Negative Volume Index (NVI) — Indicator by HPotter

The theory behind the indexes is as follows: On days of increasing
volume, you can expect prices to increase, and on days of decreasing
volume, you can expect prices to decrease. This goes with the idea of
the market being in-gear and out-of-gear. Both PVI and NVI work in similar
fashions: Both are a running cumulative of values, which means you either
keep adding or subtracting price rate of change each day to the previous day`s
sum. In the case of PVI, if today`s volume is less than yesterday`s, don`t add
anything; if today`s volume is greater, then add today`s price rate of change.
For NVI, add today`s price rate of change only if today`s volume is less than
yesterday`s.

Open-source script

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////////////////////////////////////////////////////////////
//  Copyright by HPotter v1.0 11/06/2014
// The theory behind the indexes is as follows: On days of increasing 
// volume, you can expect prices to increase, and on days of decreasing 
// volume, you can expect prices to decrease. This goes with the idea of 
// the market being in-gear and out-of-gear. Both PVI and NVI work in similar 
// fashions: Both are a running cumulative of values, which means you either 
// keep adding or subtracting price rate of change each day to the previous day`s 
// sum. In the case of PVI, if today`s volume is less than yesterday`s, don`t add 
// anything; if today`s volume is greater, then add today`s price rate of change. 
// For NVI, add today`s price rate of change only if today`s volume is less than 
// yesterday`s. 
////////////////////////////////////////////////////////////
study(title="Negative Volume Index", shorttitle="Negative Volume Index")
EMA_Len = input(255, minval=1)
xROC = roc(close, 1)
nRes = iff(volume < volume[1], nz(nRes[1], 0) + xROC, nz(nRes[1], 0))
nResEMA = ema(nRes, EMA_Len)
plot(nRes, color=red, title="NVI")
plot(nResEMA, color=blue, title="EMA")