By Stephen Dowling BBC News

Homer Simpson and family have notched up 350 episodes

The Simpsons is marking its 350th episode - placing it third in the league of long-running US sitcoms. So what is the secret of TV longevity?

TV critic Clive James once said of TV shows: "If you have the right format, it will stand up like an iron suit."

After 16 years and 350 episodes, animated comedy The Simpsons is still standing firm in the schedules.

And creator Matt Groening has said he does not "see any end in sight".

Writers are already working on the programme's 17th season - making it the longest-running TV sitcom in history - the show is perceived to be as much of a sitcom as a cartoon series.

'Know your audience'

In an age when TV shows appear and disappear almost overnight, longevity is something to which most programmes are not given the chance to aspire.

TV Quick writer Clair Woodward says the key to success is knowing what your audience wants and being able to deliver it.

"The Simpsons has the format of a family sitcom, and they have an amazing team of writers, which is what a comedy needs," she says.

The "classic" cast from Last of the Summer Wine: Compo, Foggy and Clegg

Another show which shares that factor is a radically different sitcom aimed at an altogether different audience - the idle-pensioners British comedy, BBC One's Last of the Summer Wine.

In its 26th season, it has been running since 1973.

Its Sunday teatime slot is perfect timing for its older audience, too, Ms Woodward says - another indication of a show understanding its market.

Some shows, though, run the risk of relying too long on the same formula - which can turn off the audience that once loved it, she adds.

The US comedies Friends and Frasier, she believes, were guilty of that.

No formula

"Friends was going off the boil by about series seven because people couldn't believe in the characters anymore," Ms Woodward says. "These people had perfect hair and perfect teeth and they couldn't get boyfriends or girlfriends."

Frasier lost the interest of viewers when Niles and Daphne got together, adds Sara Nathan, The Sun's TV editor.

The show had sustained itself with a will-they, won't-they sexual titillation that ran out of steam when they married.

What the Simpsons has that few other shows have, Ms Nathan says, is an ability to make fun of the president and popular culture - "things that people relate to".

But she does not believe that long-running shows have any definable formula, other than "good plots and characters people care about".

Tom Baker as Doctor Who: Regeneration kept the original show fresh

TV historian Dick Fiddy believes that the qualities which make for long-living TV are intangible.

"If we knew what they were, every show would last for 20 years," he says.

"Most of the shows which have lasted are unique cases. The Simpsons, for instance, is set in a world that never changes, it doesn't age.

"The actors aren't moving on or growing older."

Soaps, of course, are a byword for longevity in TV. ITV's Coronation Street marks its 45th year on screen this year.

'Classic formats'

Mr Fiddy believes soaps are a law unto themselves because they become "habit forming. They are like doing a crossword in the morning."

Soaps become so ingrained in people's TV-watching schedules, he says, that "they have to go pretty poorly to lose audiences and run the risk of closure.

Science fiction shows have the asset of being set in unfamiliar surroundings - and offer the option of developing their format.

The 1960s US series Star Trek morphed into Next Generation and Deep Space 9, different series united by the same concept.

Star Trek revamped itself, changing characters and settings

"And the thing with Doctor Who is the sheer good fortune of regeneration, which means [every now and again] it gets a refreshing jolt in the arm," adds Mr Fiddy.

Mr Fiddy also cites long-running factual shows such as What The Papers Say (1956) and The Sky At Night (1957) as "classic formats" that have stood the test of time because they have a simple concept.

Then again, there are those fondly remembered shows that are on our screens for the blink of an eye.

BBC Two's The Office only ran for two seasons and a Christmas special. Twenty-something drama This Life also only ran for two seasons. And Fawlty Towers? Much repeated it may be, but it ran for only 12 episodes.