Any means of obtaining psychological or social science research data other than by interviews, questionnaires, or other direct approaches, that is, without the conscious cooperation of the research participants or subjects. It is considered useful where responses might be influenced by the respondents' awareness that their behaviour is being observed, the validity of measurements in such circumstances demanding non-reactive measures. The most common unobtrusive measures are observational studies; studies of the physical traces of behaviour, such as the wear of floor tiles in front of museum exhibits as measures of the exhibits' popularity; and investigations based on archival records, including legal records, mass media, sales records, diaries, and other personal data. The term was popularized by the US psychologist Eugene J(ohn) Webb (1933–95) and several co-authors in their book Unobtrusive Measures: Nonreactive Research in the Social Sciences (1966), in which the idea itself is traced to the 1870s and attributed to the English explorer and scientist Sir Francis Galton (1822–1911). See also bogus pipeline, implicit association test, lost-letter technique, modern racism.