Definition
WordPress.org does not publish exact active-install counts. It shows a rounded tier, and the tier changes only when a plugin's installs cross one of its thresholds. This measure counts the plugins in the top 12,500 whose tier was higher at the end of the report month than at the end of the month before, and the plugins whose tier was lower. It names the largest plugins in each group with the tier they left and the tier they reached.
Population and window
Rivale measures it across the plugins in the top 12,500 at the end of the report month, reading each plugin's active install tier at the end of the previous month and at the end of the report month.
How it is computed
WordPress.org publishes active installs as rounded tiers (for example 90,000, 100,000, 200,000, 1,000,000). Rivale compares the tier shown at the end of the previous month with the tier shown at the end of the report month for every plugin in the top 12,500, counts those that moved up and those that moved down, and names the largest of each.
How to read it
Moving up a tier means the plugin crossed a WordPress.org threshold, not that it gained a fixed number of installs. Tiers widen as counts grow, so a move from 3,000,000 to 4,000,000 covers far more installs than a move from 900 to 1,000. Moving down means the count fell below a threshold. Read the named plugins with their tiers, not as a ranking of growth.
What it is not
This measure does not give exact install counts, does not measure downloads, and does not say why a count changed. A plugin that gained installs without crossing a threshold shows no move.