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As I understand it, the stock change is intended to be a measure of how much the value of a stock changes over the course of a day. So why is it calculated as the difference between the current price and the last day close, instead of the difference between the current price and the current day open?

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    Some trading can still occur between the previous day's close-of-market and the current day's market-opening, so the two prices may not be the same. – chepner Mar 6 at 15:46
  • In addition, price can gap up or gap down at the open. – Bob Baerker Mar 6 at 16:19
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the stock change is intended to be a measure of how much the value of a stock changes over the course of a day.

No - it indicates the change from one day to the next (or more generally from one period to the next). Since the price can change from the end of one day to the beginning of the next day (due to after hours trading), you'd be missing the after-market changes if you only looked at intra-day change.

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