McNemar

Test
Dr. Saadah Mohamed Akhir
Unit Rundingcara Statistik
FSK, UKM
Learning Outcomes
 At
the end of the lecture, students will be
able to
 Understand when to use McNemar test.
 Perform McNemar test using SPSS.
 Calculate the McNemar test using formula.
 Interpret and report the output from SPSS.
What is McNemar test?
 Non parametric test
 Categorical data
 a dichotomous (binary) dependent
variable between two related groups.
 paired nominal data
 toanalyze pretest-posttest (before and after)
study designs
Wilcoxon Sign Ranked Test and
McNemar Test – Any difference?
 They do not share the same type of data.
 Wilcoxon – continuous data
 McNemar – categorical data
Assumptions
 Has one categorical dependent variable
with two categories (dichotomous variable)
and one categorical independent variable
with two related groups.
 Dichotomous variable (Binary data) – eg.
Yes and No, Normal and Abnormal.
 Participants are a random sample from the
population.
 Example
 Categorical
dependent:
With Training Total
Knowledge status
Knowledge Knowledge
Improved not  yes – improved
Improved

Without Knowledge 10 5 15
 no – not improved
 Two related group
Training Improved

Knowledge
not
Improved
15 20 35
– training status
Total 25 25 50  yes – with
 no - without
Contingency Table
 Organizationof data – table type form
 Variables composed in columns and rows
 Known as cross tabulation table.
With Training Total

Knowledge Knowledge
Improved not
Improved
Without Knowledge 10 5 15
Training Improved
Knowledge 15 20 35
not
Improved
Total 25 25 50
Case Study:
Bilirubin level from experimental drug
- A study was conducted to evaluate the
toxicity side effect of an experimental
therapy.
- Patients (n=86) were treated with the
experimental drug for 3 months.
- Clinical lab measured bilirubin levels of
each patient at baseline and 3 months
after drug is given.
Data - Variables
 Dependent variable – bilirubin level
 Normallevel of bilirubin
 Abnormally high level of bilirubin

 Independent variable – related

groups
 pre – before drug is given
 Post – after drug is given
Data for McNemar statistic test
 Rawdata (n = 86) –  Summarizeddata –
86 data are contingency 2 x 2
entered. table
After
Before
Normal Abnormal

Normal 60 14

Abnormal 6 6
McNemar with SPSS
Raw Data
McNemar - SPSS
 Enterraw data
 Analyze – Non Parametric – Related
Samples
 objective – customize analysis
 Field – move pre test and post test to test
field box
 Click Run
Raw data
Analyze – Non Parametric –
Related Samples
objective – customize analysis
Field – move pre test and post test to
test field box
SPSS - output
McNemar with SPSS
Raw data
SPSS (McNemar)
 Enter raw data in SPSS
 Analyze  Descriptive statistics 
Crosstabs
 Move over the two variables to the row
and column boxes (I used rows for the
pre-test and columns for the post-test)
 Click on Statistics and check McNemar,
then click Continue.
 Click OK.
Raw data
Analyze  Descriptive statistics
 Crosstabs
SPSS
Output
McNemar with SPSS
2 x 2 contingency table
SPSS (McNemar)
 Output from raw data – contingency table
 Weight cases by frequency
 Analyze  Descriptive statistics  Crosstabs
 Move over the two variables to the row and
column boxes (I used rows for the pre-test and
columns for the post-test – it can also be the other
way round)
 Click on Statistics and check McNemar, then click
Continue.
 Click on Cells and then check Row and Column
under Percentages, then click Continue.
 Click OK.
What should be in SPSS?
 Since there are 4 cells in 2 x 2 contingency table –
there are only 4 data entry.
 How many variables? There are 3 variables
 Before
 After
 Frequency

After
Before
Normal Abnormal

Normal 60 14

Abnormal 6 6
Variable View – Data View
Create 3 columns and label

Create 3 Columns – Before,

After, Frequency
-
Label column before
1 as “N” and 2 as “Y”
- Label column after
1 as “N” and 2 as “Y”

Enter n (frequency) data in

column frequency
Variable View – Data View
Enter contingency data in SPSS
Weight Cases by frequency
Weight Cases by frequency
Analyze
 McNemar Test
– Test Pairs in SPSS
Output
McNemar - Conduct
statistical decision by using
formula
Case Study:
Bilirubin level from experimental drug
- A study was conducted to evaluate the
toxicity side effect of an experimental
therapy.
- Patients (n=86) were treated with the
experimental drug for 3 months.
- Clinical lab measured bilirubin levels of
each patient at baseline and 3 months
after drug is given.
 36

Hypothesis
null hypothesis : H 0 : p1  p2
alt. hypotehsis : H A : p1  p2
Null hypothesis: There is no change in bilirubin
level after taking experimental drug

Alternative hypothesis: There is a change in

bilirubin level after taking experimental drug
Formula
( B  C ) 2
test statistic : 2 
BC
decision rule : reject H 0 if  2 12 ( )

After Responders Non-

Before responders

Responders A B
Non-responder C D
 38

McNemar’s test
null hypothesis : H 0 : p1  p2
alt. hypotehsis : H A : p1  p2
( B  C )2
test statistic :  
2

BC
decision rule : reject H 0 if  2 12 ( )

After Responders Non-

Before responders

Responders A B
Non-responder C D
Formula – Calculate
(McNemar)
After
Normal Abnormal High(Y) null hypothesis : H 0 : p1  p2
(N) alt. hypotehsis : H A : p1  p2
Before
(B  C )2
test statistic :  
2

BC
Normal 60 (A) 14 (B) decision rule : reject H 0 if  2 12 ( )
Abnormally
High
6 (C) 6 (D)
- df = 1, the critical value =
3.84 (from the chi-square
- df = (r - 1) (c – 1) = 1
table)
- r = 2, c = 2
- calculated value of 3.20
does not exceeds this value
14 −6 2
- 2 = - there is no significant
14+6 difference in bilirubin level
after therapy
= 3.20
Chi Square table
Report (APA style)
 Eighty six participants were recruited to take
part in the toxicity side effect of an
experimental drug for 3 months. A bilirubin
levels were measured to identify the toxicity
side effect of the experimental drug. Of these,
only 6 had normal bilirubin level after taking
the experimental drug for 3 months. A
McNemar test showed that there is no
significant change of bilirubin level after
taking the experimental drug, 2 (1, N = 86) =
3.20, p = 0.115.
Exercise
THANK YOU