The Appreciation Index or AI is a measure of how much the audience enjoyed the programme. The score, out of a hundred, is compiled by a specially selected panel of around 5,000 people who go online and rate and comment on programmes.
The score of 84 is considered 'very good', and represents an above-average figure for the Capaldi era so far. It is up by one point on The Witch's Familiar, and equal to The Magician's Apprentice.
[Source: Doctor Who News]