The Engagement Index metric is a measure of how engaged your list is with your email messages. This index is calculated by factoring the Active and Inactive contacts with the Recency and Frequency of interactions, in consideration of the Opt-out Rate. You may use the resulting index to measure the relative engagement level of your lists.  When monitored over time, this index provides an indication of the success of your email program, and the overall effectiveness of your communications to drive interaction.