We get this question a lot: what's the difference between the preliminary numbers you post ~11:30a ET every day and the final numbers that are usually posted ~5 hours later? Why are their differences in the ratings between the preliminary and final ratings and why do some shows get adjusted when others don't?
The preliminary ratings we post (the "Nielsen Fast National Ratings" in Nielsen jargon) just measure whatever aired on local affiliates in the time period. Most of the time that works pretty well. But there are a few fairly common situations that increase the likelihood of adjustments to the preliminary Fast National Ratings in the National Final Ratings:
1. Live events: the Fast National ratings only measure what was on in the local time period. So if something airs coast-to-coast at 8pm ET and 5pm PT (like a sporting event) the 5pm viewing on the west coast isn’t included in the fast nationals. It is added to the final program ratings though.
2. Local preemptions: the fast nationals measure whatever aired in the time period. So if NBA basketball or MLB baseball aired on the CW affiliate in Chicago, the basketball or baseball viewing *is* included in the fast national ratings but it is stripped out in the finals. When this happens, the numbers in the preliminary (Fast National) ratings are often inflated, and then appropriately "deflated" in the National Finals.
3. Overruns: the fast nationals are time period measures only, so if Modern Family runs from 9:00-9:31, in the fast nationals, the extra minute goes into the show airing at 9:30p. In the final national ratings that minute gets stripped out of whatever aired at 9:30 and is added into Modern Family where it belongs.
It's major local preemptions that usually cause the most false hope and confusion. To our consternation, despite posting red, bold-text, italicized disclaimers at the top of the preliminary (Fast National) ratings noting major preemptions and that there is a greater likelihood of adjustments, many fans completely ignore the warning.
So even when we disclaim that the Thursday Night Football game aired on the ABC affiliate in the Philadelphia market and to take the preliminary numbers with a grain of salt, there are still plenty of "Woo hoo, Scandal at season highs, baby!" comments on the preliminary post only to have Scandal adjusted down three tenths, to just below season highs with adults 18-49 in the finals.