The post interaction shows how active the fans engage with posts of a page. It shows the average amount of all interactions (reactions (likes, love, wow, haha etc.), comments and shares) per fan per post. As opposed to the Engagement Rate, the Post Interaction ignores days without any posts.
Page A yields the following interaction values:
Monday: 20 interactions and 3500 fans and 2 posts
Tuesday: 0 interactions and 3590 fans and 0 posts
Wednesday: 37 interactions and 3700 fans and 1 post
Thursday: 100 interactions and 3750 fans and 3 posts
Friday: 0 interactions and 3755 fans and 1 post
Saturday: 0 interactions and 3780 fans and 0 posts
Sunday: 2 interactions and 3783 fans and 3 posts
Now we sum up all interactions by fans for every post and divide it by the amount of posts. ( (20/3500) + (37/3700) + (100/3750) + (2/3783) ) / (2+1+3+1+3) = 4.29% Total Interaction Rate / 10 Posts = 0.429% That means, in this week each fan interacted 0.00429 times with each post of the page.