The sampling of rare and elusive populations is difficult because the costs of locating such populations are substantial and can exceed actual interviewing costs. There are efficient probability methods that have been developed recently that reduce these costs. If the special populations are geographically clustered, efficient sampling involves the rapid location of segments in which no members of the special population are located with the use of Census data, telephone screening, or incomplete lists. Populations that are not geographicaily clustered can be located by network sampling and use of large previously gathered samples. Characteristics of mobile populations such as the homeless can be estimated by capture-recapture methods.

objective: To evaluate the association between BMI (kg/m 2 ) and headaches among women. Methods and Procedures:Cross-sectional analysis of 11 datasets identified after searching for all large publicly available epidemiologic cohort study datasets containing relevant variables. Datasets included National Health Interview Survey (NHIS): 1997-2003, the first National Health Examination and Nutrition Survey, Alameda County Health Study (ACHS), Tecumseh Community Health Study (TCHS), and Women's Health Initiative (WHI). The women (220,370 in total) were aged 18 years or older and had reported their headache or migraine status. Results: Using nonlinear regression techniques and models adjusted for age, race, and smoking, we found that increased BMI was generally associated with increased likelihood of headache or severe headache among women. A BMI of ~20 was associated with the lowest risk of headache. Relative to a BMI of 20, mild obesity (BMI of 30) was associated with roughly a 35% increase in the odds for experiencing headache whereas severe obesity (BMI of 40) was associated with roughly an 80% increase in odds. Results were essentially unchanged when models were further adjusted for socioeconomic variables, alcohol consumption, and hypertension. Being diagnosed with migraine showed no association with BMI. Discussion: Among US women, a BMI of ~20 (about the 5th percentile) was associated with the lowest likelihood of headache. Consistently across studies, obese women had significantly increased risk for headache. By contrast, the risk for diagnosed migraine headache per se was not obviously related to BMI. The direction of causation and mechanisms of action remain to be determined.

This issue is of particular relevance to the interpretation of incidence measures, computed from the SEER data, for which there is no national database. Future efforts should be directed at a better understanding of how the SEER population differs from the U.S. population so that SEER rates can be adjusted to be more nationally representative.

Background-Numerous public health organizations have adopted national physical activity recommendations. Despite these recommendations, over half of the US population does not meet the minimum recommendation for physical activity, with large variations across individual US states.

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