SII or Social Interaction Index is available for Facebook Pages and illustrates the intensity of engagement generated by fans and other people who interacted with the page. Unlike Engagement Rate, it takes into consideration not only comments, reactions and shares of posts published by the page, but also posts published by fans on this page, as well as interactions with those posts.
Here's the formula we use to calculate Social Interaction Index:
Each type of interaction is assigned with different weights:
likes, or more precisely - reactions, have a weight of 1, as they are the most common types of interactions,
comments have a weight of 3 - they require more effort and tend to trigger more interactions from other users as well,
shares have a weight of 9 since they highly influence the post reach and illustrates fan involvement,
fan posts published on a page have a weight of 0.1 as they do not necessarily bring higher engagement to a page itself nor increase page reach; however, SII also includes comments and reactions to such posts (in the parameters above).
The sum of interactions multiplied by their weight is divided by the element of the fourth degree from the number of fans. This allows you to directly compare engagement levels for pages with very different community sizes.
The formula might seem complex, but it is the outcome of various econometric analyses. We performed them to calculate the best formula that would allow you to directly compare engagement on pages of different sizes, factor in the quality of engagement (thus the weights for different actions) and account for situations when users interact by publishing posts on pages rather than comment on their posts.