The time dimension in measurements of population health

As recently attested by the Millennium Declaration (United Nations, 2000), the health of populations is a concern for both governments and civil society: three of the eight Millennium Development Goals are defined in terms of health objectives. It is therefore reasonable to enquire what it means when we claim population health improves or declines. We should also know how to measure such changes. Since the Millennium Declaration does not answer these questions, we infer that concern for population health on the part of governments and civil society is not necessarily accompanied by clear concepts regarding its definition and measurement. Nevertheless, substantial technical progress on population health measurement has been made recently (see, for example, Murray and Lopez, 1996). In addition, there is a substantial body of older work on life expectancy and related measures (for an overview, see Keyfitz and Caswell, 2005). Life expectancy, in addition to being one of the oldest population health measures, might also be the only one to have successfully established itself in popular and political conceptions. As we argue in the following, this very success conceals subtle technical points that have confounded, at least to some degree, further developments in population health measurement