I'll go out on a limb with regard to (TV) ratings, citing only the side I know quite well, the RADIO side of Nielsen, which bought the radio ratings business known as Arbitron. There have to be many similarities in the business model, especially considering the purchase/merger of the two.
Posts thus far have, in my opinion, NAILED the flaws that exists in TV and in RADIO ratings! Now, from the "other" side of the MASS media, here's how SLANTED ratings really are in radio.
In RADIO, you could be a regular listener to us at WION on the web, or over the air.
First of all, there may be, say, ONE thousand diaries sent out for the area including my county. 8 HUNDRED go into the Grand Rapids metro area, (which we don't reach very strongly except online.) TWO hundred may get to 4 other surrounding counties, at 50 per county. FIVE might make it to Ionia City, and ONE gets filled-out but not in Ionia, but by a listener who drives OUT of our area to go to work each day, nearer to Lansing or a county to our North or South. As you can see, the farther from the larger metro area, the less accurate the sample where small market RADIO is concerned.
THEN....if my station doesn't BUY the results, literally paying around 3 thousand dollars, we don't get to SEE the results, cannot USE the results publicly without being sued (even if leaked to us by agencies who would then buy)....AND...if we don't BUY the ratings, online listenership is thrown out of the mix! You could listen 24/7 to us locally or at any distance, but unless we're subscribing, online listenership is not counted, and we're not allowed to report to Nielsen (who bought Aribitron's system) that we simulcast 24/7 on the web which now IS a factor in daily listening habits! Basically, it's a racket. "Pay to play"....and with horribly skewed distribution of potential samples, let alone the unreliable return of samples (surveys.)
That having been said, if you like a local TV station, especially if you have a remaining independent one, (there's few like that left now) and assuming the same general conditions apply on the TV side of ratings (to which I cannot speak with any credibility) it's important that you DO turn in the survey. Granted, it's a pain...but, short of electronically tracking your viewing (which, to a degree, would have people screaming "privacy invasion") it's the second of two ways to support local terrestrial broadcasting. The FIRST of course, in TV and in my medium of radio is to shop with advertisers, and tell them you KNOW they're on the local TV or Radio station.
In the end it's an interpretation "game'....and one has to remember the old phrase, "Figures Lie, and Liars (can) Figure!"
PS...in smaller (radio) markets, most advertising clients don't ask "What are your ratings"...they simply get RESULTS from well done advertising and KNOW that radio works, and when it does, they renew contracts and stay "on the air" with their chosen local stations. Again, TV is probably similar. If I'm wrong and we have some TV execs reading, please chime in....but the COMPANY doing the surveys is now the same, and I know from 11 years of ownership how the radio side works. There have to be SOME similarities. And....many shortcomings.
By the way, in 11 years here, NO TV ratings packets have come to me at this address, nor my former home for more than 20 years! (both are small towns.) If one came, I'd find it VERY interesting!