Bypassing the selection rule in choosing controls for a case-control study

Objectives It has been argued that in case–control studies, controls should be drawn from the base population that gives rise to the cases. In designing a study of occupational injury and risks arising from long-term illness and prescribed medication, we lacked data on subjects' occupation, without which employed cases (typically in manual occupations) would be compared with controls from the general population, including the unemployed and a higher proportion of white-collar professions. Collecting the missing data on occupation would be costly. We estimated the potential for bias if the selection rule were ignored. Methods: We obtained published estimates of the frequencies of several exposures of interest (diabetes, mental health problems, asthma, coronary heart disease) in the general population, and of the relative risks of these diseases in unemployed versus employed individuals and in manual versus non-manual occupations. From these we computed the degree of over- or underestimation of exposure frequencies and exposure ORs if controls were selected from the general population. Results: The potential bias in the OR was estimated as likely to fall between an underestimation of 14% and an overestimation of 36.7% (95th centiles). In fewer than 6% of simulations did the error exceed 30%, and in none did it reach 50%. Conclusions: For the purposes of this study, in which we were interested only in substantial increases in risk, the potential for selection bias was judged acceptable. The rule that controls should come from the same base population as cases can justifiably be broken, at least in some circumstances. <br/

New York City's unemployment picture

This issue of Second District Highlights briefly examines several explanations that have been offered for the rise in unemployment in New York City from 1994 to 1997.Unemployment - New York (N.Y.) ; New York (N.Y.) ; Federal Reserve District, 2nd

Brazilian biofuels policies and impacts on world agricultural trade

Agricultural and Food Policy, Resource /Energy Economics and Policy,

Learning from 20 Years of Payments for Ecosystem Services in Costa Rica

Costa Rica's Payments for Ecosystems Services (PES) programme has become something of an icon in the world of conservation. Its innovative blend of economic and regulatory instruments - and its hitches and successes - provide a valuable source of inspiration for other countries that are looking for effective ways to conserve and regenerate ecosystems. Since 1997, nearly one million hectares of forest in Costa Rica have been part of the PES programme at one time or another, and forest cover has now returned to over 50 per cent of the country's land area, from a low of just 20 per cent in the 1980s. What lessons can be learnt from the 20 years since it was founded? Also published in Spanish, this paper is for local practitioners, international researchers and donors who are interested in the Costa Rican experience