[R] McNemar test in R & SPSS

[Peter Dalgaard]

Bob Green wrote:
> Peter,
>
> Thanks for your reply. To perform the analysis in R, I used the table 
> format suggested in the book by Everitt & Hothorn, whereas in SPSS the 
> analysis was performed directly from the 2 variables, rather than using 
> count data.
>   
You still need the right table.

matrix(c(128,29,331,430), ncol =2,....)

consists of the two marginal tables, which has strictly less information than the crosstabulation of pre and post values. I expect every text on the McNemar test makes this point, and I'd be highly surprised if E&H really suggested that you should use the table that you did.

> There is still something I don't understand  - I tried to replicate the 
> syntax from your e-mail but my computer just kept waiting - what am I still 
> doing wrong?
>   
You need to give it the data when it starts with the "0:"-style prompt, 
end with a blank line, as shown. It's just a device for cutting and 
pasting your table. I might as well have used

d <- matrix(c(311,119,20,9), ncol=2)

Or, with the raw data to hand

d <- table(preMHT, postMHT)


>
>   mctest <- as.table(matrix(c(128,29,331,430), ncol =2, dimnames = 
> list(group=c("preMHT","postMHT"), assault=c("yes","no"))))
>  > d <- read.table(stdin())
> 0: mcnemar.test(as.matrix(d),correct=F)
> 1:
>
> Thanks again
>
> Bob
>
>
> At 10:52 PM 26/12/2006 +0100, Peter Dalgaard wrote:
>   
>> Bob Green wrote:
>>     
>>> Hello,
>>>
>>> I am hoping someone can clarify why I might obtain a quite different 
>>> value in R & SPSS  for a McNemar test I ran.
>>>
>>> Firstly, here is the R syntax & output
>>>
>>>
>>> R OUTPUT
>>>
>>>  > mctest <- as.table(matrix(c(128,29,331,430),
>>> + ncol =2, dimnames = list(group=c("preMHT","postMHT"),
>>> + assault=c("yes","no"))))
>>>
>>>  > mctest
>>>                assault
>>> group       yes  no
>>>    preMHT  128 331
>>>    postMHT  29 430
>>>
>>>  >  mcnemar.test(mctest,correct=F)
>>>
>>>          McNemar's Chi-squared test
>>>
>>> data:  mctest
>>> McNemar's chi-squared = 253.3444, df = 1, p-value < 2.2e-16
>>>
>>>
>>> SPSS OUTPUT
>>>
>>> The same data was inputted in SPSS. Regarding the first table SPSS 
>>> generated - it lists the number of cases in each combination of 
>>> categories. The diagonal contains the number of cases with the same 
>>> response on both variables, while the off diagonal contains cases that 
>>> have different responses on the 2 variables. The overall chisquare value 
>>> is much lower than the value obtained using R, though still significant.
>>>
>>>
>>> pre02 & post02
>>>
>>> pre02   post02
>>>
>>>          0       1
>>> 0       311     20
>>> 1       119     9
>>>
>>>
>>> Test Statistics(b)
>>>
>>>   pre02 & post02
>>>
>>> N               459
>>> Chi-Square(a)   69.094
>>> Asymp. Sig.     .000
>>>
>>> a  Continuity Corrected
>>> b  McNemar Test
>>>
>>>
>>> Any assistance is much appreciated,
>>>
>>>       
>> Well, you can't expect R to give the correct result if you feed it the 
>> wrong matrix, can you?
>>
>>     
>>> d <- read.table(stdin())
>>>       
>> 0:          0       1
>>
>> 1: 0       311     20
>>
>> 2: 1       119     9
>>
>> 3:
>>     
>>> d
>>>       
>>   X0 X1
>>
>> 0 311 20
>>
>> 1 119  9
>>
>>     
>>> mcnemar.test(as.matrix(d),correct=F)
>>>       
>>        McNemar's Chi-squared test
>>
>> data:  as.matrix(d)
>> McNemar's chi-squared = 70.5108, df = 1, p-value < 2.2e-16
>>
>>     
>>> mcnemar.test(as.matrix(d),correct=T)
>>>       
>>        McNemar's Chi-squared test with continuity correction
>>
>> data:  as.matrix(d)
>> McNemar's chi-squared = 69.0935, df = 1, p-value < 2.2e-16
>>
>>
>>
>>     
>>> Bob Green
>>>
>>> ______________________________________________
>>> R-help at stat.math.ethz.ch mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>>       
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>