Using the $C$% confidence interval for $\mu_1  \mu_2$ to perform two sided two sample $z$ test: full explanation
If 0 is not in the $C$% confidence interval, the difference between the two sample means is significantly different from 0 at significance level $\alpha = 1  C/100$
If 0 is not in the $C$% confidence interval, the difference between the two sample means is significantly different from 0 at significance level $\alpha = 1  C/100$. In order to understand why, we have to be aware of the following three points, which are illustrated in the figure above:
