A market breadth indicator that is based on the difference between the number of advancing and declining issues on the NYSE. It is primarily used for short and intermediate term trading. To calculate subtract a 39 day EMA (of advancing issues - declining issues) from a 19 day EMA (of advancing issues - declining issues). Simplified, it looks as follows: (19 Day EMA of Advances - Declines) - (39 Day EMA of Advances - Declines) Usually, a small number of stocks making large gains characterizes a weakening bull market. This gives the perception that the overall market is healthy, but in reality it isn't, as rising prices are being driven by a small number of stocks. Conversely, when a bear market is still declining, but a smaller amount of stocks are declining, an end to the bear market may be near.