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The term unobtrusive measures was first coined by Eugene Webb, Campbell, Schwartz, and Sechrest in a 1966 book, Unobtrusive methods: Nonreactive research in the social science ,  in which they described methods that don't involve direct induction of data from research subjects. For example, the evidence people leave behind as they traverse their physical environment is unobtrusive. Unobtrusive methods get around biases, such as the selection bias and the experimenter's bias , that result from the researcher and his intrusion. Consequently, however, these methods reduce the researcher's control over the type of data collected.