T-Test (Dependent Measures)

Hypothesis Testing• If the null hypothesis is true, this is the expected distribution

of means:

• Does my actual sample fall in the “more likely” area?

More Likely Outcomes

Less Likely Outcomes

When Do We Use• A t-test for independent samples?

• Two samples (maybe males and females)• Continuous Dependent Variable (maybe height)• We ask “Are the sample means (basically) the same or different?”

What about T test for Dependent Measures• Classic example: • Give a scale before a treatment and then again after.

• (Pre test- post test design)• Ask “Did this one group change from the pre-test to the post

test?”

Subject Pre Test Post Test DifferenceA 8 10 2B 6 5 -1D 5 8 3C 7 2 -5

• Its all about those difference scores

• The null hypothesis is the mean of the difference scores is zero

Effect Size• What does the p-test tell us about our results?

• What is it we really want to know about the results?

One measure of Effect Size: Cohen’s d

D

Cohen’s d

• Very simple way of standardizing the effect size