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]] ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see 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.