Why is there a gap between APM and eAPM?

To understand why there can be differences in the gap sizes between APM and eAPM, let's break down each term.

APM, or Attention Per Mille, is the estimated attention generated per 1,000 visible impressions.

Now, eAPM, or effective Attention Per Mille, represents the estimated attention generated per 1,000 impressions that were actually purchased.

For instance, imagine 1,000 impressions were bought, but only 70% were actually seen. In this case, the eAPM would be 30% lower than the APM score.

Therefore, the less percentage of impressions that are viewable, the wider the gap between APM and eAPM gets.

I hope this brief explanation was helpful to you. If you wish to dive a bit deeper into the topic, we also have Adnami's Attention White Paper, which you can download by clicking on this link.