Negative Volume Index (NVI)

Paul Dysart's cumulative index (popularised by Norman Fosback) that only updates when volume contracts. The hypothesis is that smart-money accumulation happens on quiet days, so NVI tracks the "smart money" leg of price action while ignoring volume-spike days that retail tends to chase.

Quick reference

Item	Value
Family	Volume
Input type	Candle (uses close, volume)
Output type	f64
Output range	unbounded (anchored at 1000.0)
Default parameters	none
Warmup period	2
Interpretation	"Smart money" trend on quiet days

Formula

Seed: NVI = 1000.0

NVI_t = NVI_{t-1} · (1 + (close_t - close_{t-1}) / close_{t-1})
        if volume_t < volume_{t-1}

NVI_t = NVI_{t-1}    otherwise

The starting value 1000 matches Fosback's textbook convention. See crates/wickra-core/src/indicators/nvi.rs.

Parameters

None — Nvi::new() takes no arguments.

Inputs / Outputs

Indicator<Input = Candle, Output = f64>. Python: Nvi().batch(close, volume) returns a 1-D np.ndarray. Node: same.

Warmup

warmup_period() == 2. First bar seeds; bar 2 emits the first non-trivial value.

Edge cases

• Volume equal. No update — index unchanged.
• Equal volume + equal close. No update either way.
• Long quiet stretches. NVI moves frequently; long noisy stretches leave it unchanged for many bars.
• Reset. Resets to 1000 and clears the prior bar.

Examples

Rust

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

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let candles: Vec<Candle> = (0..20).map(|i| {
        let b = 100.0 + f64::from(i);
        let v = if i % 3 == 0 { 500.0 } else { 200.0 };  // alternating volume
        Candle::new(b, b + 0.5, b - 0.5, b, v, i as i64).unwrap()
    }).collect();
    let mut nvi = Nvi::new();
    println!("last = {:?}", nvi.batch(&candles).last());
    Ok(())
}

Python

import numpy as np
import wickra as ta

close = 100 + np.arange(20, dtype=float)
vol = np.where(np.arange(20) % 3 == 0, 500.0, 200.0)
nvi = ta.Nvi()
print(nvi.batch(close, vol)[-1])

Node

const wickra = require('wickra');
const nvi = new wickra.Nvi();
// feed c, v

Streaming

use wickra::{Candle, Indicator, Nvi};

let mut nvi = Nvi::new();
let candle_stream: Vec<wickra::Candle> = Vec::new(); // your live OHLCV candle feed
for bar in candle_stream {
    if let Some(v) = nvi.update(bar) {
        // v is the smart-money cumulative index
    }
}

Interpretation

• NVI trend. Rising NVI = price advances on quiet days = smart-money accumulation. Falling NVI = price falls on quiet days = smart-money distribution.
• NVI vs 255-day EMA. Fosback's classic rule: NVI above its 255-day EMA = "smart money is bullish, bull market 95% likely per Fosback's stats".
• Pair with PVI. PVI and NVI together cover all-day flow. Most informative when they diverge.

Common pitfalls

• Frequency dependence. "Quiet day" means a lower-volume bar than the immediately prior bar. Intraday NVI on minute bars behaves differently from daily NVI.
• Volume data quality. Stale or aggregated volume produces spurious NVI moves. Best on bar-level direct exchange data.
• Anchor 1000. Relative magnitude meaningless; only the direction and crossings of trend lines (e.g. its own EMA) matter.

References

• Paul L. Dysart, 1936 — original Negative Volume Index.
• Norman G. Fosback, Stock Market Logic (1976) — popularised NVI and PVI together as a smart-money / crowd analysis system.

See also

• Pvi — complementary crowd-money cousin.
• Obv — simpler all-day cumulative volume.
• Adl — alternative cumulative flow.
• Indicators-Overview — full taxonomy.