title: Reliability of retrospectively assessed categorical menstrual cycle length data.
abstract: OBJECTIVE
  Prospective diaries have been shown to be more reliable than retrospective questionnaires for the assessment of menstrual cycle length collected as continuous data. We investigated the reliability of retrospectively collected categorical cycle length data.
  
  
  METHODS
  In 2005, 67 women from Illinois and Vermont completed a menstrual diary and retrospective questionnaire. Based on commonly used cutpoints for normal cycle length, we created 2-, 3- and 4-category cycle length variables, calculated percent agreement, Cohen's kappa and prevalence index and investigated factors associated with accurate reporting.
  
  
  RESULTS
  Overall agreement between diary and questionnaire responses was highest for the 2-category cycle length variable, <=35 vs. > 35 days (93%), and the 3-category variable, <25, 25-35 and > 35 days (82%). While Cohen's kappa fell below 0.6, its magnitude was limited by a high prevalence of "normal"" cycle length. Women with regular cycles were slightly more likely to have concordant responses.
  
  
  CONCLUSIONS
  In epidemiologic studies where cycle length categories are of interest, the limitations of prospective diaries in terms of cost, compliance, attrition and power must be carefully weighed against the potentially decreased reliability of questionnaire data.
authors.affiliation: Department of Environmental and Radiological Health Sciences, Colorado State University, Fort Collins, CO 80523-1681, USA. annette.bachand@colostate.edu
authors.forename: Annette M
authors.initials: AM
authors.lastname: Bachand
authors.forename: Lori A
authors.initials: LA
authors.lastname: Cragin
authors.forename: John S
authors.initials: JS
authors.lastname: Reif
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D007087:Illinois; D008499:Medical Records / Q000592:standards; D008597:Menstrual Cycle / Q000502:physiology*; D015203:Reproducibility of Results; D012107:Research Design; D012189:Retrospective Studies; D011795:Surveys and Questionnaires / Q000592:standards; D013997:Time Factors; D014707:Vermont; D055815:Young Adult