Networks Compared to Previous Seasons
There were two huge winners on an A18-49+ basis this season. The first one is ABC, which won the season in originals for the first time in the 14-year era. It was ABC's best relative original average since the peak Grey's Anatomy season in 2006-07 and second-best of the whole era. This was a particularly impressive show of consistent strength since the network didn't have a single megahit, didn't really have any sports inflation, and aired one of the heaviest volumes of original programming in history (more on that at the bottom).
Still, it should be noted that this win came in one of the most tightly-packed seasons ever, even moreso than the already tight 2013-14 season. ABC's 103 was the smallest relative rating for a #1 network in the era, finishing comfortably behind CBS 2013-14 and CBS 2004-05 (each at 106). Fox's 94 made it the second-strongest #4 network in the era, behind only 2004-05 ABC (96).
The other huge winner was the CW, which rode massive newbie The Flash (86) to its best relative rating since the 41 in the opening year of its existence. It's definitely questionable whether this is actually the network's strongest season; the network had the same original percentage when it averaged 41 in 2006-07, and that was while it was still programming three hours on the very problematic Sunday. But I think the real takeaway here is not 2014-15 vs. 2006-07, but 2014-15 vs. 2011-12 and 2012-13. The fact that it has gone from 29 and 30 in those two years all the way up to 41 on a young-skewing network with basically the same original volume is pretty striking.
Networks Across the Season
NBC's highest average came at the very beginning of the season (week 2) when The Voice was at its strongest. They were still comfortably in the lead as of the end of the fall (week 14), but spiraled downward during The Voice's two months off. NBC bottomed out at 98 in week 22, the week before The Voice returned. They recovered from there but still finished in third place, very narrowly behind CBS (100.36 to 100.30).
ABC was above 100 at the end of all 35 weeks of the season, but they were closest to falling below the mark at the end of the TGIT hiatus (week 18) when the unrounded number was 100.14. This was also the time of year when CBS' average peaked at 102, as they also benefited from The Voice's hiatus and the usual January bounce for their regular programs.
Fox had its weakest fall since 2006-07, but thanks to Gotham, MasterChef Junior and the NFL-inflated cartoons it was still not nearly as weak as some of the autumn averages the network had in the mid-aughts. The network got its usual surge in the winter months, though the ever-declining American Idol played less of a role than usual. This time, a new megahit fueled much of the surge, and the network's peak at 97 lined up perfectly with the finale of Empire in week 26.
A Deeper Look at Networks
It was a very tight three-way race when just looking at original series, but NBC had a runaway win in the overall ratings race due to Sunday Night Football and the Super Bowl. Usually it takes a Super Bowl for CBS to have a larger overall average than original average, but Thursday Night Football helped CBS pull it off this time, and thus finish second overall. ABC and Fox both lacked major sports inflations, so their overall numbers significantly trailed their original-only ones.
No network in the era has won the original series race while finishing fourth in scripted originals, but NBC in the last couple years has been pretty close. Last year, NBC very nearly got past Fox in the scripted department, but NBC inched down from 83 to 81 this time while Fox rode Empire to a nice year-to-year increase (85 -> 93). This marks a full decade for NBC in the scripted cellar; they haven't beaten another big four network in scripted original average since 2004-05.
ABC's comedy average surged this year, almost back to its 119 zenith in 2011-12 when Modern Family was peaking and none of the shows aired on Friday. Also, NBC's comedy average went down again even as they continued airing fewer and fewer sitcoms. They still haven't quite gotten as dead comedy-wise as ABC during the mid-aughts, but they'd probably be right there if not for The Voice's Tuesday lead-in.
As noted earlier, CBS and NBC had virtually identical original averages. Most people would take CBS if asked "Which is the stronger network?" because CBS has much stronger and deeper scripted programming. In my view, The Voice is not a "cheat" per se, but one thing to consider is that it does loom larger in this comparison because CBS also airs a lot more originals. The Voice gets more credit because it's a bigger part of a 60% original slate than it would in a 70% one. Hitting league average while devoting much more time to originals is what really makes CBS the stronger network.
The real estate comparison doesn't fly so much when comparing ABC and CBS, though, as both networks were right at 70% original. These 70% numbers were the two highest original percentages for a single network in the 14-year A18-49+ era, suggesting there is some real movement toward the year-round scheduling and timeslot-sharing tendencies the nets have long been touting. So you can't say ABC ducked its way into the 103 average!
NBC was also up year-to-year in original volume, but largely because 2013-14 was an Olympic year. (The network was at 59% in 2012-13 and 56% last year.) Fox was the only network that truly bucked the trend, going down in originals (64% -> 61%) and up in reruns (21% -> 24%).
Here's the now updated A18-49+ Networks post.
Here's the now updated A18-49+ Deeper Networks post.