During the past several decades, birth rate and death rate have remarkably declined in both developed and developing countries. As the result, the age structure of any countries have been rapidly changing, and it is difficult to grasp such changing patterns in the age structure by means of any demographic indices used so far (e.g. age dependency ratio and mean age). In 1980, Kazumasa Kobayashi applied the two-parameter logit model developed by William Brass to the analysis of the age structures for 12 Asian countries . He judged that this mathematical model provided more suitable outcomes than any other demographic methods did. Considering his early application of this model, Kobayashi's analysis was judged to have led to fairly good results. However, it is noted that the Brass model sometimes fails to fullyreflect the mortality patterns of childhood and old ages, implying that there is still room for improvement in the analysis of age structure by means of mathematical models . The four-parameter logit model developed by Douglas Ewbank and his colleagues was desigened to cope with the problems which were incorporated in the Brass model . The purpose of this paper is to ascertain the validity and applicability of the Ewbank model, while analyzing the data of age structure for the 12 Asian countries . This study concluded that this model is far more valid and applicable than the Brass model .
