Note: This is a brief introduction to observational designs. For more on this type of study, see books by Paul Rosenbaum: Observational Studies and, for a less mathematical approach Design of Observational Studies
In statistics and research design, there are two types of study: Experiments and observational studies. Some people also use the term “quasi-experiment” but I do not like it. In an experiment, the key thing is <em>randomization</em>. We assign subjects (e.g. people) to different conditions (e.g. drug and placebo) randomly. Often, though, such assignment is not possible or not ethical. In social sciences, it is rarely possible. We cannot, for example, randomly assign people to different levels of education. We can only observe relationships between (say) education and political party.