Add Alternative Variance Estimates to Models Estimates

A simple object-orientation infrastructure to add alternative standard errors, e.g. sandwich estimates or New-West standard errors to fitted regression-type models, such as fitted by lm() or glm().

Usage

withSE(object, vcov, ...) 

withVCov(object, vcov, ...)

# S3 method for lm
withVCov(object, vcov, ...)

# S3 method for withVCov
summary(object, ...)
# S3 method for withVCov.lm
summary(object, ...)

Arguments

object

a fitted model object

vcov

a function that returns a variance matrix estimate, a given matrix that is such an estimate, or a character string that identifies a function that returns a variance matrix estimate (e.g. "HAC" for vcovHAC).

...

further arguments, passed to vcov() or, respectively, to the parent method of summary()

Details

Using withVCov() an alternative variance-covariance matrix is attributed to a fitted model object. Such a matrix may be produced by any of the variance estimators provided by the "sandwich" package or any package that extends it.

withVCov() has no consequences on how a fitted model itself is printed or represented, but it does have consequences what standard errors are reported, when the function summary() or the function mtable() is applied.

withSE() is a convenience front-end to withVCov(). It can be called in the same way as withVCov, but also allows to specify the type of variance estimate by a character string that identifies the function that gives the covariance matrix (e.g. "OPG" for vcovOPG).

Value

withVCov returns a slightly modified model object: It adds an attribute named ".VCov" that contains the alternate covaraince matrix and modifies the class attribute. If e.g. the original model object has class "lm" then the model object modified by withVCov has the class attribute c("withVCov.lm", "withVCov", "lm").

Examples

## Generate poisson regression relationship
x <- sin(1:100)
y <- rpois(100, exp(1 + x))
## compute usual covariance matrix of coefficient estimates
fm <- glm(y ~ x, family = poisson)

library(sandwich)
fmo <- withVCov(fm,vcovOPG)
vcov(fm)
#>              (Intercept)            x
#> (Intercept)  0.004336442 -0.003335415
#> x           -0.003335415  0.007876288
vcov(fmo)
#>              (Intercept)            x
#> (Intercept)  0.004033961 -0.002251858
#> x           -0.002251858  0.006595366

summary(fm)
#> 
#> Call:
#> glm(formula = y ~ x, family = poisson)
#> 
#> Coefficients:
#>             Estimate Std. Error z value Pr(>|z|)    
#> (Intercept)  1.02270    0.06585   15.53   <2e-16 ***
#> x            0.93317    0.08875   10.52   <2e-16 ***
#> ---
#> Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
#> 
#> (Dispersion parameter for poisson family taken to be 1)
#> 
#>     Null deviance: 243.06  on 99  degrees of freedom
#> Residual deviance: 114.31  on 98  degrees of freedom
#> AIC: 381.24
#> 
#> Number of Fisher Scoring iterations: 5
#> 
summary(fmo)
#> 
#> Call:
#> glm(formula = y ~ x, family = poisson)
#> 
#> Coefficients:
#>             Estimate Std. Error z value Pr(>|z|)    
#> (Intercept)  1.02270    0.06351   16.10   <2e-16 ***
#> x            0.93317    0.08121   11.49   <2e-16 ***
#> ---
#> Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
#> 
#> (Dispersion parameter for poisson family taken to be 1)
#> 
#>     Null deviance: 243.06  on 99  degrees of freedom
#> Residual deviance: 114.31  on 98  degrees of freedom
#> AIC: 381.24
#> 
#> Number of Fisher Scoring iterations: 5
#> 

mtable(Default=fm,
       OPG=withSE(fm,"OPG"),
       summary.stats=c("Deviance","N")
       )
#> 
#> Calls:
#> Default: glm(formula = y ~ x, family = poisson)
#> OPG: glm(formula = y ~ x, family = poisson)
#> 
#> =======================================
#>                 Default       OPG      
#> ---------------------------------------
#>   (Intercept)    1.023***    1.023***  
#>                 (0.066)     (0.064)    
#>   x              0.933***    0.933***  
#>                 (0.089)     (0.081)    
#> ---------------------------------------
#>   Deviance     114.309     114.309     
#>   N            100         100         
#> =======================================
#>   Significance: *** = p < 0.001;   
#>                 ** = p < 0.01;   
#>                 * = p < 0.05  

vo <- vcovOPG(fm)

mtable(Default=fm,
       OPG=withSE(fm,vo),
       summary.stats=c("Deviance","N")
       )
#> 
#> Calls:
#> Default: glm(formula = y ~ x, family = poisson)
#> OPG: glm(formula = y ~ x, family = poisson)
#> 
#> =======================================
#>                 Default       OPG      
#> ---------------------------------------
#>   (Intercept)    1.023***    1.023***  
#>                 (0.066)     (0.064)    
#>   x              0.933***    0.933***  
#>                 (0.089)     (0.081)    
#> ---------------------------------------
#>   Deviance     114.309     114.309     
#>   N            100         100         
#> =======================================
#>   Significance: *** = p < 0.001;   
#>                 ** = p < 0.01;   
#>                 * = p < 0.05