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The expected percentages of each-- 1.5% Legendary (doubled if using potions), x% Epic, y% Rare, and z% common. Anything less than a 95% confidence interval isn't considered statistically significant-- this is usually used for determining how large a sample size is needed for a research study.

This video is pretty good, and it goes into standard deviations a bit too.

Oh thanks! Can you show me the exact calculation that led you to the 4268 number?

I can't because I had my statistics professor friend do it. I text him random questions all the time-- we've been friends since we were kids.

Oh that's disappointing. As a statistician myself, I'd recommend having people explain things to you and not just give you numbers you don't know where they came from. Like your number seems too high to me, but without knowing where it came from, it's hard to understand if I'm missing something, or your friend is.