Many statistics problems require you to make comparisons. You may be interested in comparing salaries for different professions, for example, or you might want to compare test scores of children in different reading programs. Comparative studies like this fall into two categories -- experimental studies and observational studies. Note 2
The purpose of a comparative experimental study is to determine the "cause and effect" of an action. In this situation, an experiment deliberately imposes a particular treatment on a group of individuals in order to gauge their responses. This allows the investigator to determine if the treatment caused a change in the individuals' responses.
For example, to determine the effect of taking aspirin on heart disease, an experiment was conducted using a large group of doctors as subjects. Half the doctors took aspirin every other day; the other half took a pill that looked and tasted like aspirin, but was not. The experimenters imposed the treatment (aspirin) on the individuals by deciding through random selection which half would take aspirin and which half would not. After several years, the incidence of heart attacks for the two groups was compared. The analysis of the results suggested that the aspirin dosage had a positive effect -- that is, there was a lower rate of heart attacks among the doctors who received aspirin.