Independent t-test: Caffeine vs Placebo Reaction Times

Interprets SPSS output for an independent samples t-test comparing reaction times between groups, including Levene's test interpretation, Cohen's d calculation, and APA-formatted results.

Input

What you provide to the skill

Independent t-test SPSS output:
Caffeine n=30 M=412.5 SD=45.8
Placebo n=30 M=438.2 SD=52.1
Levene’s F=1.234 p=.271
t(58)=-2.103 p=.040
Interpret assumptions, calculate Cohen’s d, provide APA format

Output

What the skill generates for you

Interpretation

Test Type: Independent Samples t-test
Purpose: Comparing reaction times between caffeine (n=30) and placebo (n=30) groups

Descriptive Statistics

Caffeine: M = 412.5, SD = 45.8
Placebo: M = 438.2, SD = 52.1
Caffeine group had reaction times 25.7 ms faster on average

Assumption Check: Levene’s Test

F = 1.234, p = .271 (non-significant)
Assumption MET: variances are approximately equal
Use row: “Equal variances assumed”

Main Results

t(58) = -2.103, p = .040
Significant: p < .05
The caffeine group showed significantly faster reaction times than the placebo group

Effect Size Calculation

import math
pooled_sd = math.sqrt(((30-1)*45.8**2 + (30-1)*52.1**2) / (30+30-2))
d = (412.5 - 438.2) / pooled_sd
print(f"Cohen's d = {d:.2f}")  # d = -0.52

Result: d = -0.52 (medium effect)

APA-Formatted Results

“An independent samples t-test revealed that the caffeine group (M = 412.5, SD = 45.8) exhibited significantly faster reaction times than the placebo group (M = 438.2, SD = 52.1), t(58) = -2.103, p = .040, d = -0.52.”

What NOT to Report

Std. Error Mean (calculation artifact)
“Equal variances not assumed” row
Levene’s F in results section

Common Mistakes to Avoid

Ignoring Levene’s test when choosing which row to report
Reporting both t-test rows
Forgetting to state which group had faster reaction times
Omitting Cohen’s d effect size
Reporting p = .040 without three decimal places when available