Bias is unlikely to invalidate coho studies used to assess risk risk of exposure because-

. .coho study is a type of analytical study which is usually undeaken to supp the existence of an association between suspected cause and disease.also known as prospective study /longitudinal study/incidence study/forward looking study etc.the cohos are identified prior to the apperance of the disease under investigation..the study groups are observed over a period of time to determine the frequency of disease among them.the coho study always preceeds from cause to effect.there are three types of coho study mainly,prospective , retrospective ,and combined.the elements of a coho study are, 1.selection of study subjects. 2.obtaining data on exposure 3.selection of comparison groups 4.follow up 5.analysis.the data obtained through the coho study are analyzed in terms of incidence rates of outcomes among exposed and non exposed and estimation of risk. ref:park&;s textbook,ed 22,pg no 75