 Biases can occur at any stage of the research. During the implementation stage, the most common bias is the information bias which can be eliminated by blinding. Interviewer bias, subject bias, recall bias, hathone effect are all subsets of information bias. While blinding can effectively reduce interviewer bias and subject bias, this recall bias, hathone effect cannot be reduced by blinding. Attrition bias is another bias which is important during the conduction of the research that is due to the bias in the follow-up.