Manchester - Australia retained the Ashes after a 185-run win over England in the fourth Test at Old Trafford on Sunday left them 2-1 up with one to play in the five-match series.

Here is a look at the key reasons why Australia are keeping hold of the Ashes:

1. Steve Smith's runs

This series is destined to be known as 'Smith's Ashes' as surely as the 1981 campaign became synonymous with England great Ian Botham.

Star batsman Smith's series figures speak for themselves - 671 runs at 134.2 including three hundreds, with a highest score of 211 at Old Trafford.

And yet they don't tell the whole story.

This is Smith's first Test series since he completed a 12-month ban for his role in a ball-tampering scandal in South Africa that cost him the Australia captaincy.

But the 30-year-old's first Test innings since suspension saw him score 144 at Edgbaston, a hundred that helped Australia recover from 122-8 - and he followed that up with 142 in the second innings of a match his side eventually won by 251 runs.

In a campaign when top-order batsmen on both sides have found runs hard to come by, Smith, now back on top of the Test batting rankings, has been operating on a different plane despite missing England's dramatic one-wicket win in the third Test at Headingley with a concussion suffered when struck by a Jofra Archer bouncer in the drawn second match at Lord's.

"I thought Virat Kohli was the best batsman I've ever seen because of the way he plays in all forms, but Steve Smith... That's another level," said Australia coach Justin Langer.

2. Anderson absence

Much of England's pre-Ashes planning revolved around having their attack led by all-time leading wicket-taker James Anderson.

But the fact he bowled just four overs in the series before breaking down with a calf injury at Edgbaston arguably cost them the chance to make the most of their early dominance in that match.

Stuart Broad and Jofra Archer have bowled well in his absence but England would have been a stronger side had Anderson been fit and firing.

3. Australia's fast bowling depth

Australia came to England determined to do everything they could to compensate for the possibility of having a fast bowler breaking down by including a quintet of pacemen in their squad whom they could rotate alongside off-spinner Nathan Lyon in a four-man attack.

It's a policy that has paid dividends with the outstanding Pat Cummins, Josh Hazlewood and Mitchell Starc - the trio deployed at Old Trafford - kept on their toes by Peter Siddle and James Pattinson.

4. Catches win matches

Australia have been better in the field this series, with Cameron Bancroft and Matthew Wade leading the way at short leg, while the first innings at Old Trafford saw Archer drop a caught and bowled chance when Smith had made 65 while Jack Leach's no-ball then also cost England an opportunity to get rid of the Australia star.

5. World Cup hangover

Playing a World Cup - which England won - and an Ashes in the same season with just a short gap between the two may have made commercial sense to English cricket chiefs but it left several of their senior batsmen with precious little time to get acclimatised to the very different demands of Test cricket.

The idea that Jason Roy would simply carry his one-day form into the Test arena always seemed a stretch, and so it proved, while England captain Joe Root - a key batsman in all formats - has made three ducks this Ashes.

In contrast to Roy, Australia's Marnus Labuschagne - the first player to make 1,000 first-class runs this English season while playing for county side Glamorgan - made four successive fifties after coming in as Smith's concussion substitute at Lord's.

Root did not want to make an issue of it, insisting "we are in the position we are".