dpit: DPIT residuals for regression models with various...

In assessor: Assessment Tools for Regression Models with Discrete and Semicontinuous Outcomes

DPIT residuals for regression models with various non-continuous outcomes

Description

Calculates DPIT residuals for regression models with non-continuous outcomes. In particular, model assumptions for GLMs with discrete outcomes (e.g., binary, Poisson, and negative binomial), ordinal regression models, zero-inflated regression models, and semicontinuous outcome models can be assessed using dpit().

Usage

dpit(model, plot=TRUE, scale="normal", line_args=list(), ...)

Arguments

model	A model object.
plot	A logical value indicating whether or not to return QQ-plot
scale	You can choose the scale of the residuals among normal and uniform. The sample quantiles of the residuals are plotted against the theoretical quantiles of a standard normal distribution under the normal scale, and against the theoretical quantiles of a uniform (0,1) distribution under the uniform scale. The default scale is normal.
line_args	A named list of graphical parameters passed to graphics::abline() to modify the reference (red) 45° line in the QQ plot. If left empty, a default red dashed line is drawn.
...	Additional graphical arguments passed to stats::qqplot() for customizing the QQ plot (e.g., pch, col, cex, xlab, ylab).

Details

This function determines the appropriate computation based on the class of model. The supported model objects and outcome types are listed below.

In addition to the class-based interface, the package also provides distribution-specific DPIT calculators. If a fitted model comes from a different class but has a supported outcome distribution, users can call the corresponding distribution-based function directly. For instance, for a regression model with Poisson outcomes, one can use dpit to calculate the residuals if the model is fit using glm function, or to use dpit_pois upon supplying fitted mean values.

• Discrete outcomes

  • glm with family = binomial() (see dpit_bin).

  • glm with family = poisson() (see dpit_pois).

  • glm.nb for negative binomial regression (see dpit_nb).

  • polr for ordinal outcomes (see dpit_ordi).

• Zero-inflated discrete outcomes

  • zeroinfl with dist = "poisson" (see dpit_zpois).

  • zeroinfl with dist = "negbin" (see dpit_znb).

• Semicontinuous outcomes

  • Tobit regression via tobit from AER or vglm from VGAM (see dpit_tobit).

  • Tweedie regression via glm with a Tweedie family (see dpit_tweedie).

Formulation for Discrete and Zero-Inflated Outcomes:
The DPIT residual for the ith observation is defined as follows:

\hat{r}(Y_i|X_i) = \hat{G}\bigg(\hat{F}(Y_i|\mathbf{X}_i)\bigg)

where

\hat{G}(s) = \frac{1}{n-1}\sum_{j=1, j \neq i}^{n}\hat{F}\bigg(\hat{F}^{(-1)}(\mathbf{X}_j)\bigg|\mathbf{X}_j\bigg)

and \hat{F} refers to the fitted cumulative distribution function. When scale="uniform", DPIT residuals should closely follow a uniform distribution, otherwise it implies model deficiency. When scale="normal", it applies the normal quantile transformation to the DPIT residuals

\Phi^{-1}\left[\hat{r}(Y_i|\mathbf{X}_i)\right],i=1,\ldots,n.

The null pattern is the standard normal distribution in this case.

Formulation for Semicontinuous Outcomes:
The DPIT residuals for regression models with semi-continuous outcomes are

\hat{r}_i=\frac{\hat{F}(Y_i|\mathbf{X}_i)}{n}\sum_{j=1}^n1\left(\hat{p}_0(\mathbf{X}_j)\leq \hat{F}(Y_i|\mathbf{X}_i)\right), i=1,\ldots,n,

where \hat{p}_0(\mathbf{X}_i) is the fitted probability of zero, and \hat{F}(\cdot|\mathbf{X}_i) is the fitted cumulative distribution function for the ith observation. Furthermore,

\hat{F}(y|\mathbf{x})=\hat{p}_0(\mathbf{x})+\left(1-\hat{p}_0(\mathbf{x})\right)\hat{G}(y|\mathbf{x})

where \hat{G} is the fitted cumulative distribution for the positive data.

Value

DPIT residuals. If plot=TRUE, also produces a QQ plot.

References

L. Yang. Double probability integral transform residuals for regression models with discrete outcomes. Journal of Computational and Graphical Statistics, 33(3), pp.787–803, 2024.
L. Yang. Diagnostics for regression models with semicontinuous outcomes. Biometrics, 80(1), ujae007, 2024.

Examples

library(MASS)
n <- 500
set.seed(1234)
## Negative Binomial example
# Covariates
x1 <- rnorm(n)
x2 <- rbinom(n, 1, 0.7)
### Parameters
beta0 <- -2
beta1 <- 2
beta2 <- 1
size1 <- 2
lambda1 <- exp(beta0 + beta1 * x1 + beta2 * x2)
# generate outcomes
y <- rnbinom(n, mu = lambda1, size = size1)

# True model
model1 <- glm.nb(y ~ x1 + x2)
resid.nb1 <- dpit(model1, plot = TRUE, scale = "uniform")

# Overdispersion
model2 <- glm(y ~ x1 + x2, family = poisson(link = "log"))
resid.nb2 <- dpit(model2, plot = TRUE, scale = "normal")

## Binary example
n <- 500
set.seed(1234)
# Covariates
x1 <- rnorm(n, 1, 1)
x2 <- rbinom(n, 1, 0.7)
# Coefficients
beta0 <- -5
beta1 <- 2
beta2 <- 1
beta3 <- 3
q1 <- 1 / (1 + exp(beta0 + beta1 * x1 + beta2 * x2 + beta3 * x1 * x2))
y1 <- rbinom(n, size = 1, prob = 1 - q1)

# True model
model01 <- glm(y1 ~ x1 * x2, family = binomial(link = "logit"))
resid.bin1 <- dpit(model01, plot = TRUE)

# Missing covariates
model02 <- glm(y1 ~ x1, family = binomial(link = "logit"))
resid.bin2 <- dpit(model02, plot = TRUE)

## Poisson example
n <- 500
set.seed(1234)
# Covariates
x1 <- rnorm(n)
x2 <- rbinom(n, 1, 0.7)
# Coefficients
beta0 <- -2
beta1 <- 2
beta2 <- 1
lambda1 <- exp(beta0 + beta1 * x1 + beta2 * x2)
y <- rpois(n, lambda1)

# True model
poismodel1 <- glm(y ~ x1 + x2, family = poisson(link = "log"))
resid.poi1 <- dpit(poismodel1, plot = TRUE)

# Enlarge three outcomes
y <- rpois(n, lambda1) + c(rep(0, (n - 3)), c(10, 15, 20))
poismodel2 <- glm(y ~ x1 + x2, family = poisson(link = "log"))
resid.poi2 <- dpit(poismodel2, plot = TRUE)

## Ordinal example
n <- 500
set.seed(1234)
# Covariates
x1 <- rnorm(n, mean = 2)
# Coefficient
beta1 <- 3

# True model
p0 <- plogis(1, location = beta1 * x1)
p1 <- plogis(4, location = beta1 * x1) - p0
p2 <- 1 - p0 - p1
genemult <- function(p) {
  rmultinom(1, size = 1, prob = c(p[1], p[2], p[3]))
}
test <- apply(cbind(p0, p1, p2), 1, genemult)
y1 <- rep(0, n)
y1[which(test[1, ] == 1)] <- 0
y1[which(test[2, ] == 1)] <- 1
y1[which(test[3, ] == 1)] <- 2
multimodel <- polr(as.factor(y1) ~ x1, method = "logistic")
resid.ord1 <- dpit(multimodel, plot = TRUE)

## Non-Proportionality
n <- 500
set.seed(1234)
x1 <- rnorm(n, mean = 2)
beta1 <- 3
beta2 <- 1
p0 <- plogis(1, location = beta1 * x1)
p1 <- plogis(4, location = beta2 * x1) - p0
p2 <- 1 - p0 - p1
genemult <- function(p) {
  rmultinom(1, size = 1, prob = c(p[1], p[2], p[3]))
}
test <- apply(cbind(p0, p1, p2), 1, genemult)
y1 <- rep(0, n)
y1[which(test[1, ] == 1)] <- 0
y1[which(test[2, ] == 1)] <- 1
y1[which(test[3, ] == 1)] <- 2
multimodel <- polr(as.factor(y1) ~ x1, method = "logistic")
resid.ord2 <- dpit(multimodel, plot = TRUE)

assessor documentation built on March 23, 2026, 1:06 a.m.