Periodicity in academic library circulation: A spectral analysis

Spectral analysis was used to identify periods in four academic library circulation series (journals, faculty books, student books, and turnstile count) and four of pickup for shelving (reference books, non-reference books, bound journals, and current journals). Observed periods were compared to theoretical periods in a 364-day academic calendar: Two were expected, 182 and 7 days; one (122 days) was interpreted as a 112-day semester plus an interim of about 10 days; two were unexpected (3.5 and 2.3 days). The 182-day (about half an academic year) was strongest in the pickup series. The 7-day period was strong in all series. The 3.5 days was strong in most series. The 2.3 day, found only in circulation, is close to every-other-day usage or about 3 cycles per week, and may be due to Monday-Wednesday-Friday class scheduling, or it may be spurious. Only the 182-day period was visible in time charts. No evidence was found for a monthly period, the calendar unit in which library time series are often recorded.