Season 15 commences on the 3rd of September 1977 (21 weeks after Season 14 finshes on the 2nd of April), and begins its first story - "Horror of Fang Rock" - with an unusually low figure as 6.8 million viewers tune in.
This climbs to 7.1 by week 2, but then jumps to 9.8 million for the next fortnight. The average for the rest of the season seems to be around the 8 milion mark, with the occasional dip down to 7.5 or peak at 9 million. In most respects it seems to resemble the viewing figures for Seasons 8 and 9.
Fan wisdom would place the blame at the feet of poor stories, or Leela not being Sarah Jane. But fan wisdom will also point to the introduction of the very popular K9, and the return of the Time Lords and Sontarans as highlights of the season, as usual attempting to have their cake and eat it (although they could have a point there, as the ratings do climb to around 11 million at the start of the Time Lord 6-parter "The Invasion of Time").
However once more it does help to remember that casual non-fan viewers can leave for reasons that are not down to the show itself. Perhaps ITV, ever with an eye on the main chance, had successfully scheduled something equally popular against Doctor Who?
When compared to Season 5 and 6, Season 15 is far from a failure. However there is a tendency to compare it to Seasons 13 and 14 which were far more successful in terms of viewers. As in everything, it's all a matter of perspective. Season 15's ratings may be indicative of a decline in viewers, but they are still a healthy result.