take a look at ?relevel()
Best, Dimitris ---- Dimitris Rizopoulos Ph.D. Student Biostatistical Centre School of Public Health Catholic University of Leuven Address: Kapucijnenvoer 35, Leuven, Belgium Tel: +32/(0)16/336899 Fax: +32/(0)16/337015 Web: http://med.kuleuven.be/biostat/ http://www.student.kuleuven.be/~m0390867/dimitris.htm ----- Original Message ----- From: "Helmut Schütz" <[EMAIL PROTECTED]> To: <r-help@stat.math.ethz.ch> Sent: Wednesday, April 18, 2007 2:46 PM Subject: [R] Two sample t.test, order of comparions Dear group members, I want to compare response variables ("logAUC") of two groups (treatment "Test", "Reference") of a subset ("period == 1") in dataframe "resp" (below): sequence subject period treatment AUC logAUC 1 RT 1 1 Reference 44.1 3.786460 2 RT 1 2 Test 39.1 3.666122 3 RT 2 1 Reference 33.6 3.514526 4 RT 2 2 Test 23.8 3.169686 5 RT 3 1 Reference 45.5 3.817712 6 RT 3 2 Test 40.8 3.708682 7 TR 4 1 Test 19.5 2.970414 8 TR 4 2 Reference 21.1 3.049273 9 TR 5 1 Test 67.2 4.207673 10 TR 5 2 Reference 51.5 3.941582 11 TR 6 1 Test 25.7 3.246491 12 TR 6 2 Reference 30.1 3.404525 13 RT 7 1 Reference 35.3 3.563883 14 RT 7 2 Test 26.7 3.284664 15 RT 8 1 Reference 26.0 3.258097 16 RT 8 2 Test 36.5 3.597312 17 RT 9 1 Reference 38.2 3.642836 18 RT 9 2 Test 57.8 4.056989 19 TR 10 1 Test 33.6 3.514526 20 TR 10 2 Reference 32.5 3.481240 21 TR 11 1 Test 25.1 3.222868 22 TR 11 2 Reference 36.8 3.605498 23 TR 12 1 Test 44.1 3.786460 24 TR 12 2 Reference 42.9 3.758872 25 RT 13 1 Reference 25.6 3.242592 26 RT 13 2 Test 20.1 3.000720 27 RT 14 1 Reference 58.0 4.060443 28 RT 14 2 Test 45.3 3.813307 29 RT 15 1 Reference 47.2 3.854394 30 RT 15 2 Test 51.8 3.947390 31 TR 16 1 Test 16.5 2.803360 32 TR 16 2 Reference 21.4 3.063391 33 TR 17 1 Test 47.3 3.856510 34 TR 17 2 Reference 39.4 3.673766 35 TR 18 1 Test 22.6 3.117950 36 TR 18 2 Reference 17.3 2.850707 37 RT 19 1 Reference 17.5 2.862201 38 RT 19 2 Test 30.1 3.404525 39 RT 20 1 Reference 51.7 3.945458 40 RT 20 2 Test 36.0 3.583519 41 RT 21 1 Reference 24.5 3.198673 42 RT 21 2 Test 18.2 2.901422 43 TR 22 1 Test 36.3 3.591818 44 TR 22 2 Reference 27.2 3.303217 45 TR 23 1 Test 29.4 3.380995 46 TR 23 2 Reference 39.6 3.678829 47 TR 24 1 Test 18.3 2.906901 48 TR 24 2 Reference 20.7 3.030134 The formula method of t.test > result <- t.test(logAUC ~ treatment, data = resp, subset = (period == 1), var.equal = FALSE, conf.level = 0.90) > result gives Welch Two Sample t-test data: logAUC by treatment t = 1.1123, df = 21.431, p-value = 0.2783 alternative hypothesis: true difference in means is not equal to 0 90 percent confidence interval: -0.0973465 0.4542311 sample estimates: mean in group Reference mean in group Test 3.562273 3.383831 Now I'm interested rather in the confidence interval of "Test" - "Reference" rather than "Reference" - "Test" which is given by t.test Do you know a more elegant way than the clumsy one I have tried? > as.numeric(exp(result$estimate[2]-result$estimate[1])) > as.numeric(exp(-result$conf.int[2])) > as.numeric(exp(-result$conf.int[1])) Best regards, Helmut -- Ing. Helmut Schütz BEBAC - Consultancy Services for Bioequivalence and Bioavailability Studies Neubaugasse 36/11 1070 Vienna, Austria tel/fax +43 1 2311746 e-mail [EMAIL PROTECTED] web http://bebac.at forum http://forum.bebac.at ______________________________________________ R-help@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. Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm ______________________________________________ R-help@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.