Simple Between-Subjects ANOVA

PSYC234

When would we use between subjects ANOVA?

When there is 1 categorical IV, 3+ Independent groups and 1 continuous DV.

Difference between between and within subjects?

Between = Different ppts in each group

Within = Same ppt completes every condition

ANOVA asks…

Are the groups means different?

Null hypothesis and Alternative hypothesis?

Null: There is no significant difference between group means.

Alternative: There is a significant difference between at least 1 groups mean.

What are the assumptions for between-subjects ANOVA?

• Continuous DV

• Independent groups

• Normality (Shapiro-Wilk)

• Equal variances (Levene’s)

Shapiro-Wilk?

p > .05 = Normality met

p < .05 = Normality violated

Levene’s test?

p > .05 = Equal variances met

p < .05 = Equal variances violated

ANOVA?

p < .05 = Significant difference between groups

p > .05 = No significant difference

Key stats?

F, df, p, n squared (eta)

What is n squared?

• eta squared

• effect size

• how much of the variation in the DV is explained by IV

Effect sizes (n squared)?

Small = .01

Medium = .06

Large = .14

What if ANOVA is significant?

Run post hoc tests (Usually Bonferri) to find which groups differ.

How to interpret post-hoc test?

1. ANOVA, ask if there is a difference (p values)

2. Post HOC, ask where is the difference. p<.05 = Groups differ, p > .05 = Groups don’t differ.

Template to answer a simple BS ANOVA question?

1. Scenario (IV and DV?)

2. Assumptions met? (DV, IG, Shapiro-Wilk, Levenes)

3. ANOVA (F, df, p, np squared)

4. Post-hoc (Which p values significant)

5. Means (Which groups higher/lower)

Combine step 4 and 5

6. Hypothesis