glm {base} | R Documentation

## Fitting Generalized Linear Models

### Description

`glm` is used to fit generalized linear models, specified by giving a symbolic description of the linear predictor and a description of the error distribution.

### Usage

    glm(formula, family = gaussian, data, weights = NULL, subset = NULL,
        na.action, start = NULL, offset = NULL,
        control = glm.control(...), model = TRUE, method = "glm.fit",
        x = FALSE, y = TRUE, contrasts = NULL, ...)

    glm.fit(x, y, weights = rep(1, nrow(x)),
            start = NULL, etastart  =  NULL, mustart = NULL,
            offset = rep(0, nrow(x)),
            family = gaussian(), control = glm.control(),
            intercept = TRUE)
    glm.fit.null(x, y, weights = rep(1, nrow(x)),
            start = NULL, etastart  =  NULL, mustart = NULL,
            offset = rep(0, nrow(x)),
            family = gaussian(), control = glm.control(),
            intercept = FALSE)
    weights(object, type = c("prior", "working"), ...)

### Arguments

`formula` | a symbolic description of the model to be fit. The details of model specification are given below.
`family` | a description of the error distribution and link function to be used in the model. See `family` for details.
`data` | an optional data frame containing the variables in the model. By default the variables are taken from `environment(formula)`, typically the environment from which `glm` is called.
`weights` | an optional vector of weights to be used in the fitting process.
`subset` | an optional vector specifying a subset of observations to be used in the fitting process.
`na.action` | a function which indicates what should happen when the data contain `NA`s. The default is set by the `na.action` setting of `options`, and is `na.fail` if that is unset. The ``factory-fresh'' default is `na.omit`.
`start` | starting values for the parameters in the linear predictor.
`etastart` | starting values for the linear predictor.
`mustart` | starting values for the vector of means.
`offset` | this can be used to specify an a priori known component to be included in the linear predictor during fitting.
`control` | a list of parameters for controlling the fitting process. See the documentation for `glm.control` for details.
`model` | a logical value indicating whether model frame should be included as a component of the returned value.
`method` | the method to be used in fitting the model. The default (and presently only) method `glm.fit` uses iteratively reweighted least squares (IWLS).
`x, y` | For `glm`: logical values indicating whether the response vector and model matrix used in the fitting process should be returned as components of the returned value. For `glm.fit`: `x` is a design matrix of dimension `n * p`, and `y` is a vector of observations of length `n`.
`contrasts` | an optional list. See the `contrasts.arg` of `model.matrix.default`.
`object` | an object inheriting from class `"glm"`.
`type` | character, partial matching allowed. Type of weights to extract from the fitted model object.
`intercept` | logical. Should an intercept be included?
`...` | further arguments passed to or from other methods.

### Details

A typical predictor has the form `response ~ terms` where `response` is the (numeric) response vector and `terms` is a series of terms which specifies a linear predictor for `response`. For `binomial` models the response can also be specified as a `factor` (when the first level denotes failure and all others success) or as a two-column matrix with the columns giving the numbers of successes and failures. A terms specification of the form `first + second` indicates all the terms in `first` together with all the terms in `second` with duplicates removed.

A specification of the form `first:second` indicates the the set of terms obtained by taking the interactions of all terms in `first` with all terms in `second`. The specification `first*second` indicates the cross of `first` and `second`. This is the same as `first + second + first:second`.

`glm.fit` and `glm.fit.null` are the workhorse functions: the former calls the latter for a null model (with no intercept).

If more than one of `etastart`, `start` and `mustart` is specified, the first in the list will be used.

### Value

`glm` returns an object of class `glm` which inherits from the class `lm`. See later in this section.
The function `summary` (i.e., `summary.glm`) can be used to obtain or print a summary of the results and the function `anova` (i.e., `anova.glm`) to produce an analysis of variance table.
The generic accessor functions `coefficients`, `effects`, `fitted.values` and `residuals` can be used to extract various useful features of the value returned by `glm`.
`weights` extracts a vector of weights, one for each case in the fit (after subsetting and `na.action`).
An object of class `"glm"` is a list containing at least the following components:

`coefficients` | a named vector of coefficients
`residuals` | the working residuals, that is the residuals in the final iteration of the IWLS fit.
`fitted.values` | the fitted mean values, obtained by transforming the linear predictors by the inverse of the link function.
`rank` | the numeric rank of the fitted linear model.
`family` | the `family` object used.
`linear.predictors` | the linear fit on link scale.
`deviance` | up to a constant, minus twice the maximized log-likelihood. Where sensible, the constant is chosen so that a saturated model has deviance zero.
`aic` | Akaike's An Information Criterion, minus twice the maximized log-likelihood plus twice the number of coefficients (so assuming that the dispersion is known.
`null.deviance` | The deviance for the null model, comparable with `deviance`. The null model will include the offset, and an intercept if there is one in the model
`iter` | the number of iterations of IWLS used.
`weights` | the working weights, that is the weights in the final iteration of the IWLS fit.
`prior.weights` | the case weights initially supplied.
`df.residual` | the residual degrees of freedom.
`df.null` | the residual degrees of freedom for the null model.
`y` | the `y` vector used. (It is a vector even for a binomial model.)
`converged` | logical. Was the IWLS algorithm judged to have converged?
`boundary` | logical. Is the fitted value on the boundary of the attainable values?
`call` | the matched call.
`formula` | the formula supplied.
`terms` | the `terms` object used.
`data` | the `data argument`.
`offset` | the offset vector used.
`control` | the value of the `control` argument used.
`method` | the name of the fitter function used, in R always `"glm.fit"`.
`contrasts` | (where relevant) the contrasts used.
`xlevels` | (where relevant) a record of the levels of the factors used in fitting.

In addition, non-null fits will have components `qr`, `R` and `effects` relating to the final weighted linear fit.
Objects of class `"glm"` are normally of class `c("glm", "lm")`, that is inherit from class `"lm"`, and well-designed methods for class `"lm"` will be applied to the weighted linear model at the final iteration of IWLS. However, care is needed, as extractor functions for class `"glm"` such as `residuals` and `weights` do not just pick out the component of the fit with the same name.
If a `binomial` `glm` model is specified by giving a two-column response, the weights returned by `prior.weights` are the total numbers of cases (factored by the supplied case weights) and the component `y` of the result is the proportion of successes.

### References

McCullagh P. and Nelder, J. A. (1989) Generalized Linear Models. London: Chapman and Hall.

Dobson, A. J. (1990) An Introduction to Generalized Linear Models. London: Chapman and Hall.

Venables, W. N. and Ripley, B. D. (1999) Modern Applied Statistics with S-PLUS. New York: Springer.

### See Also

`anova.glm`, `summary.glm`, etc. for `glm` methods, and the generic functions `anova`, `summary`, `effects`, `fitted.values`, and `residuals`. Further, `lm` for non-generalized linear models.

`esoph`, `infert` and `predict.glm` have examples of fitting binomial glms.

### Examples

    ## Dobson (1990) Page 93: Randomized Controlled Trial :
    counts <- c(18,17,15,20,10,20,25,13,12)
    outcome <- gl(3,1,9)
    treatment <- gl(3,3)
    print(d.AD <- data.frame(treatment, outcome, counts))
    glm.D93 <- glm(counts ~ outcome + treatment, family=poisson())
    anova(glm.D93)
    summary(glm.D93)

    ## an example with offsets from Venables & Ripley (1999, pp.217-8)

    ## Need the anorexia data from a recent version of the package MASS:
    library(MASS)
    data(anorexia)

    anorex.1 <- glm(Postwt ~ Prewt + Treat + offset(Prewt),
                family = gaussian, data = anorexia)
    summary(anorex.1)

    # A Gamma example, from McCullagh & Nelder (1989, pp. 300-2)
    clotting <- data.frame(
        u = c(5,10,15,20,30,40,60,80,100),
        lot1 = c(118,58,42,35,27,25,21,19,18),
        lot2 = c(69,35,26,21,18,16,13,12,12))
    summary(glm(lot1 ~ log(u), data=clotting, family=Gamma))
    summary(glm(lot2 ~ log(u), data=clotting, family=Gamma))

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