But we often will not know this. In fact, it's very unusual to know this. And so sometimes you will say, okay, if we don't know this, let's just figure out the sample standard deviation of our sample here. So instead, we'll say, okay, let's take our sample mean, plus or minus z star, times the sample standard deviation of our sample, which we can calculate, divided by the square root of n. Now this might seem pretty good if we're trying to construct a confidence interval for our sample, for our mean. But it turns out that this is not, not so good. Because it turns out that this right over here is going to actually underestimate the actual interval, the true margin of error you need for your confidence level. And so that's why statisticians have invented another statistic.