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Context.-Tension-type headache is a highly prevalent condition. Because few population-based studies have been performed, little is known about its epidemiology. Objectives.-To estimate the 1-year period prevalence of episodic tension-type headache (ETTH) and chronic tension-type headache (CTTH) in a populationbased study; to describe differences in 1-year period prevalence by sex, age, education, and race; and to describe attack frequency and headache pain intensity. Design.-Telephone survey conducted 1993 to 1994. Setting.-Baltimore County, Maryland. Participants.-A total of 13 345 subjects from the community. Main Outcome Measures.-Percentage of respondents with diagnoses of headache using International Headache Society criteria. Workdays lost and reduced effectiveness at work, home, and school because of headache, based on self-report. Results.-The overall prevalence of ETTH in the past year was 38.3%. Women had a higher 1-year ETTH prevalence than men in all age, race, and education groups, with an overall prevalence ratio of 1.16. Prevalence peaked in the 30-to 39-year-old age group in both men (42.3%) and women (46.9%). Whites had a higher 1-year prevalence than African Americans in men (40.1% vs. 22.8%) and women (46.8% vs 30.9%). Prevalence increased with increasing educational levels in both sexes, reaching a peak in subjects with graduate school educations of 48.5% for men and 48.9% for women. The 1-year period prevalence of CTTH was 2.2%; prevalence was higher in women and declined with increasing education. Of subjects with ETTH, 8.3% reported lost workdays because of their headaches, while 43.6% reported decreased effectiveness at work, home, or school. Subjects with CTTH reported more lost workdays (mean of 27.4 days vs 8.9 days for those reporting lost workdays) and reduced-effectiveness days (mean of 20.4 vs 5.0 days for those reporting reduced effectiveness) compared with subjects with ETTH. Conclusions.-Episodic tension-type headache is a highly prevalent condition with a significant functional impact at work, home, and school. Chronic tension-type headache is much less prevalent than ETTH; despite its greater individual impact, CTTH has a smaller societal impact than ETTH.

The use and functionality of electronic health records (EHRs) have increased rapidly in the past decade. Although the primary purpose of EHRs is clinical, researchers have used them to conduct epidemiologic investigations, ranging from cross-sectional studies within a given hospital to longitudinal studies on geographically distributed patients. Herein, we describe EHRs, examine their use in population health research, and compare them with traditional epidemiologic methods. We describe diverse research applications that benefit from the large sample sizes and generalizable patient populations afforded by EHRs. These have included reevaluation of prior findings, a range of diseases and subgroups, environmental and social epidemiology, stigmatized conditions, predictive modeling, and evaluation of natural experiments. Although studies using primary data collection methods may have more reliable data and better population retention, EHR-based studies are less expensive and require less time to complete. Future EHR epidemiology with enhanced collection of social/behavior measures, linkage with vital records, and integration of emerging technologies such as personal sensing could improve clinical care and population health.