Dedicated followers of Soccernomics principles would tell you that a new manager can affect the team he inherits with no more than a 10% bounce.
Sometimes the eye deceives us. A new manager might win his first couple of games and be heralded a genius, but his side often reverts to the level of results which ultimately cost his predecessor his job.
But there are exceptions and few more obvious in the Premier League this season than the impact of Tony Pulis at Crystal Palace.
Palace looked dead and buried, and certainties for the drop. The bookmakers had them as short as 1/20 to be relegated before Pulis replaced his old friend Ian Holloway.
To say the impact was instant is not strictly true though; the process of change had already started under Keith Millen's steady stewardship. Palace had already looked more unified and become a much more solid proposition.
It's just that under Pulis they've progressed from a hard to beat team to a side capable of winning big matches.
To this extent when Manchester United visit Selhurst Park on Saturday night, Sky Bet's offer of 11/2 for the home side winning looks very generous indeed.
The new additions have helped too: there were dream debuts for Scott Dann plus goalscorers Joe Ledley and Tom Ince against West Brom a fortnight ago.
But Pulis is a master at maximising the talent he has at his disposal and creating the kind of team spirit essential to a squad fighting relegation.
The same qualities one would have associated with David Moyes at Everton, but they have been less evident so far in his relatively short time at the helm of the champions Manchester United.
Perhaps though - after so many disappointing results - there is now a realism at Old Trafford about the huge rebuilding task that lies ahead.
The question is how far can United fall? Already 23 points behind the pace they set last season and 11 behind fourth placed Liverpool, they are closer to the bottom half than they are to the leaders Chelsea.
But there is still talent in their team: in Wayne Rooney, Robin Van Persie, Juan Mata and Adnan Januzaj there is enough to beat anyone on their day. It's just that their day hasn't come around often enough this season.
The temptation must be to get those four players on the field together but at what cost to a side that showed enough defensive frailty to concede two goals at home to the bottom club Fulham two weeks ago.
Of course, United and Moyes could still end the season as champions of Europe. First, they must come through the challenge of Olympiakos in the last 16 and an awkward looking trip to Athens on Tuesday night.
It means their team selection for Saturday will be fascinating. But if United take their eye off Palace, Pulis and his team will be ready to pounce.