The Negative Volume Index (NVI) is a technical indicator that measures changes in volume to identify trends in the stock market. It was developed by Norman Fosback in the 1970s.The NVI is based on the assumption that smart money (professional traders) trade on low volume days, while the general public trades on high volume days. When the smart money is buying on low volume, it is considered a bullish signal, and when they are selling on low volume, it is considered a bearish signal.To calculate the NVI, the closing price of a stock is compared to the previous day's closing price. If the volume is lower than the previous day, the NVI is increased by the amount of the closing price change. If the volume is higher, the NVI is not changed.The NVI is a momentum indicator and is used to identify bullish and bearish trends. A rising NVI indicates that the market is trending upward, while a falling NVI indicates a downtrend. The NVI can also be used to confirm other indicators such as moving averages, and can also be used to spot divergences.It is important to note that the NVI is a lagging indicator and it may not be appropriate for short-term trading. It is also important to use it in conjunction with other technical indicators and fundamental analysis to get a more complete picture of the market.