What is the PEI?
The Publisher Earnings Index (PEI) is an indexed measure of average total ad revenue in a given day. A measurement of 1 constitutes the highest total ad revenue seen in a single day.
What it means
The PEI is a great indicator of the state of the industry, and is a great way to monitor the health of total ad revenue and to see earnings trends materialize over time.
CPM vs. PEI
The PEI is not a measure of simple "CPM", but is instead a measure of total ad revenue (thus changes in CPM show up in a multiplicative way in total ad revenue).
The PEI is valuable to look at as an indicator of how total publisher ad revenue is performing compared to past periods, allowing you to compare performance today with a point in the past.