Public Interactions Per Post per 1000 followers is a measure of normalized public interactions. This metric, available across all channels with follower, fan, or subscriber data, gives you an apples to apples comparison between brands’ performance on a given channel by removing the effect a brand’s audience size has on it’s ability to receive engagement.
For example, McDonald's currently has 73 million likes on their Facebook page. In contrast, Five Guys has just over 1 million. If we were to graph based on total public interactions per post, McDonald's would beat Five Guys every single time - their audience is just that much larger. However, if we want a better understanding of the relative success of our Facebook pages, we should graph based on Public Interactions per Post per 1,000 followers, because that allows me to answer the question, "which Facebook page is receiving more interactions, assuming both pages have the same number of followers?"
This metric is calculated using the following formula:
(Number of public interactions x 1,000) / (Total posts x Number of followers)
Divide your total followers by 1,000, and multiply the result by the Public Interactions per Post, which would be Total Public Interactions divided by Total Posts.