Gianfranco Zola's troubled reign as Birmingham manager continued with defeat to QPR on Saturday, meaning he has taken just six points from his first 12 games in charge. Blues were outside the play-off places on goal difference alone when they controversially sacked Gary Rowett on December 14. They immediately appointed Zola and have since slipped to 14th in the Championship. Rowett's average of 1.62 points per game this season was more than triple that of Zola (0.5 PPG) and though Birmingham are still 10 points clear of the relegation zone, their trajectory is alarming.Here, we assess the impact of this season's managerial changes in the Premier League and Championship to see how Birmingham's compares.Premier LeagueOnly three top-flight clubs - Swansea (twice), Crystal Palace and Hull - have changed managers this season.Swansea took 0.57 PPG under Francesco Guidolin and 0.73 under his replacement Bob Bradley, the pair leaving them 17th and 19th respectively in the table. They are now 15th after winning four of their first six league games under Paul Clement (2.00 PPG).Palace have declined from 0.88 PPG under Alan Pardew this season to 0.50 under Sam Allardyce, whose fortunes have not improved since losing his England job after one match. The Eagles have dropped from 17th to 19th after taking four points in eight games since his appointment.Mike Phelan was August's Premier League manager of the month but the honeymoon quickly ended and he left Hull bottom of the table with 13 points from 20 games (0.65 PPG). Marco Silva has started with seven in five (1.40) despite facing Chelsea, Manchester United, Liverpool and Arsenal in succession.ChampionshipNine clubs - Aston Villa, Cardiff, Derby, Rotherham (twice), Wolves, Wigan, QPR, Birmingham and Nottingham Forest - have changed managers.Seven of the 10 new managers have improved their teams' points-per-game records, the exceptions being Zola, Kenny Jackett at Rotherham and QPR's Ian Holloway.Two managers, Cardiff's Neil Warnock and Derby's Steve McClaren, have more than doubled their predecessors' points-per-game totals. Warnock has 1.68 PPG (37 from 22) to Paul Trollope's 0.73 while McClaren has 1.85 (37 from 20) to Nigel Pearson's 0.91.Rotherham's three managers have each collected less than a point per game. They took six points from their first 13 games (0.46 PPG) under Alan Stubbs, one from five (0.20) in Jackett's brief reign and 10 from 15 so far (0.67) under Paul Warne. Wigan have taken 0.94 PPG since Warren Joyce took charge, compared to 0.79 this season under Gary Caldwell.QPR's two managers this season have overseen exactly the same number of league games, 16 apiece. Jimmy Floyd Hasselbaink collected 20 points (1.25 PPG) before being sacked in November, with Holloway so far managing 17 (1.06).Roberto Di Matteo drew seven of his 11 league games in charge of Aston Villa, who have improved from 0.91 PPG to 1.30 since turning to Steve Bruce.The smallest impact can be seen at Wolves, who took 16 points in 14 games under Walter Zenga and 18 in the first 15 games of Paul Lambert's reign. Caretaker Rob Edwards oversaw a draw and a defeat in between times.