zapoisson {VGAM} | R Documentation

## Zero-Altered Poisson Distribution

### Description

Fits a zero-altered Poisson distribution based on a conditional model involving a Bernoulli distribution and a positive-Poisson distribution.

### Usage

    zapoisson(lpobs0 = "logitlink", llambda = "loglink", type.fitted =
              c("mean", "lambda", "pobs0", "onempobs0"), imethod = 1,
              ipobs0 = NULL, ilambda = NULL, ishrinkage = 0.95, probs.y = 0.35,
              zero = NULL)
    zapoissonff(llambda = "loglink", lonempobs0 = "logitlink", type.fitted =
                c("mean", "lambda", "pobs0", "onempobs0"), imethod = 1,
                ilambda = NULL, ionempobs0 = NULL, ishrinkage = 0.95,
                probs.y = 0.35, zero = "onempobs0")

### Arguments

`lpobs0` | Link function for the parameter pobs0, called `pobs0` here. See `Links` for more choices.
`llambda` | Link function for the usual lambda parameter. See `Links` for more choices.
`type.fitted` | See `CommonVGAMffArguments` and `fittedvlm` for information.
`lonempobs0` | Corresponding argument for the other parameterization. See details below.

`imethod, ipobs0, ionempobs0, ilambda, ishrinkage` | See `CommonVGAMffArguments` for information.
`probs.y, zero` | See `CommonVGAMffArguments` for information.

### Details

The response Y is zero with probability pobs0, else Y has a positive-Poisson(lambda) distribution with probability 1-pobs0. Thus 0 < pobs0 < 1, which is modelled as a function of the covariates. The zero-altered Poisson distribution differs from the zero-inflated Poisson distribution in that the former has zeros coming from one source, whereas the latter has zeros coming from the Poisson distribution too. Some people call the zero-altered Poisson a hurdle model.

For one response/species, by default, the two linear/additive predictors for `zapoisson()` are (logit(pobs0), log(lambda))^T.

The VGAM family function `zapoissonff()` has a few changes compared to `zapoisson()`. These are: (i) the order of the linear/additive predictors is switched so the Poisson mean comes first; (ii) argument `onempobs0` is now 1 minus the probability of an observed 0, i.e., the probability of the positive Poisson distribution, i.e., `onempobs0` is `1-pobs0`; (iii) argument `zero` has a new default so that the `onempobs0` is intercept-only by default. Now `zapoissonff()` is generally recommended over `zapoisson()`. Both functions implement Fisher scoring and can handle multiple responses.

### Value

An object of class `"vglmff"` (see `vglmff-class`). The object is used by modelling functions such as `vglm`, and `vgam`.

The `fitted.values` slot of the fitted object, which should be extracted by the generic function `fitted`, returns the mean mu (default) which is given by

mu = (1-pobs0) * lambda / [1 - exp(-lambda)].

If `type.fitted = "pobs0"` then pobs0 is returned.

### Note

There are subtle differences between this family function and `zipoisson` and `yip88`. In particular, `zipoisson` is a mixture model whereas `zapoisson()` and `yip88` are conditional models.

Note this family function allows pobs0 to be modelled as functions of the covariates.

This family function effectively combines `pospoisson` and `binomialff` into one family function. This family function can handle multiple responses, e.g., more than one species.

### Author(s)

T. W. Yee

### References

Welsh, A. H., Cunningham, R. B., Donnelly, C. F. and Lindenmayer, D. B. (1996) Modelling the abundances of rare species: statistical models for counts with extra zeros. Ecological Modelling, 88, 297–308.

Angers, J-F. and Biswas, A. (2003) A Bayesian analysis of zero-inflated generalized Poisson model. Computational Statistics & Data Analysis, 42, 37–46.

Yee, T. W. (2014) Reduced-rank vector generalized linear models with two linear predictors. Computational Statistics and Data Analysis, 71, 889–902.

### See Also

`rzapois`, `zipoisson`, `genapoisson`, `pospoisson`, `posnegbinomial`, `binomialff`, `rpospois`, `CommonVGAMffArguments`, `simulate.vlm`.

### Examples

    zdata <- data.frame(x2 = runif(nn <- 1000))
    zdata <- transform(zdata, pobs0  = logitlink( -1 + 1*x2, inverse = TRUE),
                              lambda = loglink(-0.5 + 2*x2, inverse = TRUE))
    zdata <- transform(zdata, y = rzapois(nn, lambda, pobs0 = pobs0))

    with(zdata, table(y))
    fit <- vglm(y ~ x2, zapoisson, data = zdata, trace = TRUE)
    fit <- vglm(y ~ x2, zapoisson, data = zdata, trace = TRUE, crit = "coef")
    head(fitted(fit))
    head(predict(fit))
    head(predict(fit, untransform = TRUE))
    coef(fit, matrix = TRUE)
    summary(fit)

    # Another example ------------------------------
    # Data from Angers and Biswas (2003)
    abdata <- data.frame(y = 0:7, w = c(182, 41, 12, 2, 2, 0, 0, 1))
    abdata <- subset(abdata, w > 0)
    Abdata <- data.frame(yy = with(abdata, rep(y, w)))
    fit3 <- vglm(yy ~ 1, zapoisson, data = Abdata, trace = TRUE, crit = "coef")
    coef(fit3, matrix = TRUE)
    Coef(fit3)  # Estimate lambda (they get 0.6997 with SE 0.1520)
    head(fitted(fit3), 1)
    with(Abdata, mean(yy))  # Compare this with fitted(fit3)

[Package VGAM version 1.1-1 Index]