An unobtrusive measure of observation is any method of observation that directly removes the observer from the set of interactions or events being studied. Public archival documents represent one major class of unobtrusive measures; the conditions that lead to their production are in no way influenced by an intruding sociological observer. Unobtrusive measures range from the public and private archive to simple behavior observations of persons at work or play, from contrived observations based on mechanical equipment (such as tape recorders and video cameras) to physical trace analysis. When observers begin to sample by locale, time and subject statistical models take on their greatest relevance. The student employing unobtrusive methods should search for naturally occurring groups and locations for his studies, and abandon the college student and housewife as the main classes of subjects. A viable causal hypothesis will demonstrate covariance between the variables examined while establishing that the independent variable preceded the dependent variable in time.