<prompt> Non - bias study is -Case control studyCoho studyRandomized controlled trialsUnrandomized trials  <response>: .randomised control trials mainly include dtrawing up a protocol,selecting reference and experimental populations ,randomisation,manipulation or intervention ,follow up, assessment of outcome.RCT s are generally described as non biassed studies as they give no chance of any systemic error in the determination of association between exposure and disease.ref:park;s textbook of preventive and social medicine,22 nd edition ,pg no 70 