The Mass Index(MI) is an indicator whick identifies trend reversals by using the changes of daily price ranges to identify reversals in trends.

As the price ranges narrow, the Mass Index decreases. As the price ranges widen, the Mass Index increases. 

A significant pattern to watch for is the "reversal bulge." These occur when on a 25-period plot the Mass Index surpasses 27 then falls past 26.5. 

A 9-period Exponential Moving Average of prices is often used to determine if the reversal bulge indicates a buying or selling. If a reversal bulge occurs, buy if the moving average is trending down (expecting a reversal) and sell if it is trending up.

This indicator was developed by Donald Dorsey.