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If I have a dataset of size $N$, and choose $n$ samples, where $k$ of the samples are OK, how confident can I be that the dataset is OK?

To make it more concrete, suppose I have a dataset D1 with 5,000 values. I choose 20 of these and check each one, finding that all 20 are OK. I have dataset D2 with 5,000 values, I choose 20 of these and find out that 18 of them are OK.

I want to answer the question of "how good is the data", based on the sample that I've taken. Perhaps a better question would be - what sample size do I need to take in order to have a confidence interval of $\pm$ values about the proportion given by the sample? Although I'm not sure what an interval would mean in the case of the sample being all correct. Say I take a sample of 20 from 5000 and they're all correct, I can't then infer that the data has between (4900, 5100) correct samples with some degree of confidence (because the maximum is 5000 of course).

I feel that it's probably something binomial, but am unsure.

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