Users are often curious why the intraday high/low data we show for the Dow Jones Industrial Average (^DJI) on the quote summary page is different from the historical high/low data we show for ^DJI on the historical prices page. The short answer is that there are two different methods to calculate high and low prices for the index—Theoretical and Actual—and we choose to display the Theoretical high/low data in historical prices since the data ranges back to 1928, versus 1992 for the Actual data.

The intraday data we display for ^DJI on the quote summary page is Actual data. Explanations of both calculation methods are below.

The Theoretical Dow Jones Industrial Average assumes all index components hit their high or low at the same time during the day. At the end of the trading day, the daily highs for all 30 Dow components are used to calculate the index high for the day, and the same is performed for the index low.

In 1992, Dow Jones started using the Actual method, which calculates the index every 10 seconds throughout the trading day. Before 1992, the Actual method did not exist. Since all index components almost never hit their highs and lows at the same time, the Theoretical high is almost always higher than the Actual, and the Theoretical low is almost always lower than the Actual.