Hi all:
I have a question about multi-comparison.
The data is in the attachment.
My purpose:
Compare the predicted means of the 3 methods(a,b,c) pairwisely.
I have 3 ideas:
#idea1
result_aov<-aov(y~ method + x1 + x2)
TukeyHSD(result_aov)
diff lwr upr p adj
b-a 0.845 0.5861098 1.1038902 0.0000001
c-a 0.790 0.5311098 1.0488902 0.0000002
c-b -0.055 -0.3138902 0.2038902 0.8578386
#idea2
library(multcomp)
summary(glht(result_aov,linfct=mcp(method="Tukey")))
Estimate Std. Error t value Pr(>|t|)
b - a == 0 0.3239 0.1402 2.309 0.0683 .
c - a == 0 -0.3332 0.1937 -1.720 0.2069
c - b == 0 -0.6570 0.1325 -4.960 <0.001 ***
#idea3
#ref=a
dat$method <- relevel(dat$method, ref="a")
lm_ref_a<-lm(y~method + x1 + x2)
summary(lm_ref_a)
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 0.92202 0.64418 1.431 0.1647
methodb 0.32389 0.14025 2.309 0.0295 *
methodc -0.33316 0.19372 -1.720 0.0978 .
x1 0.57935 0.09356 6.192 1.78e-06 ***
x2 0.13596 0.11563 1.176 0.2507
#ref=b
dat$method <- relevel(dat$method, ref="b")
lm_ref_b<-lm(y~method + x1 + x2)
summary(lm_ref_b)
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 1.24591 0.73770 1.689 0.1037
methoda -0.32389 0.14025 -2.309 0.0295 *
methodc -0.65705 0.13248 -4.960 4.14e-05 ***
In summary:
idea1:
a vs b:pvalue=0.0000001
a vs c:pvalue=0.0000002
b vs c:pvalue=0.8578386
idea2:
a vs b:pvalue=0.0683
a vs c:pvalue=0.2069
b vs c:pvalue<0.001
idea3:
a vs b:pvalue=0.0295
a vs c:pvalue=0.0978
b vs c:pvalue=4.14e-05
So the result of 3 ideas are different,and I don't know which one is correct.
Many thanks for your help.
My best
method x1 x2 y
a 4.1 5.3 3.95
a 4.6 5 4.35
a 4.8 6 4.55
a 5.4 6.2 4.7
a 5.2 6.1 4.9
a 5.7 5.9 4.9
a 6 6 5
a 5.9 6.1 5.5
a 4.2 5.2 4.05
a 4.6 5 4.3
b 5.5 6.2 5.1
b 5 7.1 5.3
b 6 7 5.5
b 6.2 6.1 5.65
b 5.9 6.5 5.7
b 5.2 6.8 5.4
b 6.4 7.1 5.95
b 5.4 6.1 4.95
b 6.1 6 5.55
b 5.8 6.4 5.55
c 6.1 7.1 5.05
c 6.3 7 5.2
c 6.5 6.2 5.45
c 6.7 6.8 5.55
c 7 7.1 5.85
c 6.5 6.9 4.9
c 7.1 6.7 6
c 6.9 7 4.9
c 6.7 6.9 5.35
c 7.2 7.4 5.85
______________________________________________
R-help@r-project.org 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.
