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Poisson {stats}

The Poisson Distribution
R 3.0.2


Density, distribution function, quantile function and random generation for the Poisson distribution with parameter lambda.


dpois(x, lambda, log = FALSE)
ppois(q, lambda, lower.tail = TRUE, log.p = FALSE)
qpois(p, lambda, lower.tail = TRUE, log.p = FALSE)
rpois(n, lambda)


vector of (non-negative integer) quantiles.
vector of quantiles.
vector of probabilities.
number of random values to return.
vector of (non-negative) means.
log, log.p
logical; if TRUE, probabilities p are given as log(p).
logical; if TRUE (default), probabilities are P[X ≤ x], otherwise, P[X > x].


The Poisson distribution has density

p(x) = λ^x exp(-λ)/x! for x = 0, 1, 2, ... . The mean and variance are E(X) = Var(X) = λ.

If an element of x is not integer, the result of dpois is zero, with a warning. p(x) is computed using Loader's algorithm, see the reference in dbinom.

The quantile is right continuous: qpois(p, lambda) is the smallest integer x such that P(X ≤ x) ≥ p.

Setting lower.tail = FALSE allows to get much more precise results when the default, lower.tail = TRUE would return 1, see the example below.


dpois gives the (log) density, ppois gives the (log) distribution function, qpois gives the quantile function, and rpois generates random deviates.

Invalid lambda will result in return value NaN, with a warning.

The length of the result is determined by n for rpois, and is the maximum of the lengths of the numerical parameters for the other functions. The numerical parameters other than n are recycled to the length of the result. Only the first elements of the logical parameters are used.

See Also

Distributions for other standard distributions, including dbinom for the binomial and dnbinom for the negative binomial distribution.



-log(dpois(0:7, lambda = 1) * gamma(1+ 0:7)) # == 1
Ni <- rpois(50, lambda = 4); table(factor(Ni, 0:max(Ni)))
1 - ppois(10*(15:25), lambda = 100)  # becomes 0 (cancellation)
    ppois(10*(15:25), lambda = 100, lower.tail = FALSE)  # no cancellation
par(mfrow = c(2, 1))
x <- seq(-0.01, 5, 0.01)
plot(x, ppois(x, 1), type = "s", ylab = "F(x)", main = "Poisson(1) CDF")
plot(x, pbinom(x, 100, 0.01), type = "s", ylab = "F(x)",
     main = "Binomial(100, 0.01) CDF")

Documentation reproduced from R 3.0.2. License: GPL-2.