Several ongoing cohort studies in Europe and US use different dietary assessment methods to increase our knowledge about diet and disease relationships concerning cardiovascular disease, osteoporosis and cancer. Diet history methods and food frequency questionnaires are commonly used methods to measure usual food intake in case-control studies and cohort studies. However, misclassification imposed by the methods limit the possibilities of collecting reliable data on dietary intake. The validity of the dietary methods depends on several components, like accurate estimations of frequency and portion sizes, the quality of food composition data bases used, and data collection procedure; all components which are crucial for interpretation of the dietary results. If the food list does not include commonly consumed food items in the chosen study population, or dietary methods with predefined questions are used, the sample characteristics must be homogeneous for several factors like gender, age, ethnicity and social class. This might introduce narrower distribution of potential explanatory variables. Non-representative study samples will also restrict valid estimations of populations at risk. This paper intends to evaluate some methodological issues of logistic models regarding misclassification and attenuation of estimated observed relative risks.Different diet assessment methods are used in some of the ongoing cohort studies and these methods have been validated in the past. A need for further development of the present dietary methods was put forward at an international meeting on diet assessment methods. Studies using semi-quantitative food frequency questionnaires were also recommended to include standardized substudies with recall or record methods to get quantitative calibrated estimates of nutrient intakes.2 Other methods, for example meal-based methods like dietary record, diet histories and 24-hour recalls are more timeconsuming but they have some advantages since they cover the consumption of all foods.3 Although, a gold standard is lacking for validation of the different diet assessment methods, comparison of the method's
Bias in Diet Assessment MethodsConsequences of Collinearity and Measurement Errors on Power and Observed Relative Risks SÖLVE ELMSTÅHL AND BO GULLBERGElmståhl S (Department of Community Medicine, Lund University, S-205 02 Malmö, Sweden) and Gullberg B. Bias in diet assessment methods-consequences of collinearity and measurement errors on power and observed relative risks. International Journal of Epidemiology 1997; 26: 1071-1079. Background. If several risk factors for disease are considered in a regression model and these factors are affected by measurement errors, the observed relative risk will be attenuated. In nutritional epidemiology, several nutrient variables show strong correlation, described as collinearity. The observed relative risk will then depend not only on the validity of the chosen diet assessment method but also on collinearity between variables in the model. Methods. Th...