Age at menarche in Warsaw girls in 1965.

of March in 1965. Many earlier investigations utilized the age at menarche as remembered by the girl or the woman. The age at menarche was calculated as an arithmetic mean on the basis of these data. With this procedure one must remember that the persons examined might have given, intentionally or unintentionally, false dates. What is more, if investigations of this type are carried out in groups of girls who are not fully mature, the absence of data from girls maturing later causes an erroneous lowering of the mean age at menarche. The probit method used in recent years avoids these dangers. The starting material in this method is represented by the proportions of girls who are menstruating in the various age groups. The distribution, the mean and other characteristics of the age at menarche can then be determined. This method of collecting data was used in our investigations. The sample of Warsaw girls investigated was selected in the following manner. Twenty-one primary schools, eight general-education secondary schools and fifteen technical secondary schools were drawn at random from the list of all Warsaw schools. In these schools one of the forms of each age group (about 20 girls) was investigated, beginning with the third form of the primary school. If a form had less than fifteen girls, the girls in a second form of the same age group were also investigated. A total of 3918 girls aged 8J to 19 years was concerned: of these 2080 were between 11.0 and 16.0. The wide limits used were in order to detect possible cases of very early or very late menstruation. The investigation was carried out by three ladies trained for the