The confidence interval of two populations means a) it must pool the sample variances
A confidence interval provides a range of possible values for the difference between two population means. For example, a 95% confidence level indicates that given 100 random samples from the population, you can expect about 95 samples to yield intervals that include the population difference.
If the confidence interval for the between-groups difference contains zero, it means that if you run the experiment again, it is likely that you will not find a difference between the groups. Two populations from which two independent samples are selected are known to have the same variance.
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