you can't do that on one week only.... last week even had a major 1st party release for N in Europe and japan, this would affect 3rd party sales.
The Fury is right however, this should be compared on multi platform games and per region (so a ratio, thinking of GH3 there that sells nothing in JP) and of course keeping in mind the release dates.
You could take a sample of games and use them as a benchmark (including some shovelware as it is a factor)
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