[Franek Grabowski]

Craig addressed some important issues.
It is impossible to assess individual performance by statistics, indeed, because it is against the rules of statistics.
Plenty of German data comes from highest levels of command, hence they are prone to errors, passed by various chains of command. There are no combat reports, which should perhaps be considered most accurate, though not necessarily could be. Pilots could not remember the exact details, or the person filing them could misunderstand them, or even the pilots did not assess the situation accurately. BTW I understand that Russell's data are coming from germandocsinrussia site, and several daily reports held there.

It is not possible to assess results of air combats by simple correlation of time and place.
It is a necessity to compare detailed information from both sides to draw accurate conclusions, and even if fully documented, it is often not possible to recreate events.
It is always worth to remember that claims and credited victories are two different things, the latter being the effect of activity of a bureaucratic machinery, rather than pilots themselves. Neither claim nor credited victory is equal to actual loss.

Thus perhaps it is easier to discuss such issues on a unit level rather than individual level.
My personal observation is, that the German units on the western front overclaimed to a much higher degree, that widely thought. It is believed that the victories credited to JG 26 are highly accurate. It looks like the number of credited victories matches to the number of aircraft lost. The problem is, that accurate scrutiny of allied losses show, that plenty of those losses were to various different reasons, but not necessarily German action. Thus the one may assume that there was a preliminary selection of claims, which were then reduced to the number of wrecks found with no available paper record. Without the initial claim list it is not possible to properly assess the issue. The otherproblem is, that available victory lists are highly inaccurate, missing important details, or obviously erroneous, this becoming obvious after careful scrutiny of each combat. In the effect, it is often not possible to say, that one victory is valid, and the other is not.
Finally, about Marseille. I do not know, how accurately allied losses in Africa were recorded. This is a common problem for overseas units, that the paperwork was not kept accordingly, and they often were not very attractive for researchers, thus available information might not be the best to say the least. Such massive projects like Fighters over the Desert are often not accurate because of numbers of data involved, not possible to verify by a limited number of researchers. So the rule of the limited trust is always valid. The additional problem is, that most of Marseille's combats were not on equal terms. He always had a technical edge, and also the tactics of the Desert Air Force was not much to expectations. As soon as Spitfires appeared in the theatre, flown by experienced pilots aimed at scoring victories, the joy has ended. I presume, that Marseille would not last long on the ETO, where things were much more equal. Thus he was a highest scoring pilot, perhaps, but not necessarily the best one.