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Statistics Resources

This guide contains all of the ASC's statistics resources. If you do not see a topic, suggest it through the suggestion box on the Statistics home page.

Testing Assumptions

Think of assumptions as prerequisites. These conditions must be met in order for the t-test to be used. Before running the t-test, you would want to test the appropriate assumptions. Most of the t-tests have the same basic assumptions, but there are some slight differences based on the type of t-test being conducted.

General Assumptions
  1. The dependent variable is continuous (interval or ratio).
    - review the ASC guide on Levels of Measurement
    - attend the What's a Variable group session
  2. Each observation is independent of other observations.
    - evaluate the study design - Does any person's score depend on someone else's?
    *this important assumption is often not an issue*
  3. No significant outliers.
    Sage Publications resource
  4. The dependent variable should be approximately normally distributed.
    Laerd Statistics guide
    - for the Independent Samples T-test: each group should be assessed individually
    - for the Dependent Samples T-test: the differences should be assessed for normality
Test-Specific Assumptions
  • Independent groups (Independent Samples T-test)
    - categorical independent variable with exactly two levels (i.e. yes/no, male/female)
    - participants only belong to one group or level of the IV
  • Paired or related groups (Dependent Samples T-test)
    - categorical independent variable with exactly two levels (i.e. pre-test/post-test)
    - the same participants provide scores for both groups or levels of the IV
  • Homogeneity of Variances (Independent and Dependent Samples T-tests)
    - are the variances of the two groups approximately the same?
    - use Levene's Test by checking the appropriate box in SPSS when running the test

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