点击显示 收起
Department of Epidemiology
German Institute of Human Nutrition
Arthur-Scheunert-Allee 114-116
14558 Nuthetal
Germany
E-mail: khoff{at}mail.dife.de
Dear Sir:
I read with interest the recent article by Gillespie et al (1) suggesting a new method for predicting the mean retinol concentration of a person from a single measurement. The basic idea is to adjust for intraindividual variation. Gillespie et al applied the method to reduce prevalence estimates of inadequate serum retinol concentrations by using laboratory-quality data and a subsample of repeated measurements. The centerpiece of the method is the linear regression model,
Second, the linear regression model should predict a person's "usual" serum retinol concentration defined as the long-term daily average. Although Gillespie et al adopt a similar interpretation of their approach, they actually use the mean of only 2 measurements as the dependent variable in the model. Because the mean of 2 repeated measurements still has an intraindividual variance component, this component must be subtracted before regression analysis is applied. Principally, the usual retinol concentration has to be estimated in a preliminary step. The problem of estimating usual or long-term exposure by repeated short-term measurements has been intensively studied in food-consumption and environmental surveys, and several statistical methods aimed at eliminating the intraindividual variance component are available (2-4).
In summary, I propose applying a modified regression model with another dependent variable and another independent variable. Regression of the estimated usual retinol concentration on a single concentration by using all single measurements allows elimination of the distracting effect of intraindividual variation. Applying this modified model should yield lower prevalence estimates of inadequate serum retinol concentrations than those obtained by Gillespie et al.
REFERENCES