Elder Ray Index (Bull Power / Bear Power) (ERI)

Summary

Elder Ray Index: Alexander Elder's Bull Power / Bear Power pair from Trading for a Living (1993) — how far the bar's high and low sit from an EMA of the close. Bulls strong enough to push the high above the average read as positive Bull Power; bears dragging the low below it read as negative Bear Power.

Formula

Bull Power = High − EMA(Close, n) and Bear Power = Low − EMA(Close, n), both lines against the same EMA. Bull ≥ Bear on every bar since high ≥ low. TradingView's built-in Bull Bear Power — which its own support page calls "otherwise known as the Elder-Ray Index" — plots only the sum of the two, not the pair; StockCharts, TC2000 and pandas-ta all ship the two lines.

Because the underlying average is an EMA, ERI inherits its unstable period: the warm-up consumes TA_GetUnstablePeriod(TA_FUNC_UNST_EMA) extra bars, exactly as EMA itself does.

Inputs

inHigh — High price series
inLow — Low price series
inClose — Close price series

Outputs

outBullPower — High minus the EMA of close
outBearPower — Low minus the EMA of close

Parameters

Parameter	Type	Default	Accepted values	Description
`optInTimePeriod`	integer	13	1–100000	Number of bars in the EMA of close

Notes

ERI is a cancelling difference: near the zero crossings that carry its signal, tiny EMA discrepancies are amplified without bound in relative terms. Compare against external values with an absolute tolerance.
No MAType parameter: every canonical source fixes the EMA, and a selectable average would invent a variant nobody ships.

Properties

Numerical Stability: Initial Unstable Period — Inherited from EMA, which ERI computes internally; tunable via EMA's unstable period.


☐ Overlap Input
✅ Independent Y-Axis i
☐ Candlestick
☐ Can Output NaN or ±Inf
☐ Identity at Period 1

Implementation

TA-Lib Definition: eri.c · eri.yaml

Native	File
C	`ta_ERI.c`
Rust	`eri.rs`
Java	`Core_ERI.java`

TA-Lib is also available for Python, R and more using a wrapper.

Aliases

Elder-Ray Index · Bull Power / Bear Power

References

Alexander Elder, Trading for a Living (Wiley, 1993), the Elder-Ray chapter