[R] polr question

[Troels Ring]

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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