Live-plus-same-day. Live-plus-three. Live-plus-seven. Industry insiders and fans alike are drowning in the deluge of TV ratings data that pours in every day now that measuring TV viewing has become a game of three-dimensional chess. But there’s one batch of numbers that never get much media attention, despite being more important than any of the above to the financial health of ad-supported networks.

Most TV ad deals are based on Nielsen’s commercial ratings for the 18-49 demographic—how many 18-49ers watched the ads that aired during a show, whether they watched them live or in the three or seven days after. These are C3 and C7 ratings, and they’re far closer to live ratings than networks probably want you to know.

Commercial ratings (hence the “C”) measure the average viewership of all of the commercials that air within a program. While the bread-and-butter of the TV ad marketplace is C3, an increasing number of deals are being done based on C7 ratings, though C7 ratings rarely differ more than a tenth or two of a point from C3.

Many shows routinely see lifts of 50% or more once all viewing within three or seven days — whether on a TV screen or on one of the digital platforms counted by Nielsen — is accounted for.

But C3 gains are far more modest.

An example: ABC’s new Kiefer Sutherland drama “Designated Survivor” had a respectable L+SD start, premiering to a 2.2 demo rating. That increased to a 3.7 after three extra days of viewing. But the shows C3 rating was a 2.7.

C3 data isn’t something networks tout, and it takes longer than L+3 and L+7 data for ratings provider Nielsen to calculate. But Variety has obtained the first batch of Nielsen’s C3 data for the 2016-17 TV season, which covers premiere week (Sept. 19-25).

CBS averaged a 2.1 C3 rating in the 18-49 demographic over that week, excluding its “Thursday Night Football” telecast. With the “TNF” game, it logged a 2.4.

The interesting thing about CBS is that it doesn’t see the needle move much from L+SD to C3. For instance: “The Big Bang Theory” had an initial L+SD rating of 3.8 for its premiere on Sept. 19. With an additional three days of viewing, that number jumped to a 5.4. And yet its C3 rating was a 3.8, indicating that none of those latecomers watched the ads.

The biggest C3 surprise for CBS is the three-tenths of a point freshman series “Macgyver” tacked on to its rating, ending up with a 2.0—not bad for a Friday night show.

NBC is the best-off of the big four broadcasters. Without its “Sunday Night Football” game that week, it came in with a 2.2 C3 demo rating. With “SNF,” it’s at a 2.6.

NBC’s scripted shows see a bit more of a C3 lift than CBS, but the increases are more along the lines of the rich getting richer, i.e. bona fide hit “This Is Us” tacking on an extra four tenths of a point and ending up with a 3.2 for its premiere, or “Law & Order: SVU” doing the same for a 2.2 in C3.

ABC is in a bit of a pickle, “Designated Survivor” aside—its C3 average for premiere week was a 1.7, a 26% drop from the comparable week in 2015 (2.3).

How to Get Away With Murder,” a former bright spot, has fallen precipitously: Last year, the Shondaland soap premiered to a 3.1 C3 rating. This year, it bowed to a 1.9 in C3, a gain of half a point on its same-day number.

But Fox is perhaps in the roughest place right now with C3, averaging a 1.6 over premiere week. That’s down 33% from last year’s 2.4.

Fox’s most-touted newcomer, “Pitch,” only added three-tenths of a ratings point to its premiere, bringing its C3 rating up to a 1.4.

Last year, Fox’s “Scream Queens” saw healthy gains in C3, going from a 1.7 to a 2.1. This year’s premiere only added two-tenths of a point, going from a 1.0 to a 1.2.

On the other end of the scale, Fox’s “Empire” is also facing declines. A year ago, “Empire’s” sophomore season opened to a whopping 7.9 C3 rating. Season three started off with a 4.8, a decline of almost 40%.

That said, “Empire’s” 4.8 is nothing to sneeze at. The show remains well ahead of the next highest scripted series, “The Big Bang Theory.”

The declines in the C3 numbers do not necessarily mean the networks are having to fork over extra commercial time to make up for a shortfall in ratings deliveries, aka “makegoods,” at least not yet. The stakeholders in the industry are well aware of year-over-year viewership declines, and ratings guarantees made by the networks have been adjusted accordingly.

Still, when it comes to assessing the performance of new and returning series, including C3 ratings in the conversation is a must.