[R] Two sample t.test, order of comparions
Dimitris Rizopoulos
dimitris.rizopoulos at med.kuleuven.be
Wed Apr 18 15:32:58 CEST 2007
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" <helmut.schuetz at bebac.at>
To: <r-help at 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 helmut.schuetz at bebac.at
web http://bebac.at
forum http://forum.bebac.at
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