R: Zero-Altered Poisson Distribution

Zapois {VGAM} | R Documentation

## Zero-Altered Poisson Distribution

### Description

Density, distribution function, quantile function and random generation for the zero-altered Poisson distribution with parameter `pobs0`.

### Usage

    dzapois(x, lambda, pobs0 = 0, log = FALSE)
    pzapois(q, lambda, pobs0 = 0)
    qzapois(p, lambda, pobs0 = 0)
    rzapois(n, lambda, pobs0 = 0)

### Arguments

`x, q` | vector of quantiles.
`p` | vector of probabilities.
`n` | number of observations. If `length(n) > 1` then the length is taken to be the number required.
`lambda` | Vector of positive means.
`pobs0` | Probability of zero, called pobs0. The default value of `pobs0 = 0` corresponds to the response having a positive Poisson distribution.
`log` | Logical. Return the logarithm of the answer?

### Details

The probability function of Y is 0 with probability `pobs0`, else a positive Poisson(lambda).

### Value

`dzapois` gives the density, `pzapois` gives the distribution function, `qzapois` gives the quantile function, and `rzapois` generates random deviates.

### Note

The argument `pobs0` is recycled to the required length, and must have values which lie in the interval [0,1].

### Author(s)

T. W. Yee

### See Also

`zapoisson`, `dzipois`.

### Examples

    lambda <- 3; pobs0 <- 0.2; x <- (-1):7
    (ii <- dzapois(x, lambda, pobs0))
    max(abs(cumsum(ii) - pzapois(x, lambda, pobs0)))  # Should be 0
    table(rzapois(100, lambda, pobs0))
    table(qzapois(runif(100), lambda, pobs0))
    round(dzapois(0:10, lambda, pobs0) * 100)  # Should be similar

    ## Not run:  x <- 0:10
    barplot(rbind(dzapois(x, lambda, pobs0), dpois(x, lambda)),
            beside = TRUE, col = c("blue", "green"), las = 1,
            main = paste("ZAP(", lambda, ", pobs0 = ", pobs0, ") [blue] vs",
                       " Poisson(", lambda, ") [green] densities", sep = ""),
            names.arg = as.character(x), ylab = "Probability")
    ## End(Not run)

[Package VGAM version 1.0-3 Index]