When a scientific experiment occurs outside a controlled setting, usually in the course of normal social events; sometimes because of a natural event or a political intervention; Have to wait until right circumstance comes along and cannot just create. Weaknesses of experiments don"t in a lab, don"t know if it really represents the real world. Three dimensions of validity: measurement validity, generalizability, casual validity (or internal validity) Difference between survey and experiment, experiments manipulate the independent variable. Exists when a measure measures what we think it measures. Exists when a conclusion holds true for the population, group, setting or event that we say it does, given the conditions that Exists when the conclusion that a leads to, or causes b, is correct. A systematic explanation for the observations that relate to a particular aspect of life. Systematic sets of interrelated statements intended to explain some aspect of social life.