Interpreting age, period and cohort effects in plasma lipids and serum insulin using repeated measures regression analysis: The CARDIA study

Observed changes in health-related behaviours and disease risk factors may arise from physiological or environmental changes, or from biases due to sa mpling or measurement errors, We illustrate problems in the interpretation of such changes with longitudinal data from the Coronary Artery Risk Develo pment in Young Adults (CARDIA) study. Mean plasma cholesterol was 14 mg/dl higher in 27- than in 20-year-old black men cross-sectionally, but longitud inally it declined by 4 mg/dl during the 7 years. To sort out these contrad ictory assessments of the effect of age/passage of time, we estimated age a nd period effects under the assumptions that age effects are a smooth funct ion of age independent of period, and that period effects are changes commo n to persons across all ages. Simple estimates the age effect, such as the cross-sectional age slopes, may be confounded by cohort effects, by interac tions of time and age after baseline, or by the occurrence of non-lineariti es in response after baseline. We note examples of each potential type of b ias. The data and background literature support the assumption that cohort effects do not seriously compromise interpretation for these variables in t he CARDIA study. Strong secular decreases in plasma cholesterol apparently due to population-wide dietary change, mask increases with ageing. Age incr eases in triglycerides are largely explained by increases in body fatness. For these data, we cautiously accept the cross-sectionl age slope as an est imate of ageing and the age-matched time trend as an estimate of secular tr end. Copyright (C) 1999 John Wiley & Sons, Ltd.