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It is useful to distinguish two kinds of definitions, “center-focused definitions” and “boundary-focused definitions.”
A center-focused definition is intended to describe the “ideal type” of what is defined, a standard against which other examples may be measured. It is not intended to identify precisely what items do and not belong in a class, but to suggest the ideas that are central they are to it. As a result it tends to be vague around the edges.
For example, a “wardrobe” is a collection of garments, but can two jackets hanging on adjacent hooks constitute a wardrobe? Or must the garments belong to the same person or have some other relationship to each other? A “wardrobe” can also be a cabinet designed to contain clothing, but is it still a wardrobe if it is filled with dishes?
A boundary-focused definition, as the name implies, identifies what does or does not belong in the class of ideas or objects being identified.
For example, a person is or is not a registered student. A senator represents California or New Mexico, but not both.
We might speak of the Classic Period in pre-contact Mexico as “the period in which the great city of Teotihuacan rose to prominence and influenced many other peoples across central Mexico.” Such a definition is center-focused. Exactly what dates fall into the Classic and what dates do not becomes impossible to say, since it is unclear just what constitutes “influence,” and since the influence would have been different and occurred at different times in different places. On the other hand it becomes meaningful to speak of one or another artifact or site as being “more perfectly Classic” than another, or “further from the center of Classic influences,” or the like.
We use a boundary-focused definition when we say, for example, that the “Classic” period in Mexican archaeology is “that period which extends from the year 300 to the year 900.” An event that takes place in 901 is by definition post-Classic. An event in 899 is by definition Classic. There is no being “more Classic” or “less Classic” with this kind of definition.
Most everyday definitions are center-focused, not boundary-focused. It is hard to get too rigid about the definition of “nerd,” for example, or of “party” or of “yellow.” But boundary-focused identifications are necessary in law (Was it a parking violation or not?), in science (Is the eggshell color normal or abnormal?), and in many other areas (Is the respondent a potential voter or not?). This is especially critical if quantitative measurements are needed. (E.g., what percentage of voters make decisions based on negative campaign ads?)
Creating boundary-focused definitions is called “operationalization.” Operationalization is a critical process in setting out a plan of research because it is how you decide what information to collect, and how to classify the facts you assemble.
For example, in archaeology we may want to know whether ancient “cities” are significantly associated with “river basin adaptations,” but to be precise about it we need to operationalize these terms, that is, to create boundary-focused definitions of them so we can count examples, calculate correlations, and so on. Some instances that we force into our definitions will not fit comfortably, but the strength of the method comes from our ability to count cases and thereby to see patterns that would otherwise be invisible.
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