Negative Volume Index
is based on the assumption that active investors dominate trading on quiet days. And the related Positive Volume Index is based on the assumption that uninformed investors dominate trading on active days.
NVI is usually charted with a 255 day EMA representing 1 year. A NVI above it's 255 EMA could indicate a higher likelihood of a bull market.
1. start with a value of 100
2. for each day if volume < volume-1, NVI = NVI-1 + (((close - close-1) / close-1) * NVI-1), otherwise no change.