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In the stock market, why is the “open” price value never the same as previous day's “close”?
Nobody has mentioned the futures market yet. Although the stock market closes at 4pm, the futures market continues trading 24 hours a day and 5.5 days a week. Amongst the products that trade in the future market are stock index futures.  That includes the Dow Jones, the S&P 500. These are weighted averages of stocks and their sectors. You would think that the price of the underlying stock dictates the price of the average, but in this day and age, the derivative actually changes the value of the underlying stock due to a very complex combination of hedging practices. (this isn't meant to be vague and mysterious, it is "delta hedging") So normal market fluctuations coupled with macroeconomic events affect the futures market, which can ripple down to individual stocks. Very popular stocks with large market caps will most certainly be affected by futures market trading. But it is also worth mentioning that futures can function completely independently of a "spot" price. This is where things start to get complicated and long winded. The futures market factor is worth mentioning because it extends even outside of the aftermarket and pre-market hours of stock trading.