different year classes and can thus be used for age distribution- and growth estimates of the fish population. Sediments are usually distributed in a bimodal fashion. When sampling mining galleries crossing either the host rock and the mineralized veins, the distribution of geochemical variables would be bimodal. Bimodal distributions are also seen in traffic analysis, where traffic peaks in during the AM rush hour and then again in the PM rush hour. This phenomenon is also seen in daily water distribution, as water demand, in the form of showers, cooking, and toilet use, generally peak in the morning and evening periods. Econometrics In econometric models, the parameters may be bimodally distributed. Origins Mathematical A bimodal distribution most commonly arises as a mixture of two different unimodal distributions (i.e. distributions having only one mode). In other words, the bimodally distributed random variable X is defined as with probability or with probability where Y and Z are unimodal random variables and is a mixture coefficient. Mixtures with two distinct components need not be bimodal and two component mixtures of unimodal component densities can have more than two modes. There is no immediate connection between the number of components in a mixture and the number of modes of the resulting density. Particular distributions Bimodal distributions, despite their frequent occurrence in data sets, have only rarely been studied. This may be because of the difficulties in estimating their parameters either with frequentist or Bayesian methods. Among those that have been studied are Bimodal exponential distribution. Alpha-skew-normal distribution. Bimodal skew-symmetric normal distribution. A mixture of Conway-Maxwell-Poisson distributions has been fitted to bimodal count data. Bimodality also naturally arises in the cusp catastrophe distribution. Biology In biology five factors are known to contribute to bimodal distributions of population sizes: the initial distribution of individual sizes the distribution of growth rates among the individuals the size and time dependence of the growth rate of each individual mortality rates that may affect each size class differently the DNA methylation in human and mouse genome. The bimodal distribution of sizes of weaver ant workers arises due to existence of two distinct classes of workers, namely major workers and minor workers. The distribution of fitness effects of mutations for both whole genomes and individual genes is also frequently found to be bimodal with most mutations being either neutral or lethal with relatively few having intermediate effect. General properties A mixture