poisson.test
Exact Poisson tests
Performs an exact test of a simple null hypothesis about the rate parameter in Poisson distribution, or for the ratio between two rate parameters.
- Keywords
- htest
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
- 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.
"htest" containing the following components:
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.
See Also
Examples
library(stats)
### 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))