expectedDeviance {statmod}

Expected Value of Scaled Unit Deviance for Linear Exponential Families

Description

Expected value and variance of the scaled unit deviance for common generalized linear model families.

Usage

expectedDeviance(mu, family="binomial", binom.size, nbinom.size, gamma.shape)

Arguments

mu

numeric vector or matrix giving mean of response variable.

family

character string indicating the linear exponential family. Possible values are "binomial","gaussian", "Gamma", "inverse.gaussian", "poisson" or "negative.binomial".

binom.size

integer vector giving the number of binomial trials when family = "binomial". Equivalent to the "size" argument of pbinom.

nbinom.size

numeric vector giving the negative binomial size parameter when family = "negative.binomial", such that the variance of the response variable is mu + mu^2 / nbinom.size. Equivalent to the "size" parameter of pnbinom.

gamma.shape

numeric vector giving the gamma shape parameter when family = "Gamma", such that the variance of the response variable is mu^2 / gamma.shape. Equivalent to the "shape" parameter of pgamma.

Details

For a generalized linear model (GLM), the scaled unit deviances can be computed using d <- f$dev.resids(y, mu, wt=1/phi) where f is the GLM family object, y is the response variable, mu is the vector of means and phi is the vector of GLM dispersions (incorporating any prior weights).

The scaled unit deviances are often treated as being chiquare distributed on 1 df, so the mean should be 1 and the variance should be 2. This distribution result only holds however when the saddlepoint approximation is accurate for the response variable distribution (Dunn and Smyth, 2018). In other cases, the expected value and variance of the unit deviances can be far from the nominal values. The expectedDeviance function returns the exact mean and variance of the unit deviance for the usual GLM familes assuming that mu is the true mean and phi is the true dispersion.

When family is "poisson", "binomial" or "negative.binomial", the expected values and variances are computed using Chebyshev polynomial approximations. When family = "Gamma", the function uses exact formulas derived by Smyth (1989).

Value

A list with the components

mean

expected values

variance

variances

both of which have the same length and dimensions as the input mu.

Author(s)

Lizong Chen and Gordon Smyth

References

Dunn PK, Smyth GK (2018). Generalized linear models with examples in R. Springer, New York, NY. doi:10.1007/978-1-4419-0118-7

Smyth, G. K. (1989). Generalized linear models with varying dispersion. J. R. Statist. Soc. B, 51, 47-61. doi:10.1111/j.2517-6161.1989.tb01747.x

See Also

family, meanval.digamma, d2cumulant.digamma.

Examples

# Poisson example
lambda <- 3
nsim <- 1e4
y <- rpois(nsim, lambda=lambda)
d <- poisson()$dev.resids(y=y, mu=rep(lambda,nsim), wt=1)
c(mean=mean(d), variance=var(d))
unlist(expectedDeviance(mu=lambda, family="poisson"))

# binomial example
n <- 10
p <- 0.01
y <- rbinom(nsim, prob=p, size=n)
d <- binomial()$dev.resids(y=y/n, mu=rep(p,nsim), wt=n)
c(mean=mean(d), variance=var(d))
unlist(expectedDeviance(mu=p, family="binomial", binom.size=n))

# gamma example
alpha <- 5
beta <- 2
y <- beta * rgamma(1e4, shape=alpha)
d <- Gamma()$dev.resids(y=y, mu=rep(alpha*beta,n), wt=alpha)
c(mean=mean(d), variance=var(d))
unlist(expectedDeviance(mu=alpha*beta, family="Gamma", gamma.shape=alpha))

# negative binomial example
library(MASS)
mu <- 10
phi <- 0.2
y <- rnbinom(nsim, mu=mu, size=1/phi)
f <- MASS::negative.binomial(theta=1/phi)
d <- f$dev.resids(y=y, mu=rep(mu,nsim), wt=1)
c(mean=mean(d), variance=var(d))
unlist(expectedDeviance(mu=mu, family="negative.binomial", nbinom.size=1/phi))

# binomial expected deviance tends to zero for p small:
p <- seq(from=0.001,to=0.11,len=200)
ed <- expectedDeviance(mu=p,family="binomial",binom.size=10)
plot(p,ed$mean,type="l")