propodds {VGAM} | R Documentation

## Proportional Odds Model for Ordinal Regression

### Description

Fits the proportional odds model to a (preferably ordered) factor response.

### Usage

    propodds(reverse = TRUE, whitespace = FALSE)

### Arguments

`reverse, whitespace` | Logical. Fed into arguments of the same name in `cumulative`.

### Details

The proportional odds model is a special case from the class of cumulative link models. It involves a logit link applied to cumulative probabilities and a strong parallelism assumption. A parallelism assumption means there is less chance of numerical problems because the fitted probabilities will remain between 0 and 1; however the parallelism assumption ought to be checked, e.g., via a likelihood ratio test. This VGAM family function is merely a shortcut for `cumulative(reverse = reverse, link = "logit", parallel = TRUE)`. Please see `cumulative` for more details on this model.

### Value

An object of class `"vglmff"` (see `vglmff-class`). The object is used by modelling functions such as `vglm`, and `vgam`.

### Warning

No check is made to verify that the response is ordinal if the response is a matrix; see `ordered`.

### Author(s)

Thomas W. Yee

### References

Agresti, A. (2010) Analysis of Ordinal Categorical Data, 2nd ed. Hoboken, NJ, USA: Wiley.

Yee, T. W. (2010) The VGAM package for categorical data analysis. Journal of Statistical Software, 32, 1–34. http://www.jstatsoft.org/v32/i10/.

Yee, T. W. and Wild, C. J. (1996) Vector generalized additive models. Journal of the Royal Statistical Society, Series B, Methodological, 58, 481–493.

### See Also

`cumulative`.

### Examples

    # Fit the proportional odds model, p.179, in McCullagh and Nelder (1989)
    pneumo <- transform(pneumo, let = log(exposure.time))
    (fit <- vglm(cbind(normal, mild, severe) ~ let, propodds, data = pneumo))
    depvar(fit)  # Sample proportions
    weights(fit, type = "prior")  # Number of observations
    coef(fit, matrix = TRUE)
    constraints(fit)  # Constraint matrices
    summary(fit)

    # Check that the model is linear in let ----------------------
    fit2 <- vgam(cbind(normal, mild, severe) ~ s(let, df = 2), propodds, data = pneumo)
    ## Not run:  plot(fit2, se = TRUE, lcol = 2, scol = 2)

    # Check the proportional odds assumption with a LRT ----------
    (fit3 <- vglm(cbind(normal, mild, severe) ~ let,
                  cumulative(parallel = FALSE, reverse = TRUE), data = pneumo))
    pchisq(deviance(fit) - deviance(fit3),
           df = df.residual(fit) - df.residual(fit3), lower.tail = FALSE)
    lrtest(fit3, fit)  # Easier

[Package VGAM version 1.0-3 Index]