Ans: comparison group is one of the main features of analytical epidemiology. There was an outbreak of hepatitis b in Pennsylvania. It was found that there was a large group of people visiting the same restaurant for more than four weeks. Before the research was made, it was unknown, which worker was serving foods and which food was contaminated. The researchers made a comparison on which people had eaten in the restaurant and did not fall sick. Causes and effects are the base of research in analytical epidemiology. Epidemiology itself cannot prove the outcome of any disease hence it requires the help of analytical epidemiology.