# vcov: Calculate Variance-Covariance Matrix for a Fitted Model...

Description Usage Arguments Details Value

## Description

Returns the variance-covariance matrix of the main parameters of a fitted model object. The “main” parameters of model correspond to those returned by `coef`, and typically do not contain a nuisance scale parameter (`sigma`).

## Usage

 ```1 2 3 4 5 6 7 8``` ```vcov(object, ...) ## S3 method for class 'lm' vcov(object, complete = TRUE, ...) ## and also for '[summary.]glm' and 'mlm' ## S3 method for class 'aov' vcov(object, complete = FALSE, ...) .vcov.aliased(aliased, vc, complete = TRUE) ```

## Arguments

 `object` a fitted model object, typically. Sometimes also a `summary()` object of such a fitted model. `complete` for the `aov`, `lm`, `glm`, `mlm`, and where applicable `summary.lm` etc methods: logical indicating if the full variance-covariance matrix should be returned also in case of an over-determined system where some coefficients are undefined and `coef(.)` contains `NA`s correspondingly. When `complete = TRUE`, `vcov()` is compatible with `coef()` also in this singular case. `...` additional arguments for method functions. For the `glm` method this can be used to pass a `dispersion` parameter.
 `aliased` a `logical` vector typically identical to `is.na(coef(.))` indicating which coefficients are ‘aliased’. `vc` a variance-covariance matrix, typically “incomplete”, i.e., with no rows and columns for aliased coefficients.

## Details

`vcov()` is a generic function and functions with names beginning in `vcov.` will be methods for this function. Classes with methods for this function include: `lm`, `mlm`, `glm`, `nls`, `summary.lm`, `summary.glm`, `negbin`, `polr`, `rlm` (in package MASS), `multinom` (in package nnet) `gls`, `lme` (in package nlme), `coxph` and `survreg` (in package survival).

(`vcov()` methods for summary objects allow more efficient and still encapsulated access when both `summary(mod)` and `vcov(mod)` are needed.)

`.vcov.aliased()` is an auxiliary function useful for `vcov` method implementations which have to deal with singular model fits encoded via NA coefficients: It augments a vcov–matrix `vc` by `NA` rows and columns where needed, i.e., when some entries of `aliased` are true and `vc` is of smaller dimension than `length(aliased)`.

## Value

A matrix of the estimated covariances between the parameter estimates in the linear or non-linear predictor of the model. This should have row and column names corresponding to the parameter names given by the `coef` method.

When some coefficients of the (linear) model are undetermined and hence `NA` because of linearly dependent terms (or an “over specified” model), also called “aliased”, see `alias`, then since R version 3.5.0, `vcov()` (iff `complete = TRUE`, i.e., by default for `lm` etc, but not for `aov`) contains corresponding rows and columns of `NA`s, wherever `coef()` has always contained such `NA`s.