We can identify any resource as a unique instance or as a member of a class of resources. The size of this class—the number of resources that are treated as equivalent—is determined by the properties or characteristics we consider when we examine the resources in some domain. The way we think of a resource domain depends on context and intent, so the same resource can be thought of abstractly in some situations and very concretely in others. As we discussed in Chapter 4, “Resource Description and Metadata”, this influences the nature and extent of resource description, and as we have seen in this chapter, it then influences the nature and extent of categories we can create.

Consider the regular chore of putting away clean clothes. We can consider any item of clothing as just that—a member of a broad category whose members are any kind of garment that a person might wear. Using one category for all clothing, that is, failing to distinguish among the various items in any useful or practical way would likely mean that we would keep our clothes in a big unorganized pile.

However, we cannot wear any random combination of clothing items—we need a shirt, a pair of pants, socks, and so on. Clearly, our indiscriminate clothing category is too broad for most purposes. So instead, most people organize their clothes in more fine-grained categories that fit the normal pattern of how they wear clothes. For example, everyone probably separates their shirts, pants, and socks when they put away their clothes after doing their laundry. Some pants and shirts may merit wooden hangers; others may rest in special drawers.

In §6.3.2, “Single Properties” we described an organizing system for the shirts in our closet, so let us talk about socks instead. When it comes to socks, most people think that the basic unit is a pair because they always wear two socks at a time. If you are going to need to find socks in pairs, it seems sensible to organize them into pairs when you are putting them away. Some people might further separate their dress socks from athletic ones, and then sort these socks by color or material, creating a hierarchy of sock categories analogous to the shirt categories in our previous example. We note, parenthetically, that not everyone works this hard when putting their clothes away; some people toss all the single, unpaired socks in a drawer and then rummage around when they need to find a matching pair of socks. People differ in their preferences or tolerances for the amount of granularity in an organizing system and we need to expect and respect these differences.

Questions of resource abstraction and granularity also emerge whenever the information systems of different firms, or different parts of a firm, need to exchange information or be merged into a single system. All parties must define the identity of each thing in the same way, or in ways that can be related or mapped to each other either manually or electronically.

For example, how should a business system deal with a customer’s address? Printed on an envelope, “an address” typically appears as a comprehensive, multi-line text object. Inside an information system, however, an address is stored as separate information components for each printed line, or as a set of distinctly identifiable information components. This fine-grained organization makes it easier to sort customers by city or postal codes, for sales and marketing purposes. Incompatibilities in the abstraction and granularity of these information components, and the ways in which they are presented and reused in documents, will cause interoperability problems when businesses need to share information, some of which may be difficult to detect because of the vocabulary problem.365[Com]

Bowker and Star point out that there is often a pragmatic tradeoff between precision and validity when defining categories and assigning resources to them, particularly in scientific and other highly technical domains. More granular categories make more precise classification possible in principle, but highly specialized domains might contain instances that are so complex or hard to understand that it is difficult to decide where to organize them.365a[LIS]

As an example of this real-world messiness that resists precise classification, Bowker and Star turn to medicine and the World Health Organization's International Classification of Diseases (ICD), a system of categories for cause-of-death reporting. The ICD requires that every death be assigned to one and only one category out of thousands of possible choices, which facilitates important uses such as statistical reporting for public health research.

In practice, however, doctors often lack conclusive evidence about the cause of a particular death, or they identify a number of contributing factors, none of which could properly be described as the sole cause. In these situations, less precise categories would better accommodate the ambiguity, and the aggregate data about causes of death would have greater validity. But doctors have to use the ICD's precise categories when they sign a death certificate, which means they sometimes record the wrong cause of death just to get their work done.

Category abstraction is normally described in terms of a hierarchy of superordinate, basic, and subordinate category levels. “Clothing,” for example, is a superordinate category, “shirts” and “socks” are basic categories, and “white long-sleeve dress shirts” and “white wool hiking socks” are subordinate categories. Members of basic level categories like “shirts” and “socks” have many perceptual properties in common, and are more strongly associated with motor movements than members of superordinate categories. Members of subordinate categories have many common properties, but these properties are also shared by members of other subordinate categories at the same level of abstraction in the category hierarchy. That is, while we can identify many properties shared by all “white long-sleeve dress shirts,” many of them are also properties of “blue long-sleeve dress shirts” and “black long-sleeve pullover shirts.”

Psychological research suggests that some levels of abstraction in a system of categories are more basic or natural than others. An implication for organizing system design is that basic level categories are highly efficient in terms of the cognitive effort they take to create and use.366[Cog]

The abstraction level we choose determines how precisely we identify resources. When we want to make a general claim, or communicate that the scope of our interest is broad, we use superordinate categories, as when we ask, “How many animals are in the San Diego Zoo?” But we use precise subordinate categories when we need to be specific: “How many adult emus are in the San Diego Zoo today?”

If we return to our clothing example, finding a pair of white wool hiking socks is very easy if the organizing system for socks creates fine-grained categories. When resources are described or arranged with this level of detail, a similarly detailed specification of the resources you are looking for yields precisely what you want. When you get to the place where you keep white wool hiking socks, you find all of them and nothing else. On the other hand, if all your socks are tossed unsorted into a sock drawer, when you go sock hunting you might not be able to find the socks you want and you will encounter lots of socks you do not want. But you will not have put time into sorting them, which many people do not enjoy doing; you can spend time sorting or searching depending on your preferences.

If we translate this example into the jargon of information retrieval, we say that more fine-grained organization reduces recall, the number of resources you find or retrieve in response to a query, but increases the precision of the recalled set, the proportion of recalled items that are relevant. Broader or coarse-grained categories increase recall, but lower precision. We are all too familiar with this hard bargain when we use a web search engine; a quick one-word query results in many pages of mostly irrelevant sites, whereas a carefully crafted multi-word query pinpoints sites with the information we seek. We will discuss recall, precision, and evaluation of information retrieval more extensively in Chapter 9, “Interactions with Resources”.

This mundane example illustrates the fundamental tradeoff between organization and retrieval. A tradeoff between the investment in organization and the investment in retrieval persists in nearly every organizing system. The more effort we put into organizing resources, the more effectively they can be retrieved. The more effort we are willing to put into retrieving resources, the less they need to be organized first. The allocation of costs and benefits between the organizer and retriever differs according to the relationship between them. Are they the same person? Who does the work and who gets the benefit?

The ways in which people categorize depend on the goals of categorization, the breadth of the resources in the collection to be categorized, and the users of the organizing system. Suppose that we want to categorize languages. Our first step might be determining what constitutes a language, since there is no widespread agreement on what differentiates a language from a dialect, or even on whether such a distinction exists.

What we mean by “English” and “Chinese” as categories can change depending on the audience we are addressing and what our purpose is, however.367[Ling] A language learning school’s representation of “English” might depend on practical concerns such as how the school’s students are likely to use the language they learn, or which teachers are available. For the purposes of a school teaching global languages, and one of the standard varieties of English (i.e., those associated with political power), or an amalgamation of several standard varieties, might be thought of as a single instance (“English”) of the category “Languages.”

Similarly, the category structure in which “Chinese” is situated can vary with context. While some schools might not conceptualize “Chinese” as a category encompassing multiple linguistic varieties, but rather as a single instance within the “Languages” category, another school might teach its students Mandarin, Wu, and Cantonese as dialects within the language category “Chinese,” that are unified by a single standard writing system. In addition, a linguist might consider Mandarin, Wu, and Cantonese to be mutually unintelligible, making them separate languages within the broader category “Chinese” for the purpose of creating a principled language classification system.

In fact languages can be categorized in multitude of ways. If we are concerned with linguistic diversity and the survival of minority languages, we might categorize some languages as endangered in order to mobilize language preservation efforts. We could also categorize languages in terms of shared linguistic ancestors (“Romance languages,” for example), in terms of what kinds of sounds they make use of, by how well we speak them, by regions they are commonly spoken in, whether they are signed or unsigned, and so on. We could also expand our definition of the languages category to include artificial computer languages, or body language, or languages shared by people and their pets—or thinking more metaphorically, we might include the language of fashion.

If people could only categorize in a single way, the Pyramid game show, where contestants guess what category is illustrated by the example provided by a clue giver, would pose no challenge. The creative possibilities provided by categorization allow people to order the world and refer to interrelationships among conceptions through a kind of allusive shorthand. When we talk about the language of fashion, we suggest that in the context of our conversation, instances like “English,” “Chinese,” and “fashion” are alike in ways that distinguish them from other things that we would not categorize as languages.