• There are four tests of means; each test asks, in some form, whether what is typical of a sample matches what we expect in a population.
  
  
• The one-sample z test is used when we know the population mean (μ) and standard deviation (σ); the one-sample t test is used when we only know the population mean (μ).
  
  
• The independent-means t test is used when we have two separate samples of data in our study and group membership does not overlap in any way.
  
  
• Dependent-means t tests are used when we have two samples of data in our study, but the data are linked either naturally in the population, through matching, or by studying the same people over time.

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