Today, in the United States, the profits of listed companies are less than half of the profits of all enterprises. In the past few decades, this proportion has been shrinking. According to Wilshire's statistics, 1997, there were more than 7,400 listed companies in the United States, but today there are only 3,600. The remaining corporate profits are obtained by sole proprietorships, partnerships and private holding companies, ranging from hot dog stalls with only one person to private companies employing tens of thousands of employees. It is very difficult to hold a diversified portfolio that reflects the whole economic situation, and it takes too much time and professional knowledge to buy private enterprises directly, so investors have only the following three choices-first, buy private equity funds or venture capital funds. These funds directly invest in private enterprises, but they are expensive, and only some of them are value for money. It is even more difficult for small investors to get a good private equity fund, and the liquidity of this investment is still very poor. Secondly, invest in listed companies that manage private equity funds. In this way, they can own shares in the funds they manage in disguise, but the performance of these companies after listing is very bad. Finally, use professional index funds or exchange-traded funds to build a tradable asset portfolio. In this way, we can track the whole macroeconomic situation, not just the listed stock market. This method may not be perfect, but it is low in cost and simple to operate, which can help investors diversify their investments and has sufficient liquidity.