Advance/Decline Indicator
Advance/Decline measures the ratio of rising stocks to falling stocks on - in Zweig's examples - the NYSE. It excludes stocks whose price does not change. If 2000 stocks rise and 500 stocks fall, the A/D ratio would be 4.
Zweig uses the example of the rare event of a 10 day A/D ratio of two or more.
If you had invested in the market as a whole each time this has happened, in the following 6 months, your investment would have risen by an average of 19 percent.