Abstract
There are two major errors in medical research. Random errors, which we named as error by chance, and systematic errors, which we called as bias, affect the internal and external validity of researches. The aim of this technical note is to give some information about different bias causes under two major bias headings as “selection and information bias”. It is possible to decrease the probability of random error by increasing the sample size. But systematic error is not prevented by increasing sample size unless the representativeness is not provided. Also information bias, which will occur during planning and conducting of research, will make us away from the truth. Therefore it is very important to consider probable bias causes during planning and conducting of any research from beginning to the end.

Key words: bias, representativeness, truth