I don't know. I know that GS tries to be objective in its reviews, (Greg Kasavin confirmed this a few years back) but I think the very nature of game reviews makes this impossible.
For example, Yoshi's Island DS. It's a terrible game; redundant, easy levels, absolutely atrocious music, and lackluster graphics. But GS game it a 9.4... Now, that's fine, people can disagree with me. But the problem is that GS says that a 9.4 game is better than a 9.0 game. Yet we all know (i.e. most of us will agree) that Bioshock is not an inferior game to Yoshi's Island DS.
So what's going on here? Simple. GS is rating different genres of games on different criteria. They thought Yoshi's Island did what 2D platform games are supposed to do, and did it really, really well. OK, but how does that mean that it's a better game than Bioshock? Because it does what "it's supposed to do" better than Bioshock does what it's "supposed to do"? How can you be objective about something like that? You can't (at least not that precisely); it's a subjective opinion.
I think reviews are fundamentally subjective, and that the best review will typically relate a personal experience had while playing the reviewed game. This makes reviews more reliable, also, because when someone reads the subjective account of a reviewer's gaming experience, they will know right away if they agree or disagree with the reviewer. For example, when someone gives a good review of a rollercoaster, and says it's awesome because it has some really intense drops, a listener will know that those intense drops are things that he wouldn't enjoy. So even though the ride was getting a good review, he'd know it wasn't for him. It's all about relating to the reviewer.
Gamespot tries to standardize its ratings into one system whereby all games can be compared to one another, but I think games are a subjective thing which can't be measured objectively.
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