McNemar procedure demonstrated with an example

  1. The McNemar's test
  2. Analyzing data with McNemar test
  3. Output, interpretation and assumption checking

The McNemar's test

Simar to the contigency test, McNemar's test can be used to analyze categorical data in survey and questionnarie. But when the data are dependent, McNemar's test is more appropiate. For example, assume that each of several test subjects is afflickted with tinea pedis(athlete's foot) on each foot, and each subject is given a treatment X on one foot and Y on the other foot. Frequency count from matched pairs of foot is measured for both cured and not cured. Because left and right foot of the same subject are dependent, contingency test cannot be used and McNemar's test should be used. The table below demonstrates the frequency count.

		2 X 2 Table with Frequency Counts from Mathed Pairs
	___________________________________________________________________
						Treatment X
					___________________________________
					    Cured		Not Cured
	___________________________________________________________________
				 Cured	      a			   b		
		Treatment
			     Not cured	      c			   d
	___________________________________________________________________


In summary, McNemar's test is appropiate if the following assumptions are met:

  1. The sample data have been randomly selected.
  2. The sample data consist of matched pairs.
  3. The frequency count can be organized into a 2 X 2 table shown above. There should be 2 variables (Treatment and Result), each has two categories (X/Y; cured/not cured).
  4. As shown in the table above, the frequencies are big enough such that b + c &ge 10.

Analyzing a survey data with McNemar's test

The clinical Trials for treatments for athelete's foot is in the file "athelete.csv".

The question is to test whether treatments X or Y matter to the results.

Open the data set from SAS. Or import with the following command. Note that the procedure "format" organize the lable from 'c' to 'cured' and 'nc' to 'not cured'.

 
  	proc format;
		value $result 'c'='cured'
				  'nc'='Not cured';
 

	data athelete;
		infile "H:\sas\data\athelete.csv" dlm=',' firstobs=2;
		input subject  treatX $ treatY $;
		format treatX treatY $result. ;
	run;

The McNemar's test can be run as follows. Note that the "agree" option on the tables statement produces a both McNemer's test statistics as well as a measure of agreement called Kapped, which is often used as a measure of interrater reliability.

	proc freq data=athelete;
		title "McNemar's test for Paired Samples";
		tables treatX*treatY /agree expected norow nocol nopercent; 
		run;

The control option "agree" requests tests and measures of classification agreement for square tables. it provides McNemar's test for 22 tables and Bowker's test of symmetry for tables with more than two response categories.

The control option "expected" asks for expected value for assumption checking.

The other options such as "norow" eliminate unwanted outputs for clarity.

Reading the output

            		McNemar's test for Paired Samples                              
                                                                     

                                       The FREQ Procedure

                                    Table of treatX by treatY

                              treatX     treatY

                              Frequency 
                              Expected  cured   Not cured  Total
                         ______________________________________________
                              cured        12      8 	     20
                                           13      7  
                         ______________________________________________    
                              Not cured    40      20        60
                                           39      21 
                         ______________________________________________
                              Total        52      28       80


                            Statistics for Table of treatX by treatY

                                         McNemar's Test
                                    __________________________
                                    Statistic (S)    21.3333
                                    DF                     1
                                    Pr > S            <.0001


                                    Simple Kappa Coefficient
                               ____________________________________
                                Kappa                    -0.0435
                                ASE                       0.0821
                                95% Lower Conf Limit     -0.2044
                                95% Upper Conf Limit      0.1174

                                        Sample Size = 80




Interpreting the result

This output shows McNemar's test statistics is 21.33, with a corresponding p-value <0.0001. Thus we conclude that the treatment make a difference in the result.