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Background & Aim: Randomized clinical trials, unlike observational studies, have less potential for bias therefore, they are considered as a golden standard to compare the effects of different types of interventions in the health siences. The purpose of this study was to investigate the concept of bias in randomized clinical trials, the causes and types of bias and ultimately provide effective strategies for managing and reducing the effects of various bias. Materials & Methods: This study is a narrative review using existing resources. Search for articles and related books were done in international databases including Medline, SOUPUS, Google Books, Google Scholar, Web of Science, and national databases including Magiran and SID without any time limitation, in both Farsi and English languages. At the end, 36 article and 10 books related to the topic were found that results were presented at different levels of a clinical practice. Results: In this review article we discussed different types of bias in the various phases of a clinical trial, including planning, implementation, outcome measurement, data analysis and interpretation, reporting of results, publication and uptake phases. Conclusion: There are various types of bias in the scientific research, which had different impact on the study results. Bias may occur in the planning to post- publication of a study phases. They can damage to the study results seriously and leading to inaccurate conclusions and undermining the internal validity of the study therefore, the results of these studies should be interpreted with caution.  

Keywords: Clinical trials, Bias

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