For better or worse — and there are good arguments both ways — the DVR has forever changed the primetime economic model.

Americans are watching more primetime television than ever thanks to the time-shifting gizmos — but not necessarily live, or even the same night. And the networks reap ad revenue only from commercials viewed within three days of a telecast.

As 2010 comes to a close, the networks are confronted with some sobering statistics detailing just how much time-shifting is taking place.

An analysis of the fall season by Brad Adgate, senior VP of research for Horizon Media, found that a whopping 19 primetime broadcast series now derive more than 30% of their audience from same-week DVR playback, up from 13 in 2009 — and none in 2008. National penetration of DVR homes stood at 29% at this time two years ago, before rising to 34% in the fourth quarter of 2009 and to 38% at present.

There are signs that increases in DVR playback may be slowing, though, as roughly 70% of households that have digital video recorders have only one — a figure that has held steady throughout 2010. Only 5% of homes have three or more DVRs, though this number could grow as more cable and satellite providers introduce multiroom recorders.

The Nielsen Co. last week released some data that seemed designed to refute the notion that the value of the traditional television advertising spot has been dramatically diminished by DVRs.

While acknowledging that many viewers opt to bypass commercials in playback mode, Nielsen pointed out that among DVR homes, three-day playback lifts commercial ratings by 44% among adults 18-49. Among all adults 18-49, DVR playback — which peaks in the 9-11 p.m. hours — adds 16% to commercial ratings after three days.

Looking at this fall’s programs, NBC’s “The Office” was the most time-shifted program of the season on a percentage basis, with 38.8% of its overall “live plus 7-day” audience opting to watch either delayed on Thursday or within seven days of its premiere telecast. This marks the third straight fall that the comedy, which has consistently faced tough timeslot competish from the likes of “Grey’s Anatomy” and “CSI,” has garnered the least percentage of live viewing of any series.

Among other findings from the Horizon analysis:

• ABC’s “Modern Family” had the highest number of time-shifted viewers in the fall of 2010 with 5.47 million; the network’s “Grey’s Anatomy” (5.07 million) is the only other show to top 5 million. Four other shows — Fox’s “Glee” and “House” and CBS’ “The Big Bang Theory” and “NCIS” — add 4 million or more viewers in same-week DVR playback.

• The time-shift growth for “Big Bang Theory” is particularly impressive, as it is an example of a blazing hot show that came to peak during the three-year period of the Horizon study. In 2008, DVR playback boosted the show by 1.3 million viewers (or 13.1%), and in 2009 it picked up 3.22 million in time-shifting (21.9%); and this year, as it changed timeslots from Monday at 9:30 p.m. to Thursday at 8 p.m., DVR viewers add on 4.69 million viewers, or a 30.6% gain.

• Not surprisingly, “Modern Family” and “Glee” — the hottest new shows of the past couple of seasons — are also the sophomore series highest on DVR playback lists. “Modern Family” has doubled its time-shifted audience from 2.61 million to 5.47 million, while “Glee” has mushroomed from 2.97 million to 4.80 million.

Other sophomores up nicely are CBS’ “The Good Wife” (2.05 million to 3.11 million) and ABC comedy “The Middle” (1.14 million to 2.14 million).

• Among first-year shows, NBC’s “The Event” had the highest percentage of time-shifted viewers (34%), likely due to its serialized storyline and tough timeslot opposite must-watch-live shows including “Monday Night Football” and “Dancing With the Stars.” Another Peacock show, the Thursday comedy “Outsourced,” ranked second (29%).

• As expected, the median age for virtually every program drops when time-shifting is factored in — including 10 series that have a median age three years or more younger in DVR playback vs. live.

The biggest differences are seen in a pair of 8 p.m. comedies — NBC Thursday entry “Community” (38.6 in time-shifting vs. 43.5 for live only) and CBS Monday half-hour “How I Met Your Mother” (43.4 vs. 48.0) — reflecting how difficult it is to get an especially young live audience at the outset of primetime.

• Live sporting events like NBC’s “Sunday Night Football” (whose audience is 2.7% time-shifted, or 97.3% live), newsmags like CBS’ “60 Minutes” (5.9% time-shifted) and unscripted shows like Fox’s “Cops” (5.2% time-shifted) and ABC’s “America’s Funniest Home Videos” (6.2%) are the least likely programs to be played back later.

Reality competition series also rank relatively low, especially ABC’s “Dancing With the Stars” (12.2% of its audience is time-shifted), though CBS’ “Survivor” (28.8% time-shifted) and Fox’s “Hell’s Kitchen” (28.0%) derive a good chunk of their viewership via the DVR.