The FREQ Procedure

Example 6: Computing Cochran-Mantel-Haenszel Statistics for a Stratified Table

Procedure features:

TABLES statement options:

	CMH
	NOPRINT

WEIGHT statement

This example

creates stratified two-way contingency tables using existing cell counts
suppresses the display of the contingency tables
computes Cochran-Mantel-Haenszel statistics adjusting for the effects of a stratification variable.

Program

options nodate pageno=1 linesize=80 pagesize=60; data migraine; input Gender $ Treatment $ Improve $ Frequency @@; datalines; female Active Better 16 female Active Same 11 female Placebo Better 5 female Placebo Same 20 male Active Better 12 male Active Same 16 male Placebo Better 7 male Placebo Same 19 ;

proc freq data=migraine; weight frequency;

tables gender*treatment*improve/cmh noprint;

title1 'Clinical Trial for Treatment of Migraine Headaches'; run;

Output

PROC FREQ computes Cochran-Mantel-Haenszel statistics, controlling for Gender. For stratified 2×2 contingency tables, these statistics include estimates of the common relative risk and the Breslow-Day test for homogeneity of the odds ratios. For a stratified 2×2 table, the three CMH statistics test the same hypothesis. The significant p-value (.004) indicates that the association between treatment and response remains strong after adjusting for gender.
The large p-value for the Breslow-Day test (.222) indicates no significant gender difference in the odds ratios. Because this is a prospective study, the relative risk estimate assesses the effectiveness of the new drug. The probability of migraine improvement with the new drug is just over two times the probability of improvement with the placebo.

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