1. t-Tests: Measuring the Differences Between Group Means

2. t-test Comparing group means
What is the probability of getting a difference this large by chance alone?

3. t-test Type of Data Required - Nominal-level independent variable
with two levels
- Continuous dependent variable

4. t-test Assumptions - Two mutually exclusive groups
- Normally distributed dependent variable
- Homogeneity of variance

5. t-tests Formulas Pooled or equal variance Separate or unequal variance
Correlated or paired

6. t-test
Alpha level
Power
One vs. two-tailed
Effect size
Sample size

7. Cohen’s Effect Sizes - t-test
Small = .2
Moderate = .5
Large = .8

8. Effect sizes
GRE Scores
Mean = 500
Standard Deviation = 100

9. Effect sizes LOPSS Difference between groups = 17 points
Standard deviation = 24

10. Exercises
1. Do individuals who have drug A differ from those who have Placebo on the following two measures: Heart Rate and Weight?
2. Do current ratings of satisfaction with weight differ significantly from ratings of weight at age 18?

11. SPSS - t-test, 2 groups ANALYZE Compare Means
Independent Samples T Test

12. SPSS - t-test, one group measured twice
ANALYZE
Compare Means
Paired Samples T Test