Randomization is useful to eliminate
Randomization: The groups should be alike with regard to ceain variables or characteristics that might affect the outcome of experiments The entire study population can be stratified into subgroups according to the variable Individuals within each subgroup can then be randomly allocated into study and control groups Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 90
Observer bias
Confounding factors
Patient bias
Sampling bias
Epidemiology