[R] polr question
Troels Ring
tring at mail1.stofanet.dk
Sat Mar 11 22:51:31 CET 2000
Dear friends.
Do Polr in Mass change the sign of the coefficients ? Example (McCullagh 1980)
options(contrasts=c("contr.treatment","contr.poly"))
library(Mass)
freq <- c(19,29,24,497,560,269)
yy <- ordered(gl(3,1,6))
z4 <- polr(yy~x,weights=freq)
> z4
Call:
polr(formula = yy ~ x, weights = freq)
Coefficients:
x2
-0.6026492
Intercepts:
1|2 2|3
-1.1111584 0.7600705
Residual Deviance: 2955.49
AIC: 2961.49
- since as specified the logit for the non-carrier for being in the lower categories is higher ?
as also got from Lindsey's library:
library(gnlm)
y <- c(0:2,0:2) #must start at zero
x <- gl(2,3,6)
z3 <- ordglm(y~x,weights=freq)
z3
Call:
ordglm(y ~ x, weights = freq)
-Log likelihood 1477.747
AIC 1480.747
Deviance 0.3022138
df 3
Iterations 4
Location coefficients
estimate s.e.
(Intercept) -1.1120 0.2255
x2 0.6035 0.0676
Intercept contrasts
estimate s.e.
b[2] 1.871 0.2225
Correlation matrix
1 2 3
1 1.0000 -0.96836 -0.17098
2 -0.9684 1.00000 0.06321
3 -0.1710 0.06321 1.00000
--which also gives the correlation.
The fits are otherwise very similar, but why the difference in coefficients ?
Troels Ring, M.D
Department of Nephrology
Aalborg, Denmark
tring at mail1.stofanet.dk
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