Determinants of Birth Intervals and their Means.

The paper deals only with the first two of these, which, unlike duration of pregnancy, present unusual problems of estimation. The literature of the past few years contains a number of attempts to deduce values for these determinants from observational data, that is, from observed series of birth intervals, but it is one of the more frustrating aspects of such demographic analyses that the significance of observations often depends as much upon the way in which the data were collected as on the values themselves. Sets of birth intervals can be obtained in a quite remarkable variety of ways and analyzed with correspondingly varied virtuosity. The fecundabilities calculated in various tables are considerably lower than those, which have been derived from other studies. This is largely because of the inclusion in the series of the full range of women in the population; the correction of the harmonic effect also leads to a lower value. A remarkable feature of this series is that some 4% of all intervals were stated as being longer than six years. Intervals of this length are, of course, very much rarer in studies confined to women of high attained parity.