The Mass Index was developed by Donald Dorsey and analyses average high to low values to determine 'Reversal Bulges'.
A bulge occurs when the Mass Index rises above 27 and subsequently falls below 26.5.
1. calculate 9 day EMA of High - Low
2. calculate 9 day EMA of 1
3. calculate EMA Ratio = 1 / 2
4. Mass Index = EMA Ratio accumulated over 25 days
Sufficient Data for Accuracy
Mass Index is one of several indicators that include an element of prior data. As such Mass Index based on 50 days of underlying data will be significantly different to Mass Index based on 500 days of data. This site will always include enough data to ensure 'accuracy'.