To help eliminate some of these effects, the best studies are performed double blind where neither the subject nor the experimenter knows who is receiving a real treatment or who receives a placebo. More often the test is performed single blind, where only the subjects do not know who received the placebo Researchers often rely upon epidemiological studies. A cohort is a defined population group followed prospectively. Participants are carefully selected and extensively questioned about such things as life styles. Over the course of the study, individuals are monitored for disease, and attempts are made to identify factors associated with the malady.
Researchers then attempt to identify factors that determined who got sick.
These studies take less time and money to perform, but can often be difficult to interpret. Neutrality of investigators is paramount. For example, it might be difficult to evaluate the effects of tobacco if investigation is being funded by a tobacco company. A more subtle influence to assess is the impact on research if the continued funding of research is dependent upon generation of statistically relevant results. Facing loss of funding for a project that the principal investigator may have dedicated years to, may be a difficult milieu in which to function without bias.