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The advance–decline line is a stock market technical indicator used by investors to measure the number of individual stocks participating in a market rise or fall. As price changes of large stocks can have a disproportionate effect on capitalization weighted stock market indices such as the S&P 500, the NYSE Composite Index, and the NASDAQ Composite index, it can be useful to know how broadly this movement extends into the larger universe of smaller stocks. Since market indexes represent a group of stocks, they do not present the whole picture of the trading day and the performance of the market during this day. Though the market indices give an idea about what has happened during the trading day, advance/decline numbers give an idea about the individual performance of particular stocks.
The advance–decline line is a plot of the cumulative sum of the daily difference between the number of issues advancing, i.e., above the previous day's closing price, and the number of issues declining in a particular stock market index. Thus it moves up when the index contains more advancing than declining issues, and moves down when there are more declining than advancing issues. The formula for ADL is:
The Advance/Decline Line formula could be applied to volume of the advancing and declining stocks.
The ADL is one of the oldest indicators based on the Advance-Decline Data and it was the most popular of all internal indicators.