NASI & NYSI explained

The McClellan Summation Index is a market breadth indicator that is ultimately derived from the number of advancing and declining stocks in a given market. 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. Many people regard the McClellan Summation Index as an excellent indicator of the overall "health" of the market and the market's current trend. The McClellan Summation Index is derived from the McClellan Oscillator by tracking its daily accumulation or "summation". The McClellan Oscillator is a breadth indicator derived from each day's net advances, the number of advancing issues less the number of declining issues. Subtracting the 39-day exponential moving average from the 19-day exponential moving average of net advances forms the oscillator. When the 19-day EMA (shorter moving average) moves above the 39-day (longer moving average) EMA, it signals that advances are gaining the upper hand. Conversely, when the 19-day EMA declines below the 39-day EMA, it signals that declining issues are dominant. As a momentum indicator, the McClellan Oscillator attempts to anticipate positive and negative changes in the AD statistics for market timing. The Summation Index = 1000 + (10%Trend - 5%Trend) - [(10 x 10%Trend) + (20 x 5%Trend)] where: 5%Trend = 39-day EMA of (Advancers-Decliners) & 10%Trend = 19-day EMA of (Advancers-Decliners).
The $NASI is the McClellan Summation Index for the NASDAQ and the $NYSI is the McClellan Summation Index for the New York Stock Exchange. The McClellan Summation index generally oscillates between 0 and 2000 although it can move outside of this range during extreme or unusual market conditions. Historically, major market bottoms occur after the index falls below -1000. Readings above +1600 often indicate a major top is near. Top and bottom signals carry more significance if the index is also diverging from the associated market average. According to the McClellans, the beginning of a new bull market is signaled if the NYSE-based Summation index first moves below the -1200 level and then quickly rises above +2500.