Our physical environment provides various sources for us to recognize behavior without observing it directly and, therefore, without being in danger of influencing what is under investigation (Kazdin, 1979). According to Webb et al. (1966, p. 36), the measurement of these natural traces might be separated into erosion and accretion measures. Erosion refers to "the degree of selective wear on some material," whereas accretion measures record the "deposit of materials." Natural traces may be used to determine preferences and to interpret them in various situations: for studying the impact of situational variables on the viewing of erotic material (Kirschner, 1976), for example, magazines available for reading in a doctor's waiting room indicate the relative rate of being read after some time. The degree of abrasion of museum floors indicates the number of visitors attracted to a particular exhibit
(Gillespie & Perry, 1973; Webb et al., 1966; see earlier discussion). In environmental psychology, Bechtel's (1967) "hodometer research" has become quite famous: Bechtel studied the movement of visitors in art galleries by placing invisible electronic counting devices in the floor (Willems & Raush, 1969). Even temporary traces like fingerprints on glass doors may indicate the age of people using the door because they vary in height. Traces on manipulated material may also be interpreted for research purposes: Friedmann and Wilson (1975) affixed tiny glue seals between pages in textbooks to study their usage unobtrusively.
Analyses of traces are particularly interesting for environmental design and other fields concerned with the interface between people and their physical surroundings (Baxter, 1970; Kates & Adams, 1982; Rubenstein, Paradis, & Munro, 1993). Traces may be analyzed to recognize suboptimal relationships between behavior, needs, and arrangements conceptualized to meet these. Barker (1968) called this "synomorphy," a term that indicates a kind of relationship between persons (i.e., their regularly occurring behavior) and settings. Traces may indicate actions to enhance synomorphy. For example, in offices and other workplaces where possible, people rearrange the furniture and other items to work more effectively or comfortably (Davis, 1984; Schaible-Rapp & Kugelmann, 1982). Traces may also show us environmental design errors. In rest-rooms on German trains, for instance, the traces left by passengers pressing their fingers on signs indicate that the signs have been confused with the actual switches for initiating the toilet flush mechanism, sink, and electronic hand dryer. For remodeling, traces were correctly understood and the restrooms remodeled.
Nature is even more indicative of human activities. In public parks, for example, paths indicate where people regularly walk or drive. Because more solid surfaces are less likely to show signs of wear, the natural emergence of traces on lawns or topsoil may be used to determine the position of the paths, but only after the park has been used for some length of time. This unobtrusive method allows the variation of not only the route but also the width of trails. Because user needs analyses (Linneweber, 1993; Sommer, 1983) became an applied field for psychology increasingly, it is important to improve environmental design in method and theory based on these experiences. The role of psychologists in user needs analysis is to read and—even more demanding—interpret the traces and, finally, to advise environmental designers.
The nonreactive research on litter and littering behavior is well documented. As traces of consumption, packaging material, cans, empty bottles, and other types of garbage can be examined to indicate what the people who threw the items away use or prefer (see the extensive research by Rathje, 1984). Because rummaging through other people's garbage as well as applying other measures of trace analysis might appear to some readers as being just a cute technique of creative people calling themselves scientists, one cannot highlight enough the scientific importance that trace measures like the littering technique can have when investigating certain research questions. Litter analysis is the method of choice when social desirability distortions or the inaccessibility of a certain population might prevent reliable measurement of consumption patterns or attitudes. Reilly (1984), for example, described the great importance of these measures for market research. Consider a household being asked to indicate the relative amount of fast or canned food consumed per week. Because it may be (socially) desirable to underestimate this amount in favor of fresh products, an analysis of waste is much more representative (Nay, 1979; Schweigert, 1998). Cialdini and Baumann (1981) demonstrated the applicability of litter analysis as a nonreactive attitude measure. At a parking lot they placed campaign flyers of presidential candidates on car windshields and observed whether the drivers threw them away or kept them. Littering significantly correlated with the driver's previously assessed voting preference. In a second study the authors compared attitudes on topics either high or low in social desirability, measured by both a standard interview and the littering technique. Both measures provided comparable results for those attitude objects low on social desirability. However, in line with the notion of nonreactivity, both meas urement techniques lead to different attitude values when objects of high social desirability had been assessed. Here, relative to the littering results, interview data was significantly distorted into the socially desired direction.
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