# poisdisp.test: Tests for the dispersion parameter in Poisson distribution In Rfast: A Collection of Efficient and Extremely Fast R Functions

## Description

Tests for the dispersion parameter in Poisson distribution.

## Usage

 ```1 2``` ```poisdisp.test(y, alternative = "either", logged = FALSE) pois.test(y, logged = FALSE) ```

## Arguments

 `y` A numerical vector with count data, 0, 1,... `alternative` Do you want to test specifically for either over or underspirsion ("either"), overdispersion ("over") or undersispersion ("under")? `logged` Set to TRUE if you want the logarithm of the p-value.

## Value

A vector with two elements, the test statistic and the (logged) p-value.

Michail Tsagris

## References

Yang Zhao, James W. Hardin, and Cheryl L. Addy. (2009). A score test for overdispersion in Poisson regression based on the generalized Poisson-2 model. Journal of statistical planning and inference 139(4): 1514-1521.

Dimitris Karlis and Evdokia Xekalaki (2000). A Simulation Comparison of Several Procedures for Testing the Poisson Assumption. Journal of the Royal Statistical Society. Series D (The Statistician), 49(3): 355-382.

Bohning, D., Dietz, E., Schaub, R., Schlattmann, P. and Lindsay, B. (1994) The distribution of the likelihood ratio for mixtures of densities from the one-parameter exponential family. Annals of the Institute of Statistical Mathematics, 46(): 373-388.

``` poisson.mle, negbin.mle, poisson.anova, poisson.anovas, poisson_only ```

## Examples

 ```1 2 3 4 5 6 7 8 9``` ```y <- rnbinom(500, 10, 0.6) res<-poisdisp.test(y, "either") res<-poisdisp.test(y, "over") res<-pois.test(y) y <- rpois(500, 10) res<-poisdisp.test(y, "either") res<-poisdisp.test(y, "over") res<-pois.test(y) ```

### Example output

```Loading required package: Rcpp