The advance/decline line (A/D) is a technical analysis tool and is considered the best indicator of market movement as a whole. Stock indexes such as the Dow Jones Industrial Average (DJIA) only tell us the strength of 30 stocks, whereas the A/D line provides much more insight. The formula is quite simple: it is the ratio between advancing stocks and declining ones. If the markets are up but there are more declining stocks than advancing ones it's usually a sign that the markets are losing their breadth or momentum. If the number of advancing issues are dominating the declining issues, the market is said to be healthy.
Unlike the S&P futures contract, this indicator is not necessarily short term. Looking at the A/D line (not just the advance decline ratio) shows us the cumulative trend of advancers to decliners over a particular period of time. Most of the time the stock market does not turn around in an instant. Instead, the markets shift slowly, just as economic, business and market cycles would. This is why the general overall trend of the A/D line is important when determining the strength of the market.
Even so, the advancers to decliners is a tool and not a crystal ball. Sudden market shocks that result from interest rate movements, war, or other drastic events can't be detected by the A/D.
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