To achieve systematic empiricism, scientists in HDFS must be very clear about how they conceptualize the topic of study. Ambiguity is tolerable in everyday life, but specificity is required for science. First, variables can be identified by whether they refer to subjective reality (exists in the mind of the subject; calls for constructivist approach to measurement) or objective reality (exists independently of the subject's mind; calls for post-positivist approach to measurement). A mixed-methods approach uses both in the same study. Second, variables can be identified as categorical or numerical, which determines the analytical approach of the study. Third, variables must be operationally (rather than conceptually) defined. The operational definition identifies the method of measurement and the standards or criteria by which the measurement assigns a value. Operational definitions must not violate scientific consensus, but they differ significantly by context. Examples are provided of different operational definitions used for being raised by a gay or lesbian parent, as well as for sexual assault on college campuses to illustrate how profoundly the operational definition can affect the result of the study.