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Pock Lesions on Chick Embryos

Keywords: overdispersion, quasi-likelihood, Poisson, negative-binomial


A study was conducted concerning the counts of lesions produced on membranes of chick embryos by viruses of the pox group. The data give the numbers of lesions formed at a series of dilutions of the viral medium.

Variable Descriptions

Dilution Dilution of viral medium, from 1 to 32
Count Number of lesions


Data File (tab-delimited text)


Breslow, N. E. (1990). Further studies in the variability of pock counts. Statistics in Medicine 9, 615-626.

Faddy, M. J. (1997). Extended Poisson process modelling and analysis of count data. Biometrical Journal 39, 431-440.

Faddy, M. J., and Smith, D. M. (2008). Extended Poisson process modelling of dilution series data. Applied Statististics 57, 461471.


The mean is approximately proportional the log-dilution. The data appear to be overdispersed relative to the Poisson distribution but underdispersed relative to the negative binomial distribution.


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