University of California, Berkeley: Philip Stark's SticiGui: "Chapter 30: Approximate Hypothesis Tests: the z Test and the t Test"
Read Chapter 30, which will show you the difference in hypothesis testing between the z and t tests. There are several exercises and Java applets embedded in the text that are meant to further reinforce your learning. Do not skip these exercises! For instructions on how to navigate these exercises, see "Special Instructions on SticiGui Exercises and Java Applets” in the "Course Information” section above.