I have to ask, why wouldn't one be in favour of the one constant across all eras?
Ie. the best Canadians are always in the NHL and make up to 50% to all of the elite top end talent in the league.
because i think it simplifies a very complex methodological problem, and produces misleading conclusions as much as it helps. which is to say, i don't think the constant is as much of a controlled variable as you suggest.
to take the most extreme example:
1998 is the most tilted goal scoring year i can think of. top 6 scorers were all european or american. of the top 15 goal scorers, 10 were not born in canada.
meanwhile, 1981 is mostly canadian. top non-canadian scorer is #9 (kent nilsson), next one is #20 (peter and anton stastny tied).
now imagine i wanted to argue that '98 pavel bure was as good as '81 bossy. if you take out all the non-canadians, bure has a massive lead over the next group of guys.
bure: 51 (3) -- 31% of #2
nieuwendyk: 39 (7)
brind'amour: 38 (8)
allison and whitney: 33 (10)
recchi: 32 (14)
in 1981:
bossy: 68 (1) -- 17% of #2
dionne: 58 (2)
simmer: 56 (3)
gretzky and kehoe: 55 (4)
and so on.
paints a misleading picture, doesn't it? canadian-only talent level is not congruent across eras.
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but here's another objection to taking out all european and american players to level the playing field across eras: if you just subtract all of those guys from the stats sheet, that's a lot of shots and goals and minutes played that suddenly never happened. but those shots wouldn't just not be taken, those goals wouldn't just not be scored; with no non-canadians, canadians move up the lineup and have more opportunities to score.
if you take the extreme case of 1998 again, a LOT of prime scoring opportunities were taken by european and american players. top six minutes, first unit PP time, etc. not to say that those players who occupied that ice time didn't earn it, but what if you took all of those guys out of the league? not just the high end guys like selanne, jagr, bondra, tkachuk, leclair. imagine no khristiches, no amontes, no mceacherns, no kamenskys... no kapanens, kozlovs, and so on and and so on. surely some canadian players will have to fill those roles. and surely some of those canadian players moving up in the lineup -- or one-dimensional offensive guys buried in the minors moving into the lineup at all -- would score more goals than they otherwise did. and then maybe -- just maybe -- bure's fifth best season doesn't look as dominant if not more dominant than peak bossy.
just look at nieuwendyk and brind'amour. how does 31 year old joe nieuwendyk manage his best adjusted goal total ever, and his highest total overall in seven years, and all of this in only 73 games? because mike modano, the player ahead of him in the lineup, missed 30 games that year. one wonders, then, whether brind'amour might have scored more than 38 goals if there's no john leclair and rod the bod becomes the primary shooting option on the PP? then all of a sudden, bure doesn't look so much like he laps the league.
that very season, ray whitney finally finds a situation where he can do what he can do. he'd been buried for years behind guys like johan garpenlov. he couldn't stick on an oilers team where some of the wingers ahead of him were guerin, marchant, kovalenko, and zelepukin. he finds the right crappy team to give him offensive icetime and -- voila -- he finishes 10th in the league in goals. how many other ray whitneys are there in the minors or the fringes of the league: too small to play bottom six minutes, not getting a shot at the top six minutes that they might be productive at? steve sullivan and martin st. louis were a couple of years away from breaking through. but maybe yanick dube or stephane morin or some other AHL/IHL superstar could have potted big time points on the right crappy team with the right prime icetime? see: donald audette in atlanta in 2001, on pace for a 40 goal/90 point season until he was traded to a competitive team at the deadline.
EDIT: (none of this, by the way, is meant to suggest that those european and american players didn't deserve the icetime they got, or that they weren't better in those roles than the canadian guys they beat out for them. what i do, however, want to suggest is that the league leader in goals in a 1998 season with no europeans and americans probably exceeds 39.)