CARL E. TAYLOR
Some of the most important advances in improving understanding of health and family planning have come from longitudinal community-based studies. The underlying principle is that the unit studied is not an individual but a whole population. With population-based approaches, causal and associated variables can be examined both quantitatively and qualitatively in natural circumstances.
Convergence of field experience in longitudinal community-based studies has evolved from three distinct disciplines: epidemiology, demography, and health systems research. For more systematic use of these methods it see desirable to synthesize what has been learned. In this paper the historical contributions of the three disciplines are briefly described. Because of the demonstrated potential for community-based studies in improving primary health-care services, some general principles are defined to guide future efforts.
Some of the earliest longitudinal community-based studies are among the classics of epidemiology. The methodology was to follow a health problem over time as it appeared among a group of people, and make inferences about causation. For example, Snow's monograph on cholera ( Snow 1936) is considered one of the best early models of what epidemiology can do. The most famous part of Snow's monograph describes a study where he traced the case-by-case evolution of a cholera epidemic in the Broad Street area of London. From patterns of water use he concluded that transmission was from the Broad Street pump and applied the control intervention that has edified generations of epidemiology students--he took the handle off the pump. Other epidemiological studies of infectious diseases such as Budd on typhoid ( Budd 1874) and Panum on measles ( Panum 1940) made retrospective analyses of transmission patterns during epidemics in defined populations. The classic studies of Goldberger on pellagra ( Goldberger 1964) were important in developing methodology. In the southern regions of the USA he identified populations with high prevalence of pellagra which were followed carefully to demonstrate