Masking of physicians in the Growth Failure in Children with Renal Diseases Clinical Trial

Masking - hiding identities of treatments from the patient, physician and/or statistician - is a critical element in clinical trials. Wherever possible, masking is implemented to eliminate observational bias or systematic error. In this paper, general concepts of masking in clinical trials are examined. Specific masking procedures used in the "Growth Failure in Children with Renal Diseases" (GFRD) Clinical Trial are described. A method to evaluate the "success" of this masking procedure for physicians is introduced. For each randomized patient at each clinical center, the clinic director was asked to predict which treatment (1,25-dihydroxyvitamin D₃ or dihydrotachysterol) was assigned. Results showed that 72% of responses initially indicated "absolutely no idea" of treatment. Additional analyses revealed that the number and percentage of "correct" guesses were essentially equal for the two treatment groups and that a patient's time on treatment did not affect the mask. We conclude that the mask of physicians in the GFRD Clinical Trial was well maintained.