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]]

