Blizzard introduced EPM as a new stat for watchers of Starcraft2 matches, in one of the last patches. I am a bit confused. What is the difference between EPM and APM? What does each include exactly in the count?
The notes for patch 1.4.3 state:
EPM is always smaller than APM since effective actions are a subset of all actions.
Time is measured in game time (aka Blizzard time), and it is not measured in real time.
As mentioned at a Team Liquid forum post, camera location commands (aka screen bookmarks) are not included in the calculations of APM and EPM.
Note that third party analysis tools often measure APM/EPM slightly differently to the Starcraft 2 software.
Currently (as of version 126.96.36.19929) there is a bug where EPM and APM values are reversed when viewing a replay. We should expect a fix sometime in a future patch.
EPM = effective actions per minute is every valid action you make. So moving your marine will count but pressing a control group won't (Starcraft 2 default).
APM = actions per minute counts every click you make, like pressing a control group or moving a marine (old Brood War style). It should always be greater than or equal to EPM.
Note that (as of April 2012) these metrics appear reversed in replays.
APM = Actions Per Minute
EAPM or EPM = Effective Actions Per Minute
For the APM calculation all your actions are accumulated, every click and every key press is counted as 1 action, and divided by the game duration. There are a few excretions to this rule, Shift and Ctrl are not counted as an action, but Shift + 1 and Ctrl + 1 are.
This means that APM measure your average control speed, in other words how fast you can click your buttons.
Blizzard introduced the EAPM to filter out some actions they think are unimportant. While the calculation is basically the same there is a filter that removes "redundant" actions before the accumulated value is divided by the game duration.
Here are some examples what Blizzard thinks is redundant:
The exact Blizzard calculation is unknown. But SC2Gears does almost the same thing and it is well documented here.