eCPA

Effective cost per action (eCPA) is total spend divided by the number of a chosen in-app event — for example registrations, deposits or purchases — regardless of how the media was priced.

eCPA is the clearest efficiency metric for campaigns with a downstream goal. It lets you compare sources, creative and markets on what the business cares about rather than on installs. Because events happen later than installs, eCPA needs a sensible reporting window and enough volume to be stable; very small numbers of events produce noisy eCPA that can lead to premature decisions. Pair it with ROAS when the action carries different values for different users.

As a rule of thumb, wait until each source or creative has accumulated a meaningful number of events before acting on its eCPA, and compare segments over the same date range so seasonality does not distort the picture.