Intended for healthcare professionals


Bias in assignment to clinical trials

BMJ 1994; 308 doi: (Published 02 April 1994) Cite this as: BMJ 1994;308:920
  1. J Galende,
  2. J Soto,
  3. J A Sacristan
  1. General Direction for Drugs and Sanitary Products, Ministry of Health, 28014 Madrid
  2. Clinical Research Department, Abbott Laboratories, Madrid
  3. Clinical Research Department, Lilly SA, Madrid.

    EDITOR, - Despite the many biases to which clinical trials are subject, trials are still considered the best way to show the efficacy of a new therapeutic or prophylactic measure.1 Perhaps the most important biases are those of selection and measurement, as they most determine the validity of a study. The respective techniques of random assignment to study treatment groups and of blind experimentation are attempts to remove these biases.

    We have detected a type of selection bias that does not affect the assignment of subjects in a single study to …

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