Variations in rates of aging and in age distributions across local areas.
Local and community-level data on changing age distributions are a central feature of an industry that topped $18 billion in investment in 1996–the school construction industry (U.S. Department of Commerce 1999). Schools are built and re-built for a number of reasons, not all demographic (e.g., buildings wear out), but it is virtually impossible to borrow money to fund school construction absent a demographically-based enrollment projection or forecast.
Projection models are essentially the same in form: They combine the mechanical with the delphic. Existing stocks of school children are “survived forward” via grade progression ratios. This is analogous to cohort survival ratios using life tables, except that the progression ratios also contain information about migration and grade retention. Similar methods can be used to convey pre-school-age children into their school ages. All this works fairly well for five years or so into the future, but planners, builders, school boards, taxpayers, etc. are investing time and money toward a longer horizon. Where do new children come from?