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