This thing called Audience Appreciation Index. Not sure how it's counted along with viewer count. 87/84/84/85/85/85/84 points in episode order seems at glance the lowest since Series 2 and visibly lower than the series in between. Why now?
The same place the audience is going? Every episode from 'The Snowman' onwards has had a lower final figure than the one before and Series 7 now has the lowest average ratings since the series returned.
Some of it's probably just natural for any show after a while and the novelty wears off. Everyone will have their own answers for this which are probably subjective but I think the production on 7B has been a bit sloppy and that they may have hit the point of diminishing returns on mystery arcs.
Wikipedia says 85 or over is considered 'excellent' so I would not be worried just yet. Seemingly the highest Who got was 91 for The Stolen Earth/Journey's End while Love and Monsters got 76. The results for this series seem to be halfway between those two.
Both this and Series 2 eschew doom&gloom and have the relationship between the leads some kind of flirtatious. Good change of pace and should help with longevity, but I'm sure they'll go back to at least the doom&gloom next series. I'm hoping for something like a less cluttered, more planned out Series 6.
I thought the ratings have been going down since S5, though I think the way they counted ratings changed which gave it a boost in S6.
This 2nd half of the season has seen the ratings stuck at 6 million area when finals are in HOWEVER the share of the audience (which the BBC cares about more) remains around 30%. I've not seen the I-Player numbers yet, which is odd as it normally adds a fair amount onto the ratings. Worldwide, Doctor Who has never been more popular.
They're pretty meaningless since they can't tell how many people watched, if it was the first time they watched or if they watched the whole thing, Hell, I watched last week's episode on iPlayer and it took three times to get it to play properly, which will have been counted as three views of the episode.
No they didn't change how ratings were calculated - the BBC introduced a new way of looking at their programmes (live+7) but the standard way remained in place and those are the numbers that are used for comparisons across years.
Well the overnight numbers for The Name of the Doctor were good http://www.sfx.co.uk/2013/05/19/doctor-who-the-name-of-the-doctor-overnight-ratings/