The Advance/Decline Line (A/D Line) is one the most widely used measures of market breadth. As a cumulative total of the Advancing-Declining Issues indicator, the A/D Line has proven to be a very effective gauge of the market's strength.

The A/D Line is calculated from the running total of advancing stocks minus declining stocks. Designed to measure the strength of the market, a sector or industry, it makes the basic underlying assumption that as long as there are more advancing issues than declining issues, the market remains strong. While a stock index is a composite of stock prices, the A/D Line is a composite of stock movement. This gives the daily A/D Line a downward bias relative to the weekly A/D Line and the price-based indices. This downward bias is a result of the average stock tending to have as many up days as down days, but ultimately the gains will tend to accumulate faster than the losses.

The A/D Line can function as a measure of overall market strength. When more stocks are advancing than declining, the A/D Line moves up. When declining stocks outnumber advancing stocks the A/D Line will move down.

Many feel that the A/D Line shows market strength better than the more commonly reported Dow Jones Industrial Average (DJIA) or the S&P 500 Index. Studying the trend of the A/D Line can illustrate if the market is in a rising or falling trend, if the trend is still intact, or how long a current trend has prevailed.

The A/D Line can also be used to spotlight a divergence between itself the DJIA or a similar index. Often, an end to a bull market can be predicted when the A/D Line begins to round over even while the DJIA is trying to make new highs. Historically, when a divergence develops between the DJIA and the A/D Line, it is the DJIA that has changed direction and moved in the direction of the A/D Line.

Information provided by John Murphy, author of Technical Analysis of the Financial Markets and The Visual Investor.