X Factor is back: Who are its biggest hits - and misses?

• The most-watched X Factor launch show to date was in 2010. A total of 11.9 million viewers tuned in for the start of the seventh series.

Since then the audience numbers have slipped. Last year’s launch was watched by 8.3 million - the second lowest figure for eight years.

• The least-watched X Factor launch was the very first show on September 4, 2004. It was seen by 5.3 million viewers, and was the 21st most popular show on ITV that week.

It was beaten by, among others, Parkinson (6.2 million), Heartbeat (8.8 million) and Doc Martin (9.9 million).

• The X Factor launch show first topped ITV’s weekly ratings in 2007, with the first edition of the fourth series. It went on to top the weekly ratings every year until 2011.

In 2012 it slipped to third place, behind two episodes of Coronation Street. In 2013 it came second, and was back to first place in 2014.

• Leona Lewis remains the most successful X Factor winner to date, when measured by the number of weeks in the UK top 40 singles chart.

Lewis, who won in 2006, has spent 91 weeks in the top 40 so far, and has had 10 solo top 40 hits.

Little Mix, the winners in 2011, are the second most successful winning act. They have enjoyed 71 weeks in the top 40 so far, and had 10 top 40 hits.

In third place is 2008 winner Alexandra Burke. She has spent 64 weeks in the top 40 and had eight solo top 40 hits.

• Sam Bailey is the least successful X Factor winner in terms of the UK top 40.

The champion of the 2013 series has spent just four weeks in the top 40 so far, and had just one top 40 hit.

Other winning acts with modest success include Steve Brookstein, champion of the very first X Factor series in 2004, who has so far spent eight weeks in the UK top 40 and had one solo top 40 hit; and Leon Jackson, the winner in 2007, who has spent nine weeks in the top 40 and had two top 40 solo hits.

Ben Haenow, winner of last year’s series, has so far had just one top 40 hit and spent 10 weeks in the top 40.

• The most successful series to date in terms of producing top 40 hits are series five (2008) and seven (2010). Each series has been responsible for a total of 25 top 40 singles.

Series five: JLS (runners-up): 12, Alexandra Burke (winner): eight, Diana Vickers: three, Eoghan Quigg (third place): one, X Factor finalists (group): one.

Series seven: One Direction (third place): 14, Matt Cardle (winner): three, Rebecca Ferguson: (runner-up) three, Cher Lloyd: three, Aiden Grimshaw: one, X Factor finalists (group): one.

• The least successful series to date in terms of top 40 hits are series one (2004) and, perhaps not surprisingly, last year’s series. Series one has so far produced just two top 40 hits - one for winner Steve Brookstein and one for runner-up G4. Last year’s series also produced just two top 40 singles - one for winner Ben Haenow and one for Stereo Kicks.

• Simon Cowell is the most successful judge to date, having mentored three winning acts - Steve Brookstein (2004), Leona Lewis (2006) and Ben Haenow (2014).

Cheryl Fernandez-Versini has mentored two winners - Alexandra Burke (2008) and Joe McElderry (2009), as has Dannii Minogue - Leon Jackson (2007) and Matt Cardle (2010).

Tulisa Contostavlos, Sharon Osbourne, Nicole Scherzinger and Louis Walsh have each mentored one winner.

Gary Barlow, Kelly Rowland and Mel B are the only judges to have never mentored a winning act.