Rounding Behavior in the Reporting of Headache Frequency Complicates Headache Chronification Research


  • Financial Support: Research reported in this publication was supported by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health under award number R01NS065257. The original study was funded by the National Headache Foundation through a grant from Ortho-McNeil Neurologics, Inc.
  • Conflicts of Interest: Timothy T. Houle: Dr. Houle receives research support from GlaxoSmithKline and Merck; Dana P. Turner: Ms. Turner receives research support from Merck; Thomas A. Houle: Dr. Houle reports no conflicts of interest; Todd A. Smitherman: Dr. Smitherman receives research support from Merck; Vincent T. Martin: Dr. Martin is a consultant for Allergan, Merck, and Nautilus; a speaker for Allergan; and has received grants from GlaxoSmithKline and Endo Pharmaceuticals; Donald B. Penzien: Dr. Penzien receives research support from Merck; Richard B. Lipton: Dr. Richard B. Lipton receives research support from the NIH [PO1 AG03949 (Program Director), PO1AG027734 (Project Leader), RO1AG025119 (Investigator), RO1AG022374-06A2 (Investigator), RO1AG034119 (Investigator), RO1AG12101 (Investigator), K23AG030857 (Mentor), K23NS05140901A1 (Mentor), and K23NS47256 (Mentor)], the National Headache Foundation, and the Migraine Research Fund; serves on the editorial boards of Neurology and Cephalalgia and as senior advisor to Headache, has reviewed for the NIA and NINDS, holds stock options in eNeura Therapeutics (a company without commercial products); serves as consultant, advisory board member, or has received honoraria from: Allergan, American Headache Society, Autonomic Technologies, Boehringer-Ingelheim Pharmaceuticals, Boston Scientific, Bristol Myers Squibb, Cognimed, Colucid, Eli Lilly, eNeura Therapeutics, GlaxoSmithKline, MAP, Merck, Nautilus Neuroscience, Novartis, NuPathe, Vedanta, Zogenix.

Address all correspondence to T.T. Houle, Department of Anesthesiology, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157, USA, email:



To characterize the extent of measurement error arising from rounding in headache frequency reporting (days per month) in a population sample of headache sufferers.


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.


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.


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.


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.