Introduction
On Tuesday, Wayne Rooney overtook Sir Bobby Charlton's total of 49 England goals, scoring his 50th from the penalty spot in a home win against Switzerland. There has been discussion about the relative merit of each player's goalscoring record, with argument to be made for each player,for example:
- Rooney plays regularly against teams such as San Marino, who are true International minnow.
- Charlton played during an era when goals were more plentiful.
This post attempts to adjust for the factors described above to compare the players on a level playing field, adjustment for opponents made by considering Elo ratings (originally designed for chess, these ratings have since been used across sports. FiveThirtyEight provides a good explanation and justification for their use) and era-adjustment considering a long-term moving average of total goals in England international matches (i.e. both scored and conceded by England).
Sources
englandstats was used for information regarding each player's appearances and goals. Elo Ratings was used for a list of all England matches and, crucially, the Elo ratings of the teams involved.
Opponent adjustment
By taking the Elo rating of England and their opponent in each match played by each player, as well as the number of goals scored by the player, relationships can be observed between opponent quality and goals scored for each player:
Charlton
Rooney
Since the players played in different England teams (Charlton was famously part of England's 1966 World Cup winning side), the ability of the England team should be taken into account. The charts below show the relationship between Elo margin (England Elo - opposition Elo) and goals scored for each player:
Rooney 
Using linear regression with goals scored as the dependent variable and both opposition Elo and Elo margin as independent variables, the expected number of goals for each player in any match may be calculated, provided the Elo of each team is known.
Era adjustment
The work above does not consider the era of the matches, so a goal by Charlton in 1960 is weighted the same as one by Rooney 50 years later. The chart below shows the moving average number of goals in England International matches (moving average considers a time period 2 years either side of the date, so a 4 year cycle in total). Overlaid are the goals scored by Charlton and Rooney.
It is clear that, particularly during the first half of Charlton England career, goals were more plentiful. Using this moving average, the raw number of goals scored by Rooney and Charlton can be adjusted to present-day values.
Opponent and era adjustment
The two studies above can be combined by examining the number of present-day adjusted goals per game that would be expected, had Charlton played the matches played by Rooney, and vice versa. Also, to give a neutral view of matches played, the expected goals for each player can be found for all England games since 1872:
Conclusions
From this work, it appears that Rooney's goalscoring record is the more impressive, when taking opposition and era into account. While both strikers have strong records, Rooney's era- and opponent-adjusted goals per game value is higher than Charlton's when all England games are considered. Strangely, it appears that the opponents faced by Rooney's England would have favoured Charlton more than Rooney, and vice-versa.
This work does have obvious shortcomings: Charlton was a midfielder while Rooney was a striker, and the importance of the goals scored has not been taken into account, to name two. Even with these shortcomings, it provides a useful view beyond the raw goalscoring record of each player.
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