Season 10 ends on the 23rd June 1973, and Season 11 begins on 15th December the same year. That's a season break of 25 weeks.
This is Jon Pertwee's final season, so there's a new Doctor on the horizon. But as we've seen before this is currently really only of concern to fandom. The average viewer isn't that bothered about a change of Doctor yet. This may seem at odds with what we see today, but in 1973 the series may have been on TV for 10 years, but it is not quite the British Institution that it will eventually become.
By this time viewers know that the Doctor has change twice before, and it's now an accepted part of Doctor Who. The fan conceit that viewers will switch off in their droves because a popular Doctor has left just doesn't seem born out by ratings. Of course if a new Doctor is unpopular, now that might make people tune out, but it doesn't appear to be a factor beforehand.
Again we must avoid the trap of blaming scripts or the new companion (who anyway is Sarah Jane Smith, widely acknowledged as the most popular of the classic era companions) for any fall in ratings, either across the board or for individual episodes. But this also means we shouldn't immediately attribute a rise in viewers to the same thing.
Although there's no cause to worry here. Season 11 performs well (if a bit erratically), starting with 8.7 million viewers, which then drop to between 6 and 7 for weeks 2 and 3, before climbing back to 10.6 in week 4. From then on the Season hits consistent highs of between 9 and 11 million, before strangely suffering a drop to around 7.5 million a week before the start of "Death to the Daleks". It's got Daleks in the story, and Daleks in the title, yet viewers seemed more interested in the first few episodes of the preceding Dinosaur story. The figures do climb back to 10 million by the middle of the story, though, so perhaps Dalekmania isn't completely over.Audiences for the rest of the season are reasonably consistent (though less so than previous seasons), and the lowest ratings can be found during the penultimate story "The Monster of Peladon", once more proving that returning favourite monsters (in this case the Ice Warriors) do not guarantee that more people will watch. Jon Pertwee's final bow, the fan-acknowledged over-the-top six-parter "Planet of the Spiders" sees respectable figures of between 8 and 10 million, though again it would appear that more viewers tuned in to see Dinosaurs than to wave farewell to the 3rd Doctor.
At the end of the day Season 11's viewing figures average out at just below Season 10's, but this still makes it a successful part of the early '70s revival in Doctor Who's fortunes, and with the 4th Doctor about to take over, will the series be able to retain these viewers?