The Collaborative Lipid Research Clinics Program Family Study. III. Transformations and covariate adjustments of lipid and lipoprotein levels.

Several methods of transformation and covariate adjustment have been applied to the Collaborative Lipid Research Clinics Program Family Study data to facilitate analysis of lipid levels of examinees of different sex and age groups. After several exploratory analyses, cholesterol, high density lipoprotein cholesterol, and low density lipoprotein cholesterol were logarithmically transformed, and triglycerides were transformed by the power -0.25. Two types of covariate adjustment procedures, the regression and Z score methods, were used. A modified regression method was developed and was found to be preferable to both simple cubic regression and the Z score method on theoretical and empirical grounds. Refinements in this method to correct for change in variance with age, and for the effect of socioeconomic status, seasonality, and anthropometric measures were made. Methodological issues connected with the transformation and adjustment procedures are discussed.