T-tests are used when you want to examine differences but you do not know everything about the population. There are three types of t-tests that you may choose to do: one-sample t-test, independent sample t-test, or dependent sample t-test. You can calculate these by hand, in SPSS, or in Excel. The instructions below can be used for SPSS and your textbook offers instructions for using Excel.
Single-sample t-tests:
These tests are used when you want to determine the probability that a sample was drawn from a population with a known mean (μ) but with a standard deviation estimated from the sample.
Independent sample t-test:
These tests are used when you want to determine the probability that two samples were drawn from the same population with unknown means and standard deviations; both of which are estimated from the sample. No population parameters are specified.
Dependent samples t-test:
We use this t-test when we have a repeated measures design such as the same sample completes a pre and post-test and we want to know if there is a difference from one test to the other.
Let's try a few using the data below. Be sure to attach your printouts and answer the questions below.
- First, perform an independent sample t-test for gender (IV) and pretest scores (DV) Were there significant gender differences? How do you know? Interpret the results statistically and in words.
- Then perform an independent sample t-test for gender (IV) and posttest scores (DV) Were there significant gender differences? How do you know? Interpret the results statistically and in words.
- Now, perform a paired (dependent) sample t-test for pretest and posttest scores.
Were there two scores significantly different? How do you know? Interpret the results statistically and in words.
Data Set Homework 2 (*note 1 = males, 2 = females)
Gender Pretest Posttest
1.00 50.00 80.00
1.00 50.00 70.00
1.00 80.00 70.00
1.00 80.00 50.00
1.00 70.00 50.00
1.00 70.00 50.00
1.00 60.00 60.00
1.00 60.00 80.00
1.00 90.00 80.00
1.00 90.00 90.00
1.00 90.00 80.00
1.00 80.00 90.00
1.00 80.00 90.00
1.00 70.00 70.00
1.00 70.00 80.00
1.00 50.00 80.00
1.00 50.00 70.00
1.00 50.00 70.00
1.00 60.00 60.00
1.00 70.00 80.00
2.00 50.00 70.00
2.00 40.00 50.00
2.00 40.00 80.00
2.00 70.00 80.00
2.00 50.00 70.00
2.00 50.00 80.00
2.00 50.00 90.00
2.00 60.00 90.00
2.00 70.00 90.00
2.00 40.00 80.00
2.00 40.00 80.00
2.00 30.00 70.00
2.00 30.00 70.00
2.00 50.00 60.00
2.00 60.00 80.00
2.00 40.00 80.00
2.00 70.00 80.00
2.00 50.00 70.00
2.00 60.00 70.00
2.00 70.00 90.00