Zipois {VGAM} | R Documentation

## Zero-Inflated Poisson Distribution

### Description

Density, distribution function, quantile function and random generation for the zero-inflated and zero-deflated Poisson distribution with parameter `pstr0`.

### Usage

    dzipois(x, lambda, pstr0 = 0, log = FALSE)
    pzipois(q, lambda, pstr0 = 0)
    qzipois(p, lambda, pstr0 = 0)
    rzipois(n, lambda, pstr0 = 0)

### Arguments

`x, q` | vector of quantiles.
`p` | vector of probabilities.
`n` | number of observations. Must be a single positive integer.
`lambda` | Vector of positive means.
`pstr0` | Probability of a structural zero (i.e., ignoring the Poisson distribution), called phi. The default value of phi = 0 corresponds to the response having an ordinary Poisson distribution. This argument may be negative to allow for 0-deflation, hence its interpretation as a probability ceases.
`log` | Logical. Return the logarithm of the answer?

### Details

The probability function of Y is 0 with probability phi, and Poisson(lambda) with probability 1-phi. Thus

P(Y=0) = phi + (1-phi) * P(W=0)

where W is distributed Poisson(lambda).

### Value

`dzipois` gives the density, `pzipois` gives the distribution function, `qzipois` gives the quantile function, and `rzipois` generates random deviates.

### Note

The argument `pstr0` is recycled to the required length, and must have values which lie in the interval [0,1].

These functions actually allow for the zero-deflated Poisson distribution. Here, `pstr0` is also permitted to lie in the interval `[-1/expm1(lambda), 0]`. The resulting probability of a zero count is less than the nominal Poisson value, and the use of `pstr0` to stand for the probability of a structural zero loses its meaning. When `pstr0` equals `-1/expm1(lambda)` this corresponds to the positive-Poisson distribution (e.g., see `dpospois`).

### Author(s)

T. W. Yee

### See Also

`zipoisson`, `dpois`, `rzinegbin`.

### Examples

    lambda <- 3; pstr0 <- 0.2; x <- (-1):7
    (ii <- dzipois(x, lambda, pstr0 = pstr0))
    max(abs(cumsum(ii) - pzipois(x, lambda, pstr0 = pstr0)))  # Should be 0
    table(rzipois(100, lambda, pstr0 = pstr0))

    table(qzipois(runif(100), lambda, pstr0))
    round(dzipois(0:10, lambda, pstr0 = pstr0) * 100)  # Should be similar

    ## Not run:  x <- 0:10
    par(mfrow = c(2, 1))  # Zero-inflated Poisson
    barplot(rbind(dzipois(x, lambda, pstr0 = pstr0), dpois(x, lambda)),
            beside = TRUE, col = c("blue", "orange"),
            main = paste("ZIP(", lambda, ", pstr0 = ", pstr0, ") (blue) vs",
                         " Poisson(", lambda, ") (orange)", sep = ""),
            names.arg = as.character(x))

    deflat.limit <- -1 / expm1(lambda)  # Zero-deflated Poisson
    newpstr0 <- round(deflat.limit / 1.5, 3)
    barplot(rbind(dzipois(x, lambda, pstr0 = newpstr0),
                    dpois(x, lambda)),
            beside = TRUE, col = c("blue","orange"),
            main = paste("ZDP(", lambda, ", pstr0 = ", newpstr0, ") (blue) vs",
                         " Poisson(", lambda, ") (orange)", sep = ""),
            names.arg = as.character(x))
    ## End(Not run)

[Package VGAM version 1.0-3 Index]