Comment on 'The random variation in rates based on total enumeration of events' by J. R. Udry, C. Teddlie, C. M....

This comment builds on the work of Udry et al., developing a model of random variation of vital rates in small geographical areas. The model is based on the division of the variance of vital rates into three parts; that which would occur in a homogenous population; that due to population heterogeneity; and that due to yearly fluctuations in the underlying basis for the mean value of the probability of the vital event in question. Contrary to Udry et al., this model demonstrates that population heterogeneity must decrease the overall vital rate variance. Using birth rates presented in Udry et al., a test of the new model demonstrates the importance of the third factor, yearly fluctuations, in determining year-to-year variation in birth rates.