This article uses empirical data from a recent obtrusive study of reference performance to explore content validity and assumptions regarding unobtrusive studies. Data collected by author support the contention that improvements are highly desirable before conducting more unobtrusive studies of reference service. The two most important changes concern the development of test questions representing all types of queries and supplementing the correct fill rate with other measures of reference performance.
Jo Bell Whitlatch. "Unobtrusive studies and the quality of academic library reference services" College & Research Libraries (1989): 181-194.