Sometimes we use a z-test. If we're dealing with proportions. But when we deal with means, we tend to use a t-test. And the reason why is if you wanted to figure out a z-statistic, what you would do is you would take your sample mean, subtract from that the assumed mean from the null hypothesis. So mu, and I'll just put a little zero, sub zero there. So this is the assumed mean from the null hypothesis. And then you would want to divide by the standard deviation of the sampling distribution of the sample mean.