The following is a message that I just posted to a discussion group. Someone was wondering if Letterman's higher-than-usual ratings the other night with Bill Clinton would have any impact on his numbers, the way the infamous Hugh Grant appearance did with Mr. Leno, and whether Dave took viewers from Nightline. In response, I wrote…
The Hugh Grant interview actually did very little for Leno. Jay's ratings were slowly gaining before it and Dave's were slowly declining. If the Hugh Grant thing had never happened, Jay would have passed Dave a few weeks later and from there on, things would have been exactly the same. Grant just sped up the process a bit.
The Clinton interview didn't get that high a rating. In the last few years, Letterman has had shows that did much better. For the most part, "event" episodes (ones where something very special occurs that triggers a bigger-than-usual tune-in) have little long-lasting impact on the ratings. As I recall, the night Dave had Hillary Clinton on, the show got a huge rating and, the next night, the numbers were right where they were before it. Sometimes, an event bolsters the numbers for a week or two.
I suspect that what's now at work in the late night competition has very little to do with the shows taking viewers away from one another, at least not on a steady basis. There's a certain segment of the population that will flit from Jay to Dave to Ted, based on which show has the best guest or topic, or the mood of the viewer — i.e., whether they're in the mood for something serious or silly.
For the most part though, viewers have all sampled the late night shows and decided which one(s) they prefer. The ratings go up and down, depending on how many of a given show's fans tune in on a given night and — perhaps as importantly — how much of the show they watch. That is, Dave has a certain audience that, when they watch late night TV, likes to watch him. On a great night, he'll get most of them to tune in and watch most of the show. On a bad night, a lesser percentage of them will tune in and a lot will change the channel or go to bed after the Top Ten List. One of the main factors in Leno's rise in the ratings was not that he was getting more viewers to tune in but that they were sticking around longer.
So my guess is that Letterman's higher-than-usual rating that evening was mostly a matter of a higher percentage of Dave fans than usual tuning in, plus a few folks who ordinarily watch other shows. But the major factor probably was that, of folks who tuned in to watch the beginning of the show, a higher-than-usual percentage of them stayed with it until the end, or at least until the musical act. (Both Jay and Dave lose a large, noticeable chunk of their audience when the guest band starts playing.)
I would have added, if I'd thought of it before sending this message, that while a lot of numeric analysis goes into the ratings, there's also a lot of guesswork. It's what I once heard someone describe as a "sorta-exact science," meaning that there's room for speculation. For all we know, people watch whichever late night host has the loudest tie.