[R] Obtaining p values from t-test run with a by function

[Rui Barradas]

Hello,

Or in one go with *apply, function '[[':

sapply(result, '[[', 'p.value')
#         EPA            P
#2.564503e-04 4.173480e-06


Hope this helps,

Rui Barradas

Às 23:20 de 12/06/20, Sarah Goslee escreveu:
> Where you have
> 
> result[1]$p.value
> result[2]$p.value
> 
> You need
> 
> result[[1]]$p.value
> result[[2]]$p.value
> 
> to get the first component of the list.
> 
> Sarah
> 
> On Fri, Jun 12, 2020 at 5:35 PM Sorkin, John <jsorkin using som.umaryland.edu> wrote:
>>
>> Colleagues,
>> I am trying to retrieve the p values produced by a Student's t-test run using a by function, but can not do so. I can easily get the p value when I run s Student's t-test without a by function. What is the secret to obtaining results returned from a function run within a by function.
>>
>> An annotated repeatable example (including data) can be found below.
>> Thank you,
>> John
>>
>>
>> # Test data
>> mydata <- structure(list(Group = structure(c(2L, 1L, 2L, 2L, 1L, 2L, 1L,
>>             2L, 2L, 1L, 2L, 1L, 2L), .Label = c("EPA", "P"), class = "factor"),
>>             WtBaseline = c(76.6, 73.8, 77.6, 91.7, 110.3, 121.7, 82.1,
>>             82.8, 119, 88.4, 75.7, 71.4, 72.1)), class = "data.frame", row.names = c(NA,-13L))
>>
>> cat("This is what mydata looks like\n")
>> mydata
>>
>> result <- by(mydata$WtBaseline,mydata$Group,t.test)
>> cat("Student's t-test run using by command\n")
>> cat("Result has results for both groups, EPA and P\n")
>> result
>>
>> cat("I can isolate the collective results for group EPA\n")
>> result[1]
>> cat("I can isolate the collective results for group P\n")
>> result[2]
>>
>> cat("I cant get the p-values for the gruops")
>> result[1]$p.value
>> result[2]$p.value
>>
>> cat("When run without by function, one can get the p value\n")
>> xxx <- t.test(WtBaseline~Group,data=mydata)
>> cat("t-test run without by fundtion\n")
>> xxx
>> cat("p value isolated from t-test run without by function\n")
>> xxx$p.value
>>
>>
>> John David Sorkin M.D., Ph.D.
>> Professor of Medicine
>> Chief, Biostatistics and Informatics
>> University of Maryland School of Medicine Division of Gerontology and Geriatric Medicine
>> Baltimore VA Medical Center
>> 10 North Greene Street
>> GRECC (BT/18/GR)
>> Baltimore, MD 21201-1524
>> (Phone) 410-605-7119
>> (Fax) 410-605-7913 (Please call phone number above prior to faxing)
>>
>>
>>          [[alternative HTML version deleted]]
>>
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>> and provide commented, minimal, self-contained, reproducible code.
> 
> 
>