<prompt> All are disadvantages of Coho study exceptCoho study involve a large number of peopleCoho studies are expensiveRecall biasAttrition  <response>: Disadvantages of coho study Time consuming and Expensive Attrition - loss of follow up Hawthorne bias Ethical issues Not useful in rare disease Disadvantages of case control study Calculation of Odds ratio Relative risk - more accurate than OR Recall bias 