*Hacettepe University, Faculty of Medicine, Department of Biostatistics, 06100 Sıhhiye, Ankara TURKEY
oCorresponding Author e-mail: ralpar@hacettepe.edu.tr

Summary:
The term bias in clinical trials describes the systematic influence of any factors associated with the design, conduct analysis, and interpretation of teh results. The two common methods that are used to reduce the bias are randomization and blinding. Randomization aims to obviate the systematic differences (or bias) between groups due to the factors other than the treatment. Randomization gives each patient a known (usually egual) chance of being assigned to any of the groups and it is based on the premise that the group assignment cannot be predicted. Blinding (or masking) is intended to limit the occurrence of conscious and unconscious bias in the conduct and interpretation of a clinical trial arising from the influence that the knowledge of treatment may have on the recruitment and allocation of patients, the responses of subjects to the treatments, the evaluation of responses, the handling of withdrawals, the exclusion of data from analysis , and so on. There are different randomization and blinding methods. In this review, the randomization and blinding methods used to reduce bias will be discussed with appropriate examples.