Studies · Volatility
Mass Index
massIndex · Dorsey's range-expansion sum — the reversal bulge
Drawn in its own panelReads high, lowGuide lines at 26.5, 27
Hover the chart to read values · practice data: a seeded random walk with no overnight gaps
What it measures
Watches for the range of bars widening relative to its own average, ignoring direction. It sums 25 bars of (EMA of range ÷ EMA of that EMA), so a quiet market reads near 25.
How traders use it
- The "reversal bulge": the index climbs above 27 and then falls back below 26.5. Dorsey read this as a warning that the current trend is about to reverse.
- It does not say which way, so it is paired with a trend study (for example a moving average) to pick the direction.
Parameters
| Name | Value | Default | Allowed | What it does |
|---|---|---|---|---|
| emaPeriod | 9 | ≥ 1 usual 5–20 | Length of both EMAs applied to the range. Dorsey used 9. | |
| sumPeriod | 25 | ≥ 1 usual 10–50 | How many bars of the ratio are added up. Dorsey used 25. |