Advance-Decline Line

The advance-decline line asks whether the typical stock is participating in the move, not whether the index is up. An index can rise because a handful of large weights are up, while most stocks are down. In that case the cap-weighted index makes a high and the A/D line does not. That divergence is the classical warning. It is a warning about participation, not a sell signal with a date. Divergences can last for months. The line is also sensitive to the universe. A line built on every listed stock includes a crowd of small names that do not resemble the index you trade. A line built on the index's own members answers the question you actually asked.

Plot the A/D line under the index and compare swings, not the absolute level. The level depends on when you started the sum. A rising line with a rising index is broad participation. A falling line with a rising index is a narrow advance. A rising line with a falling index is a broad market holding up better than the headline, which sometimes happens when large weights are the ones under pressure. Use a moving average of the daily net advances if you want a shorter oscillator. That is a different tool. Do not smooth the cumulative line until the divergence you are supposed to see has been averaged away.

Use the A/D line as a regime filter for index trades and for how much you trust a single-stock breakout. In a narrow market, breakouts in the leaders can continue and breakouts in the average stock fail. In a broad advance, pullbacks in ordinary names are more forgivable. Do not short the index the day the divergence appears. Record the divergence, tighten the standards for new longs outside the leaders, and wait for the index's own structure to break if you want an index short. Breadth is context. Price is the trigger.

Starting the cumulative line at a convenient date so the picture matches a story. Also comparing today's A/D line with an index that has been reconstituted, and ignoring survivorship and additions. And treating a one-day net-advance spike as a turn in the cumulative line. The line turns when the sum of days turns, which is the point of cumulating it.

The A/D line weights a small stock and a giant stock as one issue each. That is the feature, and it is a distortion if you think you are measuring capital. Cap-weighted breadth measures exist for that question. The line also does not handle halted names, unusual universes, or a day when a rebalance moves many prints at the close. And a long divergence that "should have" ended a bull market and did not is a reminder: participation warnings are not clocks.