Degrees of freedom refer to the number of values in the final calculation of a statistic that are free to vary. As degrees of freedom increase, the graph of a t-distribution approaches the standard normal distribution. A z-test is used when the population standard deviation is known, while a t-test is used when the population standard deviation is unknown. The critical value is obtained from statistical tables and compared to the test statistic calculated from the sample data.

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SOURCES:2
1. What is meant by "degrees of freedom"? As the degrees of
freedom increase, what happens to the graph of a t-distribution?
2. Why would you use a z-test rather than a t-test? Which do
you think you will use more often? Why
3. What is the difference between a critical value and a test
statistic?