By Kevin Young
Entertainment reporter, BBC News
The way in which TV viewers decide which shows to watch, and when they watch them, is changing radically.
Personal video recorders (PVRs) let audiences create their own schedules, tailored to their mood.
PVRs let users create their own schedule at the touch of a button
Cynics might argue this is nothing new - after all, people have struggled to clear piles of videotapes lying around their living rooms for the past 25 years.
But there is something more ruthless about having a list of shows ready to go at a moment's notice.
The concept, dubbed "timeshifting", applies whether viewers record a show or if they see it on a separate "+1" service, where the entire output of a channel is shown again with a 60-minute delay, to maximise viewing opportunities.
"We believe timeshifting will become a more important part of the way people view television," says Bjarne Thelin, chief executive of UK ratings measurement company Barb.
"They can catalogue recordings to access the stuff they've recorded more easily."
And this counts for one of the selling points of PVRs, he says - the ability to pause a live programme if the telephone rings or to deal with another distraction.
"You might only be offsetting something by 30 seconds or a minute, but that effectively is timeshifting."
Research also suggests that people who have access to PVRs watch more TV.
Barb says that fewer than 2% of all TV programmes in the UK are timeshifted - but BSkyB suggests this rises to 12% in homes with Sky+ boxes.
Internal research by E4 found one episode of its teenage drama Skins was seen by 900,000 people when screened on a Thursday evening in February.
Drama is by far the most commonly recorded genre in Sky+ homes
But the figure grew by 305,000, or 34%, when viewing from recordings, repeats and the E4+1 service were added.
A spokesman said the show's younger audience was likely to be comfortable with the idea of finding Skins in the schedule at a time convenient for them, or recording it.
And broadcasters are keen to ensure they do not "lose" figures for non-live viewing.
From Thursday, Nielsen - the US equivalent of Barb - will begin releasing viewing figures for the 24, 48 and 72 hours following transmission, taking recordings into account.
The third of these measures - dubbed "live plus three" - has gained particular attention among networks such as ABC and CBS, with executives indicating recently that they believed this would become more of a standard assessment of true viewing figures.
Nielsen is also to start measuring ratings for commercials, something which has been done in the UK since 1981.
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"Up until the advent of the PVR, the programme rating was a pretty good surrogate for how many people were watching your advertisement," says company spokesman Gary Holmes.
"Now there's the prospect of people fast-forwarding through the commercials."
He says that networks are "very interested in making sure that they get credit for the extra viewing that takes place on PVRs", while advertisers are "not interested in paying for adverts that were fast-forwarded through".
Differences in the way UK ratings are gathered are also likely in future, according to Chris Mundy, the BBC's head of audiences.
"What we haven't spent huge amounts of time doing is analysing how many people watch programmes one day, two days, three days after transmission.
"This is going to change massively when on-demand really comes on stream, and the iPlayer launches in a couple of months' time," he says, referring to the online catch-up service which will "store" seven days of BBC output.
BSkyB describes its Sky+ box - now in a quarter of its 8.5m households - as "the iPod of TV" and has decided to drop the service's £10 monthly subscription to broaden its appeal.
Other PVR boxes are available which do not require a regular fee, such as those which work on the Freeview digital terrestrial platform.
Internal BSkyB research, from a viewing panel of 33,000 customers, suggests that drama is by far the most timeshifted genre, accounting for 40% of recordings.
News and sport, dominated by live events, sit at the bottom of the list.
Ratings for E4's Skins benefited hugely from timeshifted viewing
But while timeshifting is becoming more common, some within the industry caution that the overwhelming majority of viewing still involves live broadcasts.
There may be a hard core of timeshifting fanatics, but Barb's figures mean 98% of viewing is still "live".
Broadcaster ITV said that in terms of setting advertising rates, the "currency" of the UK market had been based on consolidated seven-day viewing figures compiled by Barb "for as long as we can remember" and was unlikely to change.
And perhaps advertisers can be reassured by BSkyB data suggesting that 40% of people with Sky+ boxes still sit through commercial breaks.
This suggests that even with the option to fast-forward, millions of viewers like watching TV just the way it always has been.