Theory: an explanation using an integrated set of principles that organizes observations and predicts behaviors or events. Hypothesis: a testable prediction, often implied by a theory. Operational definition: a statement of the procedures (operations) used to define research variables. For example, human intelligence may be operationally defined as what an intelligence test measures . Replication: repeating the essence of a research study, usually with different participants in different situations, to see whether the basic finding extends to other participants and circumstances. Case study: an observation technique in which one person is studied in depth in the hope of revealing universal principles. Naturalistic observation: observing and recording behavior in naturally occurring situations without trying to manipulate and control the situation. Survey: a technique for ascertaining the self-reported attitudes or behaviors of a particular group, usually by questioning a representative, random sample of the group. Population: all the cases in a group being studied, from which samples may be drawn.