RANDOM EFFECTS ANALYSIS OF CHILDRENS BLOOD-PRESSURE DATA

Some analyses of longitudinal blood pressure data have focussed on the question of whether a current value of blood pressure is predictive o f subsequent rate of change. A positive correlation between blood pres sure values at the beginning of a longitudinal study and rate of chang e over the course of the study has been found in studies of adults. Ne gative correlation, however, has been found in a study of children. Th ese studies, either implicitly or explicitly, rely on linear growth cu rve models in which subjects' blood pressure observations are assumed to follow simple linear regression models with slopes and intercepts v arying among subjects, but with the slopes constant over time. Our ana lysis of a longitudinal data set of 2,203 measurements of systolic blo od pressure from 216 children also provided a negative estimate of the correlation. However, smoothed plots of cross products of residuals s uggested that an alternative random effects model, in which rate of ch ange of systolic blood pressure is not treated as constant over time, might better fit the data. It is possible that the negative estimates of the correlation found in children's blood pressure data are an arti fact of assuming a constant rate of change when the data actually foll ow the alternative model. It is shown that the expected result of fitt ing the linear growth curve model to data that follow the alternative model is an apparent negative correlation between slope and intercept. In the data, the observed estimates of the parameters of the linear g rowth curve model are consistent with the observed estimates of the pa rameters of the alternative model.