The Mass Index was designed by Donald Dorsey as a measure of the range between the high and low prices. The wider the range, the higher the Mass Index. According to Dorsey a reversal bulge (when the Mass indicator exceeds 27 then dips below 26.5) signifies an imminent reversal of the trend.

The Mass Index is computed by summing the ratios of a singly and doubly smoothed exponential moving average of the range. You choose the term for the moving averages. Dorsey recommends nine days.