Another phenomenon—also from psychology—that works against an objective assessment is group thinking. As social animals, it is not easy to stick to an opinion that differs markedly from that of a majority of the group. An example with which one may be familiar is the reluctance to enter a restaurant that is empty; people generally prefer to have their opinion validated by those of others in the group.
Take for example the S&P 500 market index, which is comprised of 505 companies. Buying shares in 505 different companies would be very difficult to do. Thanks to mutual funds and ETFs, we can simply buy one single security that holds shares in all 505 companies. The largest S&P 500 mutual fund is the Vanguard 500 Index Fund Admiral Shares (VFIAX) and the largest S&P 500 ETF is the State Street Global Advisors SPDR S&P 500 ETF (SPY).
Courtyard of the Amsterdam Stock Exchange (Beurs van Hendrick de Keyser) by Emanuel de Witte, 1653. The Amsterdam Stock Exchange is said to have been the first stock exchange to introduce continuous trade in the early 17th century. The process of buying and selling the VOC's shares, on the Amsterdam Stock Exchange, became the basis of the world's first official (formal) stock market.
The New York Stock Exchange (NYSE) is a physical exchange, with a hybrid market for placing orders electronically from any location as well as on the trading floor. Orders executed on the trading floor enter by way of exchange members and flow down to a floor broker, who submits the order electronically to the floor trading post for the Designated Market Maker ("DMM") for that stock to trade the order. The DMM's job is to maintain a two-sided market, making orders to buy and sell the security when there are no other buyers or sellers. If a spread exists, no trade immediately takes place – in this case the DMM may use their own resources (money or stock) to close the difference. Once a trade has been made, the details are reported on the "tape" and sent back to the brokerage firm, which then notifies the investor who placed the order. Computers play an important role, especially for program trading.