BACKGROUND: Live-birth (male-female) ratios are a standard measure used in demography. Recently, live-birth ratios have been considered as a potential indicator for various environmental hazards. In this paper, mixture modeling is applied to analyse the geographic heterogeneity of live-births in their composition of male and female proportions (live-birth ratio) in the Kingdom of Thailand.

METHODS: Live-birth data are taken from the 1990 census of the Kingdom of Thailand. The level of aggregation is the province, of which there are 73 in Thailand. The analysis is based on the simple observation that a logical equivalent to the live-birth ratio is available, namely the proportion of male live-births. Based on this measure a simple and exact statistical model is easily derived: conditional on the number of live-births, the number of male live-births forms a binomial distribution, with parameter lambda. If there is homogeneity in the proportion of male live-births, then all provinces can be described by means of a single binomial distribution. However, if there is heterogeneity in the proportion parameter lambda, then a mixture of binomial distributions will occur.

RESULTS: For the 1990 census data, three groups could be identified: a majority group containing 84% of the provinces and a proportion parameter of lambda = 0.513, a group of five provinces having a reduced proportion parameter of lambda = 0.500 (fewer male live-births), and a group of four provinces having an increased proportion parameter of lambda = 0.525.

CONCLUSIONS: It is unclear how this can be explained, although some potential explanations are offered. The stability of these groups in time should be confirmed and regularly monitored.