 Biases can occur at any stage of the research during the planning the most common type of bias is selection bias which can be minimized by a good eligibility criteria that is a good inclusion and exclusion criteria. Following bias such as healthcare bias, length bias sampling, name and bias, survivor bias, Berksons bias, healthy worker effect, volunteer bias, exclusion bias, matching bias are considered as subsets of selection bias or related to selection bias but other types of bias which we can eliminate at the planning stage is the misclassification bias which can be eliminated or minimized by a good study protocol and the allocation bias it can be minimized only by randomization and in the planning stage we can eliminate this ascertainment bias by a good study tool. So these are all the common bias out of which selection bias is the most common bias at the planning stage.