VGAM (version 1.1-6)

# Zipois: 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?

## Value

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

## 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 $$Poisson(\lambda)$$.

zipoisson, Gaitdpois, dpois, rzinegbin.

## Examples

Run this code
# NOT RUN {
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))
# }


Run the code above in your browser using DataLab