Use 1-Sample t to estimate the mean of a population and to compare it to a target value or a reference value when you do not know the standard deviation of the population. Using this test, you can do the following:

- Determine whether the population mean differs from the hypothesized mean that you specify.
- Calculate a range of values that is likely to include the population mean.

For example, a quality analyst uses a 1-sample t-test to determine whether the average thread length of bolts differs from the target of 20 mm. If the mean differs from the target, the analyst uses the confidence interval to determine how large the difference is likely to be and whether that difference has practical significance.

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If you know the standard deviation of the population, use a 1-Sample Z because the Z-test has more power than the t-test.