Vanguard S&P 500 ETF, but I also own dozens of individual growth stocks. I think of the S&P 500 index fund as a sort of safety net, a reliable money maker in the long run.

Of course, nothing is truly guaranteed when it comes to the stock market, but the S&P 500 has undeniably produced a positive return over every rolling 20-year period since 1919. And that knowledge makes me feel comfortable taking a little more risk with my other investments.

This article was originally published on Fool.com. All figures quoted in US dollars unless otherwise stated.

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