Introduction: The need to rethink approaches to population forecasts

Three major groups call on demographers to produce medium- and long-term population forecasts at the national, regional, or global levels - or produce them themselves. They are: other scientists, government and international agencies, and the general public, including private industry. What these consumers of forecasts demand of demographers, or what demographers think that they should demand, has been changing. The types of forecasts demanded are changing, the relevant dimension of forecasts is expanding, and users are increasingly requiring that forecasts include an indication of the degree of uncertainty of the forecast. Because the demands placed on demographers for population forecasts have been changing, it is an appropriate time to rethink some of their basic aspects. In this volume we address what we see as key issues in population forecasting: in what dimensions and at what levels of disaggregation should forecasts be provided? (And, in particular, are the traditional dimensions of age and sex sufficient?) Should population forecasts take note of limits to population or interactions between population and other variables? And how should uncertainty be treated? We believe that, at least in part, these issues are driven by changes in what users of forecasts want from population forecasters. The reader will note that we have used the term "forecasts" rather than the more common "projections." Demographers claim to produce population "projections," which are correctly computed numerical outcomes of a specified algorithm whose form, initial values, and controlling parameters or transition values are specified by the analyst. By definition, a projection must be correct unless arithmetical or other errors are made. However, users of population projections require population "forecasts." Forecasts are what Donald Pittenger (1980) called a "population projection selected as a likely outcome." Thus although a demographer makes a "projection," the user employs it as a