If you want to know who is winning the evening news ratings race, don't look for a straight answer from NBC or ABC News.
NBC wrote in a press release: “NBC Nightly News” was America’s most-watched evening newscast for the week of March 16-20, 2015, according to Nielsen Media Research data. The NBC broadcast was #1 in total viewers and the younger demo A18-49, and came within a margin of 6,000 viewers A25-54.
In their press release, ABC wrote:
ABC News’ “World News Tonight with David Muir” ranked No. 1 in Adults 25-54 (1.941 million) during the week of March 16, 2015, based on Live +Same Day Data from Nielsen Media Research. Winning back-to-back weeks in the key adult news demo, “World News Tonight” beat “NBC Nightly News” (1.935 million) by 6,000 news demo viewers to post its 12th Adults 25-54 victory of the season.
ABC then added this:
NOTE: *NBC’s stated averages now include both its 6:30 p.m. E.T. airings and re-broadcasts on some stations between the hours of 2-4 a.m. E.T. NBC News has not publicly stated the markets in which the re-airs occur nor how many additional viewers they are claiming as a result.
It's all about the spin.