Self-selection occurs when individuals decide for themselves whether they are in the control group or the treatment group. Self-selection is quite common in studies of human behavior. For example, studies of the effect of smoking on human health involve self-selection: individuals choose for themselves whether or not to smoke. Self-selection precludes an experiment; it results in an observational study. When there is self-selection, one must be wary of possible confounding from factors that influence individuals' decisions to belong to the treatment group.