Main Effects & Interactions

Question: Does makeup increase perceived attractiveness?
Design: between-subjects $2 \times 1$
- IV $1$ : Makeup (present vs absent)
- IV $2$ : Rater Gender (male, female)
- Stimuli: $5$ identical female faces; attractiveness scale $1$ – $10$
Overall means (collapsed across gender)
- With makeup: $7.4$
- Without makeup: $7.3$ ⇒ no main effect of makeup
Split by gender
- Male raters: $7.9$ (makeup) vs $7.3$ (no makeup)
- Female raters: $6.8$ (makeup) vs $7.3$ (no makeup)
Conclusion: Significant Makeup × Gender interaction; effect of makeup depends on rater gender (↑ for males, ↓ for females)

Main effect: consistent influence of an IV on the DV after averaging over other IVs
Interaction: effect of one IV changes across levels of another IV
$\text{"Effect of IV}<em>1\text{ depends on IV}</em>2"$ ⇒ answers become “it depends”

Question: Does hands-free phone use impair braking, and does this vary with age?
Design: $2 \times 2$ between-subjects
- IV $1$ : Phone Use (hands-free vs none)
- IV $2$ : Age Group (Younger $18$ – $25$ vs Older $65$ – $74$ )
- DV: Brake-onset time (ms) after obstacle appears (lower = better)
Findings
- Main effect of Phone: Phone $\approx1000$ ms vs No phone $\approx850$ ms
- Main effect of Age: Older > Younger in all conditions
- No Phone × Age interaction: phone slows both age groups by similar margin
Notable: Younger drivers on phone ≈ braking speed of older drivers off phone (functional “aging” of $\sim50$ years)

Examine graph/means:
- Non-parallel lines → possible interaction
- Vertical gap between averaged lines → main effect
Reporting order: each main effect (significant/not) → interaction (significant/not)
Interpretation guide:
$\text{Interaction significant} \Rightarrow$ describe pattern within each subgroup; main effects may mislead
$\text{No interaction} \Rightarrow$ main effects generalize across other IV