Dear Bharat Rawlley,

What you tried to do appears to be nonsense. That is, you're treating PFD_n and drug_code as if they were scores for two different groups.

I assume that what you really want to do is to treat PFD_n as a vector of scores and drug_code as defining two groups. If that's correct, and with your data into Data, you can try the following:

------snip ------

> wilcox.test(PFD_n ~ drug_code, data=Data, conf.int=TRUE)

        Wilcoxon rank sum test with continuity correction

data:  PFD_n by drug_code
W = 197, p-value = 0.05563
alternative hypothesis: true location shift is not equal to 0
95 percent confidence interval:
 -2.000014e+00  5.037654e-05
sample estimates:
difference in location
             -1.000019

Warning messages:
1: In wilcox.test.default(x = c(27, 26, 20, 24, 28, 28, 27, 27, 26,  :
  cannot compute exact p-value with ties
2: In wilcox.test.default(x = c(27, 26, 20, 24, 28, 28, 27, 27, 26,  :
  cannot compute exact confidence intervals with ties

------snip ------

You can get an approximate confidence interval by specifying exact=FALSE:

------snip ------

> wilcox.test(PFD_n ~ drug_code, data=Data, conf.int=TRUE, exact=FALSE)

        Wilcoxon rank sum test with continuity correction

data:  PFD_n by drug_code
W = 197, p-value = 0.05563
alternative hypothesis: true location shift is not equal to 0
95 percent confidence interval:
 -2.000014e+00  5.037654e-05
sample estimates:
difference in location
             -1.000019

------snip ------

As it turns out, your data are highly discrete and have a lot of ties (see in particular PFD_n = 28):

------snip ------

> xtabs(~ PFD_n + drug_code, data=Data)

     drug_code
PFD_n  0  1
   0   2  0
   16  1  1
   18  0  1
   19  0  1
   20  2  0
   22  0  1
   24  2  0
   25  1  2
   26  5  2
   27  4  2
   28  5 13
   30  1  2

------snip ------

I'm no expert in nonparametric inference, but I doubt whether the approximate p-value will be very accurate for data like these.

I don't know why wilcox.test() (correctly used) and SPSS are giving you slightly different results -- assuming that you're actually doing the same thing in both cases. I couldn't help but notice that most of your data are missing. Are you getting the same value of the test statistic and different p-values, or is the test statistic different as well?

I hope this helps,
 John

John Fox, Professor Emeritus
McMaster University
Hamilton, Ontario, Canada
web: https://socialsciences.mcmaster.ca/jfox/

On 2021-01-19 5:46 a.m., bharat rawlley via R-help wrote:
  Thank you for the reply and suggestion, Michael!
I used dput() and this is the output I can share with you. Simply explained, I 
have 3 columns namely, drug_code, freq4w_n and PFD_n. Each column has 132 
values (including NA). The problem with the Wilcoxon Rank Sum test has been 
described in my first email.
Please do let me know if you need any further clarification from my side! 
Thanks a lot for your time!
structure(list(drug_code = c(0, 0, 1, 1, 1, 0, 0, 0, 1, 1, 0, 1, 0, 1, 0, 1, 1, 0, 0, 0, 1, 0, 1, 1, 0, 1, 0, 
0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0, 1, 1, 1, 1, 1, 0, 0, 1, 0, 1, 0, 0, 0, 1, 1, 0, 1, 1, 1, 0, 0, 0, 0, 1, 
1, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1, 0, 0, 1, 1, 1, 0, 1, 1, 0, 1, 1, 1, 0, 0, 1, 1, 1, 0, 0, 1, 0, 1, 1, 0, 1, 
1, 0, 1, 0, 0, 1, 1, 1, 0, 0, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 0, 1, 0, 0, 1, 1, 1, 1, 0, 0), freq4w_n 
= c(1, NA, NA, 0, NA, 4, NA, 10, NA, 0, 6, NA, NA, NA, NA, NA, 10, NA, 0, NA, NA, NA, NA, 0, NA, 0, NA, NA, 
NA, 0, NA, 0, NA, NA, NA, NA, NA, NA, NA, NA, 0, 0, 12, 0, NA, 1, 2, 1, 2, 2, NA, 28, 0, NA, 4, NA, 1, NA, 
NA, NA, NA, NA, 0, 3, 1, NA, NA, NA, NA, 4, 28, NA, NA, 0, 2, 12, 0, NA, NA, NA, 0, NA, 0, NA, NA, NA, NA, 
NA, NA, NA, NA, NA, 3, NA, NA, NA, NA, NA, NA, 6, 1, NA, NA, NA, 0, NA, NA, NA, 0, 0, NA, 0, NA, 2, 8, 3, NA, 
NA, NA, 0, NA, NA, NA, 9, NA, NA, NA, NA, NA, NA, NA, NA), PFD_n = c(27, NA, NA, 28, NA, 26, NA, 20, NA, 30, 
24, NA, NA, NA, NA, NA, 18, NA, 28, NA, NA, NA, NA, 28, NA, 28, NA, NA, NA, 28, NA, 28, NA, NA, NA, NA, NA, 
NA, NA, NA, 28, 28, 16, 28, NA, 27, 26, 27, 26, 26, NA, 0, 30, NA, 24, NA, 27, NA, NA, NA, NA, NA, 28, 25, 
27, NA, NA, NA, NA, 26, 0, NA, NA, 28, 26, 16, 28, NA, NA, NA, 28, NA, 28, NA, NA, NA, NA, NA, NA, NA, NA, 
NA, 25, NA, NA, NA, NA, NA, NA, 22, 27, NA, NA, NA, 28, NA, NA, NA, 28, 28, NA, 28, NA, 26, 20, 25, NA, NA, 
NA, 30, NA, NA, NA, 19, NA, NA, NA, NA, NA, NA, NA, NA)), row.names = c(NA, -132L), class = 
c("tbl_df", "tbl", "data.frame"))

Yours sincerely Bharat Rawlley    On Tuesday, 19 January, 2021, 03:53:27 pm IST, 
Michael Dewey <li...@dewey.myzen.co.uk> wrote:
Unfortunately your data did not come through. Try using dput() and then
pasting that into the body of your e-mail message.

On 18/01/2021 17:26, bharat rawlley via R-help wrote:
Hello,
On running the Wilcoxon Rank Sum test in R and SPSS, I am getting the following 
discrepancies which I am unable to explain.
Q1 In the attached data set, I was trying to compare freq4w_n in those with 
drug_code 0 vs 1. SPSS gives a P value 0.031 vs R gives a P value 0.001779.
The code I used in R is as follows -
wilcox.test(freq4w_n, drug_code, conf.int = T)


Q2 Similarly, in the same data set, when trying to compare PFD_n in those with 
drug_code 0 vs 1, SPSS gives a P value 0.038 vs R gives a P value < 2.2e-16.
The code I used in R is as follows -
wilcox.test(PFD_n, drug_code, mu = 0, alternative = "two.sided", correct = 
TRUE, paired = FALSE, conf.int = TRUE)


I have tried searching on Google and watching some Youtube tutorials, I cannot 
find an answer, Any help will be really appreciated, Thank you!
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