📸 Two-proportion comparison in SPSS via Crosstabs → Risk statistics
Testing the Difference Between Two Proportions
When you need to compare whether a binary outcome occurs at different rates in two independent groups, the two-proportion z-test applies. Mathematically, in a 2×2 table, the Pearson chi-square equals the squared z-statistic. SPSS additionally provides Risk Ratio (RR) and Odds Ratio (OR), which convey the magnitude of difference more clearly than chi-square alone.
Running in SPSS
Step 1: Analyze → Descriptive Statistics → Crosstabs. Group to Row; binary outcome to Column.
Step 2: Statistics: check Chi-square and Risk (for OR and RR) → Continue → OK.
📸 Two-proportion output with Risk Ratio and Odds Ratio
APA Reporting
The proportion of patients achieving recovery was significantly higher in the treatment group (68%) than in the placebo group (42%), χ²(1)=14.21, p<.001, RR=1.62 (95% CI [1.26, 2.08]).