An instrument is always something secondary: There is always a purpose for which an instrument is needed and a context in which it is going to be used (i.e., involving some sort of decision). This precipitates an idea or a concept that is the theoretical object of our interest in the respondent. Consistent with current usage, I call this the construct (see Messick, 1989, for an exhaustive analysis). A construct could be part of a theoretical model of a person's cognition—such as their understanding of a certain set of concepts or their attitude toward something—or it could be some other psychological variable such as "need for achievement" or a personality variable such as a bipolar diagnosis. It could be from the domain of educational achievement, or it could be a health-related construct such as "Quality of Life" or a sociological construct such as "rurality" or migrants' degree of assimilation. It could relate to a group rather than an individual person, such as a work group or sports team, or an institution such as a workplace, or it could be biological phenomena such as a forest's ability to spread in a new environment. It could even be a complex inanimate object such as a volcano's proclivity to erupt or the weathering of paint samples. There is a multitude of theories—the important thing here is to have one that provides motivation and structure for the construct to be measured.