The NPAR1WAY Procedure

Example 71.1 Two-Sample Location Tests and Plots

Fifty-nine female patients with rheumatoid arthritis who participated in a clinical trial were assigned to two groups, active and placebo. The response status (excellent=5, good=4, moderate=3, fair=2, poor=1) of each patient was recorded.

The following SAS statements create the data set Arthritis, which contains the observed status values for all the patients. The variable Treatment denotes the treatment received by a patient, and the variable Response contains the response status of the patient. The variable Freq contains the frequency of the observation, which is the number of patients with the Treatment and Response combination.

data Arthritis;
   input Treatment $ Response Freq @@;
Active 5 5 Active 4 11 Active 3 5 Active 2 1 Active 1 5 
Placebo 5 2 Placebo 4 4 Placebo 3 7 Placebo 2 7 Placebo 1 12 

The following PROC NPAR1WAY statements test the null hypothesis that there is no difference in the patient response status against the alternative hypothesis that the patient response status differs in the two treatment groups. The WILCOXON option requests the Wilcoxon test for difference in location, and the MEDIAN option requests the median test for difference in location. The variable Treatment is the CLASS variable, and the VAR statement specifies that the variable Response is the analysis variable.

The PLOTS= option requests a box plot of the Wilcoxon scores and a median plot for Response classified by Treatment. ODS Graphics must be enabled before producing plots.

ods graphics on;
proc npar1way data=Arthritis wilcoxon median
              plots=(wilcoxonboxplot medianplot);
   class Treatment;
   var Response;
   freq Freq;
ods graphics off;

Output 71.1.1 shows the results of the Wilcoxon analysis. The Wilcoxon two-sample test statistic equals 999.0, which is the sum of the Wilcoxon scores for the smaller sample (Active). This sum is greater than 810.0, which is the expected value under the null hypothesis of no difference between the two samples, Active and Placebo. The one-sided p-value is 0.0016, which indicates that the patient response for the Active treatment is significantly more than for the Placebo group.

Output 71.1.1: Wilcoxon Two-Sample Test

The NPAR1WAY Procedure

Wilcoxon Scores (Rank Sums) for Variable Response
Classified by Variable Treatment
Treatment N Sum of
Under H0
Std Dev
Under H0
Active 27 999.0 810.0 63.972744 37.000000
Placebo 32 771.0 960.0 63.972744 24.093750
Average scores were used for ties.

Wilcoxon Two-Sample Test
Statistic 999.0000
Normal Approximation  
Z 2.9466
One-Sided Pr > Z 0.0016
Two-Sided Pr > |Z| 0.0032
t Approximation  
One-Sided Pr > Z 0.0023
Two-Sided Pr > |Z| 0.0046
Z includes a continuity correction of 0.5.

Kruskal-Wallis Test
Chi-Square 8.7284
DF 1
Pr > Chi-Square 0.0031

Output 71.1.2 displays the box plot of Wilcoxon scores classified by Treatment, which corresponds to the Wilcoxon analysis in Output 71.1.1. To remove the p-values from the box plot display, you can specify the NOSTATS plot option in parentheses after the WILCOXONBOXPLOT option.

Output 71.1.2: Box Plot of Wilcoxon Scores

Box Plot of Wilcoxon Scores

Output 71.1.3 shows the results of the median two-sample test. The test statistic equals 18.9167, and its standardized Z value is 3.2667. The one-sided p-value Pr > Z equals 0.0005. This supports the alternative hypothesis that the effect of the Active treatment is greater than that of the Placebo.

Output 71.1.4 displays the median plot for the analysis of Response classified by Treatment. The median plot is a stacked bar chart showing the frequencies above and below the overall median. This plot corresponds to the median scores analysis in Output 71.1.3.

Output 71.1.3: Median Two-Sample Test

Median Scores (Number of Points Above Median) for Variable Response
Classified by Variable Treatment
Treatment N Sum of
Under H0
Std Dev
Under H0
Active 27 18.916667 13.271186 1.728195 0.700617
Placebo 32 10.083333 15.728814 1.728195 0.315104
Average scores were used for ties.

Median Two-Sample Test
Statistic 18.9167
Z 3.2667
One-Sided Pr > Z 0.0005
Two-Sided Pr > |Z| 0.0011

Median One-Way Analysis
Chi-Square 10.6713
DF 1
Pr > Chi-Square 0.0011

Output 71.1.4: Median Plot

Median Plot