Effective Cost Per Action (eCPA)

eCPA stands for effective cost per action, which is a related term that’s used to measure the effectiveness of advertising inventory purchased (by the advertiser) via a cost per click, cost per impression, or cost per thousand basis.

In other words, the eCPA tells the advertiser what they would have paid if they had purchased the advertising inventory on a cost per action basis (instead of a cost per click, cost per impression, or cost per mille/thousand basis).
If the advertiser is purchasing inventory with a CPA target, instead of paying per action at a fixed rate, the goal of the effective CPA (eCPA) should always be below the maximum CPA. As described by Yang’s Law, eCPA<CPA.