A sterile couple is statistically defined as a childless couple that is either biologically or electively infertile. Couples who are currently childless due to unknown reasons (by chance) are not considered as sterile couples. The completed family size data of census 1993 is shown to fit well the negative binomial distribution. Using this distribution these two types of childless couples are separated. Three known methods of fitting are used, of which the iteration one turned out to be the most appropriate. Using the most appropriate fit, an approximation of the proportion of sterile couples in Oman is provided. The consequences of assuming this distribution for the family size population are discussed. In particular it is shown that the two types of children that make up a family are positively correlated.