Why have younger viewers switched off the TV this fall? The ratings are down. So down, in fact, that network execs have begun blaming Nielsen, the ratings company. Now Nielsen has produced a report attempting to explain the dropoff. “The report from Nielsen, which has stoutly defended its data since the complaints from the networks began, found that changes in Nielsen’s methodology that started as long as a year and a half ago are partly – but not wholly – to blame for the decline in viewership of network series among men ages 18 to 34.”