R: Exact Poisson tests

poisson.test {stats}

R Documentation

Exact Poisson tests

Description

Performs an exact test of a simple null hypothesis about the rate parameter in Poisson distribution, or for the ratio between two rate parameters.

Usage

poisson.test(x, T = 1, r = 1,
    alternative = c("two.sided", "less", "greater"),
    conf.level = 0.95)

Arguments

`x`	number of events. A vector of length one or two.
`T`	time base for event count. A vector of length one or two.
`r`	hypothesized rate or rate ratio
`alternative`	indicates the alternative hypothesis and must be one of `"two.sided"`, `"greater"` or `"less"`. You can specify just the initial letter.
`conf.level`	confidence level for the returned confidence interval.

Details

Confidence intervals are computed similarly to those of binom.test in the one-sample case, and using binom.test in the two sample case.

Value

A list with class "htest" containing the following components:

`statistic`	the number of events (in the first sample if there are two.)
`parameter`	the corresponding expected count
`p.value`	the p-value of the test.
`conf.int`	a confidence interval for the rate or rate ratio.
`estimate`	the estimated rate or rate ratio.
`null.value`	the rate or rate ratio under the null, `r`.
`alternative`	a character string describing the alternative hypothesis.
`method`	the character string `"Exact Poisson test"` or `"Comparison of Poisson rates"` as appropriate.
`data.name`	a character string giving the names of the data.

Note

The rate parameter in Poisson data is often given based on a “time on test” or similar quantity (person-years, population size, or expected number of cases from mortality tables). This is the role of the T argument.

The one-sample case is effectively the binomial test with a very large n. The two sample case is converted to a binomial test by conditioning on the total event count, and the rate ratio is directly related to the odds in that binomial distribution.

Examples

### These are paraphrased from data sets in the ISwR package

## SMR, Welsh Nickel workers
poisson.test(137, 24.19893)

## eba1977, compare Fredericia to other three cities for ages 55-59
poisson.test(c(11, 6+8+7), c(800, 1083+1050+878))

[Package stats version 4.4.1 Index]