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In the stock market, why is the “open” price value never the same as previous day's “close”?
Prices reflect all available information. (Efficient markets hypothesis) A lot can happen between the time a stock closes on one day and opens on another. Particularly in a heavily traded stock such as IBM. Basically, you have a different "information set" the following day, which implies a different price. The instances where you are most likely to have a stock where the price opens at the same price is at the previous close is a thinly traded stock on which you have little information, meaning that the "information set" changes less from day to day.