Hi Everyone,
I am uncertain that I am writing the contrast statements correctly. Basically,
I'm unsure when to use a -1 and a 1 when writing the contrasts. Specifically I
am interested in comparing the slopes between different temperature regimes.
Temperature is therefore a factor. Time and percent are numerical. Using the
gmodels package I made the following model:
m2<-lm(Percent~Time+Temperature, data=Hchrys.Temp);summary(m2)
# results from m2
Call:
lm(formula = Percent ~ Time + Temperature, data = Hchrys.Temp)
Residuals:
Min 1Q Median 3Q Max
-0.098333 -0.031667 -0.003333 0.026667 0.101667
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) -0.0233333 0.0147504 -1.582 0.119413
Time 0.0007639 0.0001774 4.306 6.91e-05 ***
Temperature22:18 0.0133333 0.0170324 0.783 0.437088
Temperature22:20 0.0133333 0.0170324 0.783 0.437088
Temperature22:22 0.0666667 0.0170324 3.914 0.000252 ***
---
Signif. codes: 0 *** 0.001 ** 0.01 * 0.05 . 0.1 1
Residual standard error: 0.04665 on 55 degrees of freedom
Multiple R-squared: 0.3997, Adjusted R-squared: 0.356
F-statistic: 9.154 on 4 and 55 DF, p-value: 9.653e-06
# I then built the contrasts based on the order of the coefficients above.
cm1<-rbind("1:Temperature 20:20 v.22:22 " = c(0,0,0,0,1),
"2:Temperature 22:18 v.22:22 " = c(0,0,-1,0,1),
"3:Temperature 22:20 v.22:22 " = c(0,0,0,-1,1),
"4:Temperature 20:20 v.22:20 " = c(0,0,0,1,0),
"5:Temperature 22:18 v.22:20 " = c(0,0,-1,1,0),
"6:Temperature 20:20 v.22:18 " = c(0,0,1,0,0))
# To compare between them I used:
estimable(m2,cm1)
# The results of which are below.
Estimate Std. Error t value DF
Pr(>|t|)
1:Temperature 20:20 v.22:22 6.666667e-02 0.01703235 3.914120e+00 55
0.0002522632
2:Temperature 22:18 v.22:22 5.333333e-02 0.01703235 3.131296e+00 55
0.0027865530
3:Temperature 22:20 v.22:22 5.333333e-02 0.01703235 3.131296e+00 55
0.0027865530
4:Temperature 20:20 v.22:20 1.333333e-02 0.01703235 7.828240e-01 55
0.4370882317
5:Temperature 22:18 v.22:20 -3.469447e-17 0.01703235 -2.036975e-15 55
1.0000000000
6:Temperature 20:20 v.22:18 1.333333e-02 0.01703235 7.828240e-01 55
0.4370882317
Did I write the contrasts correctly? And does this then indicate that the slope
of 22:22 was significantly different from all others but none of the others
were different?
Help with comparing the slopes between these regressions would be wonderful.
Cheers,
-
Tyler Hallman, M.S.
Ph.D. Student
The Robinson Lab
Department of Fisheries and Wildlife
Oregon State University Corvallis
[[alternative HTML version deleted]]
______________________________________________
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.
