Firstly, such studies can seldom be made in adherence to all the requirements of experimental science since the ‘objects’ being studied are people and they come with a variety of behaviors and appetites. Secondly, the study can take place only in the conditions that have occurred, and not in a laboratory re-creation of them where certain factors can be varied systematically to determine their influence and effect. Most seriously, epidemiological studies can take decades to be completed, so that by the time a positive result becomes evident, large numbers of workers may have been damaged or injured irreversibly.