This indicator was developed by Joe Granville and aims to measure buying and selling pressure, and hence to confirm price trends and identify when a divergence may occur. Granville believed that a change in volume would lead to a change in price, rather than some theorists who believe that the price change will lead to the volume change, hence if one analyses the volume movements they can predict future price trends. The indicator does this by looking at the movements in volume - adding volume on up days and subtracting on a down day. As a result it analyses positive and negative movements in volume.
