Rating velocity (Δrating/day)

Apple does not give install counts to third parties. But rating counts can be measured every day, and because the rate at which people leave ratings is relatively stable, the daily increase moves in proportion to install volume.

A rival's Δrating/day rising usually means a metadata change or a campaign worked - that is where to look.

It needs two snapshots. It cannot be computed from a single day's data, and in that case rankcusp shows nothing rather than zero.