wickra/docs/wiki/indicators/volume/Indicator-ForceIndex.md

ForceIndex

Force Index — Alexander Elder's price change scaled by volume, then smoothed with an EMA.

Quick reference

Field	Value
Family	Volume
Sub-category	Oscillators
Input type	Candle (uses close, volume)
Output type	f64
Output range	unbounded around zero
Default parameters	period = 13 (Python)
Warmup period	period + 1
Interpretation	Conviction behind a move; sign and zero-crossings are the signal.

Formula

raw_t   = (close_t − close_{t−1}) · volume_t
Force_t = EMA(raw, period)_t

The raw force is positive on an up-close and negative on a down-close, with a magnitude that grows with the volume backing the move — a large move on heavy volume registers a large force, a large move on thin volume does not. Smoothing the raw series with an EMA turns the noisy per-bar reading into a tradeable line; Elder's classic period is 13.

Parameters

period — the EMA smoothing period. The Python binding defaults it to 13; the Rust and Node constructors require it explicitly.

Inputs / Outputs

From crates/wickra-core/src/indicators/force_index.rs:

impl Indicator for ForceIndex {
    type Input = Candle;
    type Output = f64;
    // update(&mut self, input: Candle) -> Option<f64>
}

ForceIndex is a candle-input indicator that reads close and volume. In Python the streaming update accepts a 6-tuple or a dict; the batch helper takes close, volume numpy arrays. Node and WASM expose update(close, volume) and the matching batch.

Warmup

ForceIndex::new(13).warmup_period() == 14. The first candle only establishes the previous close, so the first raw force appears on candle 2 and the first smoothed value on candle period + 1.

Edge cases

• First candle. Establishes the previous close only; emits None.
• Up- vs down-trend. A strictly rising series gives a positive force, a strictly falling series a negative one (pure_uptrend_is_positive and pure_downtrend_is_negative pin this).
• period = 1. EMA(1) has alpha = 1, so the Force Index passes the raw force through unsmoothed.
• Reset. fi.reset() clears the previous close and the EMA.

Examples

Rust

use wickra::{BatchExt, Candle, Indicator, ForceIndex};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // ForceIndex(1): EMA(1) passes the raw force through.
    let mut fi = ForceIndex::new(1)?;
    let out = fi.batch(&[
        Candle::new(10.0, 10.0, 10.0, 10.0, 100.0, 0)?,  // seeds the previous close
        Candle::new(12.0, 12.0, 12.0, 12.0, 100.0, 1)?,  // raw = (12-10)·100
        Candle::new(11.0, 11.0, 11.0, 11.0, 200.0, 2)?,  // raw = (11-12)·200
    ]);
    println!("{:?}", out);
    Ok(())
}

Output:

[None, Some(200.0), Some(-200.0)]

This matches the reference_values test in crates/wickra-core/src/indicators/force_index.rs.

Python

import numpy as np
import wickra as ta

fi = ta.ForceIndex(1)
close = np.array([10.0, 12.0, 11.0])
volume = np.array([100.0, 100.0, 200.0])
print(fi.batch(close, volume))

Output:

[  nan  200. -200.]

Node

const ta = require('wickra');
const fi = new ta.ForceIndex(1);
console.log(fi.batch([10, 12, 11], [100, 100, 200]));

Output:

[ NaN, 200, -200 ]

Interpretation

Elder reads the Force Index on two horizons. A short period (the classic 2) is a sensitive entry timer — it crosses zero often. A longer period (13) tracks the conviction behind the prevailing trend: it staying above zero confirms buyers are in control. Divergence between a 13-period Force Index and price flags an exhausting move.

Common pitfalls

• Comparing levels across instruments. The force scales with raw volume, so a value of 200 means nothing without knowing the instrument.
• Feeding it scalar prices. It needs close and volume.

References

Alexander Elder's Force Index, introduced in Trading for a Living (1993).

See also

• Indicator-Obv.md — cumulative signed volume, a coarser volume-conviction gauge.
• Indicator-VolumePriceTrend.md — cumulative volume scaled by percentage move.
• Indicators-Overview.md — the full taxonomy.