In some cases, the modeling focuses on the standardized schedules of age-specific migration flows (i.e. scaled to unit area under the curve.) There are two reasons for focusing on standardized schedules of counts (as opposed to rates or flows). The first reason is that a standardized schedule can be multiplied by an index of aggregate level to obtain the age-specific flows. This relationship makes the estimation process much simpler because it breaks up the modeling into two distinct stages: levels and shapes. Once the aggregate flows are obtained, the age patterns can be predicted

readily. The second reason is that age-specific data are sometimes provided, but they do not always match the data for the reported in- and out-migration flows. By focusing on the standardized schedule, one puts more weight on the reported age profile, not on the underlying numbers.