How do scientists determine whether a new drug will cure or help a particular disease? Usually with a study involving two groups of people. For instance, if the scientists are studying the effectiveness of a new antihistamine on allergy sufferers, they would give the trial medication to one group of patients and a placebo (sugar pill) to the other group, called the control group. Neither group is supposed to know whether they have been given the medication and the study participants are randomly assigned to each group.
This is referred to as a randomized double-blind placebo control study and is considered the gold standard of clinical trials. "Double-blind" refers to the fact that neither the scientists nor the participants know which allergy patients are in which group until after the experiment is over.
There are several reasons for doing this but one is to avoid selection bias. Let's say you want to study whether people who work at night are more likely to develop headaches. So, you recruit a group of people who work at night, and another group who work during the day, and then compare them. Your results show that the people who work at night are more likely to have aching temples.
But that doesn't necessarily mean that night work is the cause, because it could be that people who work at night tend to be poorer, have more unhealthy diets or more stress. Such factors might bias your results, unless you could make sure that the two groups are similar in every other way except for their schedules [sources: Institute for Work and Health, CIRT].