There are many ways of tracking breadth. Of all of these, I have found advance-decline lines to be among the least useful in predicting forward price movement. This reflects my more general experience that the most commonly tracked market measures are among the least useful, perhaps precisely because they are so widely tracked.
Above are three breadth measures that I have found to be useful. The first (top chart) is the number of common stocks across all changes that are making fresh three month highs minus those making new three month lows. (Data available from the Barchart site). You can see that breadth has deteriorated in recent days, leading to the most recent market weakness. You can also see that breadth has deteriorated since the early August decline, as the market rally grows increasingly selective.
The second measure (middle chart) covers all stocks listed on the NYSE and tracks the daily number that close above their upper Bollinger band minus those closing below their lower Bollinger band. (Data available from the Stock Charts site). This, too, shows a recent pattern of deterioration, even as stocks moved to new highs.
The third breadth measure (bottom chart) is specific to the universe of S&P 500 stocks. It is a composite measure of the percentages of SPX stocks trading above their 3, 5, 10, and 20 day averages. (Data available from the Index Indicators site). What we see again is a tendency for this measure to peak ahead of price, as has happened most recently.
In general, I find that strong breadth leads to short-term upside momentum, followed by reversal. Weak breadth leads to short-term downside momentum at important market bottoms (i.e., bottoms of longer-term market cycles) and short-term reversal at market corrections. In a qualitative sense, these measures give me a picture on whether markets are getting stronger or weaker day over day--very useful information for gauging where we might be at in a market cycle.
Further Reading: Useful Trading Tools: Breadth
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