Correlation (Pearson, Kendall, Spearman) Correlation is a bivariate analysis that measures the strengths of association between two variables. In statistics, the value of the correlation coefficient varies between +1 and -1.
According to the MMOHUT games list. Runescape is one of the best yet on free-to-play browser games. There are a few premium features, but there is a free membership trial for 2 weeks.
There're RPG games that's open world (Fall out, elder scroll, perhaps the new Dragon Age Inquisition); there are action adventures that's open world (Assassin's Creed); there are shooters that's open world (Borderlands)... And of course, there's GTA and Red Dead Redemption games.