distributions to the data. Assuming that the distribution is a mixture of two normal distributions then the expectation-maximization algorithm may be used to determine the parameters. Several programmes are available for this including Cluster, and the R package nor1mix. Other distributions The mixtools package available for R can test for and estimate the parameters of a number of different distributions. A package for a mixture of two right-tailed gamma distributions is available. Several other packages for R are available to fit mixture models; these include flexmix, mcclust, agrmt, and mixdist. The statistical programming language SAS can also fit a variety of mixed distributions with the PROC FREQ procedure. Example software application The CumFreqA program for the fitting of composite probability distributions to a data set (X) can divide the set into two parts with a different distribution. The figure shows an example of a double generalized mirrored Gumbel distribution as in distribution fitting with cumulative distribution function (CDF) equations: X < 8.10 : CDF = 1 - exp[-exp{-(0.092X^0.01+935)}] X > 8.10 : CDF = 1 - exp[-exp{-(-0.0039X^2.79+1.05)}] See also Overdispersion References Continuous distributions