title: Rounding behavior in the reporting of headache frequency complicates headache chronification research.
abstract: OBJECTIVE
  To characterize the extent of measurement error arising from rounding in headache frequency reporting (days per month) in a population sample of headache sufferers.
  
  
  BACKGROUND
  When reporting numerical health information, individuals tend to round their estimates. The tendency to round to the nearest 5 days when reporting headache frequency can distort distributions and engender unreliability in frequency estimates in both clinical and research contexts.
  
  
  METHODS
  This secondary analysis of the 2005 American Migraine Prevalence and Prevention study survey characterized the population distribution of 30-day headache frequency among community headache sufferers and determined the extent of numerical rounding ("heaping") in self-reported data. Headache frequency distributions (days per month) were examined using a simplified version of Wang and Heitjan's approach to heaping to estimate the probability that headache sufferers round to a multiple of 5 when providing frequency reports. Multiple imputation was used to estimate a theoretical "true" headache frequency.
  
  
  RESULTS
  Of the 24,000 surveys, headache frequency data were available for 15,976 respondents diagnosed with migraine (68.6%), probable migraine (8.3%), or episodic tension-type headache (10.0%); the remainder had other headache types. The mean number of headaches days/month was 3.7 (standard deviation = 5.6). Examination of the distribution of headache frequency reports revealed a disproportionate number of responses centered on multiples of 5 days. The odds that headache frequency was rounded to 5 increased by 24% with each 1-day increase in headache frequency (odds ratio: 1.24, 95% confidence interval: 1.23 to 1.25), indicating that heaping occurs most commonly at higher headache frequencies. Women were more likely to round than men, and rounding decreased with increasing age and increased with symptoms of depression.
  
  
  CONCLUSIONS
  Because of the coarsening induced by rounding, caution should be used when distinguishing between episodic and chronic headache sufferers using self-reported estimates of headache frequency. Unreliability in frequency estimates is of particular concern among individuals with high-frequency (chronic) headache. Employing shorter recall intervals when assessing headache frequency, preferably using daily diaries, may improve accuracy and allow more precise estimation of chronic migraine onset and remission.
authors.affiliation: Department of Anesthesiology, Wake Forest University School of Medicine, Winston-Salem, NC 27157, USA. thoule@wakehealth.edu
authors.forename: Timothy T
authors.initials: TT
authors.lastname: Houle
authors.forename: Dana P
authors.initials: DP
authors.lastname: Turner
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Houle
authors.forename: Todd A
authors.initials: TA
authors.lastname: Smitherman
authors.forename: Vincent
authors.initials: V
authors.lastname: Martin
authors.forename: Donald B
authors.initials: DB
authors.lastname: Penzien
authors.forename: Richard B
authors.initials: RB
authors.lastname: Lipton
mesh_terms: D000328:Adult; D035843:Biomedical Research / Q000379:methods*; D003625:Data Collection / Q000379:methods*; D005260:Female; D005500:Follow-Up Studies; D006261:Headache / Q000175:diagnosis / Q000453:epidemiology* / Q000517:prevention & control*; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D010147:Pain Measurement / Q000379:methods; D011159:Population Surveillance / Q000379:methods*; D015995:Prevalence; D014481:United States / Q000453:epidemiology