With such a small sample size, I doubt it's very accurate. 13000 samples out of 4 million pre orders isn't even worthy of writing an article about. No one I know has heard of slice either.
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It's a matter of a biased sample, as dallas suggested. You only need 191 people to estimate a moderate effect size in a 4x2 table (goes up to 1700 with a small effect size). The population size is irrelevant (see: central limit theorem) given an unbiased sample. A proper analysis would have involved taking current subscriber counts and assigning weights based on the users who downloaded Slice (prior to entering iPhone tracking information), then running the same table with the new weights.
iPhone 6 and iPhone 6 Plus User Thread
in Apple/iPhone/iOS
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It's a matter of a biased sample, as dallas suggested. You only need 191 people to estimate a moderate effect size in a 4x2 table (goes up to 1700 with a small effect size). The population size is irrelevant (see: central limit theorem) given an unbiased sample. A proper analysis would have involved taking current subscriber counts and assigning weights based on the users who downloaded Slice (prior to entering iPhone tracking information), then running the same table with the new weights.