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- If the data is comes from a normal distribution, then the t-test and the confidence interval are exact.
- If the data doesn’t come from a normal distribution, then the t-test and confidence interval will be approximate if
- n < 15 and the data are close to normal. If the data are clearly not normally distributed or outliers exist, do not use the t-test.
- n ≥ 15, then the t-tests can be used except in the case that the data contains outliers or the data is strongly skewed.
- n ≥ 40 (large samples), then the t-tests can be used even for skewed distributions and the results will be approximate but fairly accurate.