. Author manuscript; available in PMC: 2011 Nov 4.

Published in final edited form as: Obesity (Silver Spring). 2008 Feb;16(2):377–383. doi: 10.1038/oby.2007.32

Table 4.

Piecewise logistic regression extended model results^a

Study	Sample size	Parameter estimate	s.e.	95% CI (bootstrapped)
NHIS 1997
BMI Slope (low)^b	18,480	−0.068	0.093	(−0.224, 0.046)
Knot 1^c		20.04	1.794	(17.44, 23.23)
BMI Slope (mid)^b		0.051	0.130	(0.029, 0.125)
Knot 2		26.38	6.897	(10.71, 35.82)
BMI Slope (high)^b		0.017	0.036	(0.006, 0.030
NHIS 1998
BMI Slope	16,132	0.020	0.004	(0.014, 0.026)
NHIS 1999
BMI Slope (low)	15,421	−0.062	0.098	(−0.898, −0.004)
Knot		20.19	5.497	(12.30, 22.28)
BMI Slope (high)		0.024	0.061	(0.019, 0.032)
NHIS 2000
BMI Slope	16,205	0.029	0.003	(0.024, 0.033)
NHIS 2001
BMI Slope	16,429	0.023	0.003	(0.018, 0.028)
NHIS 2002
BMI Slope	15,077	0.024	0.004	(0.018, 0.031)
NHIS 2003
BMI Slope (low)	14,985	−0.096	0.151	(−0.809, 0.040)
Knot		19.57	2.956	(17.73, 22.34)
BMI Slope (high)		0.033	0.011	(0.028, 0.039)
ACHS
BMI Slope (low)	3,397	−0.146	0.198	(−0.441, 0.042)
Knot		19.46	3.139	(17.43, 23.81)
BMI Slope (high)		0.013	0.110	(−0.011, 0.037)
TCHS^d
BMI Slope	1,624	0.027	0.011	(0.006, 0.049)
NHANES I^e
BMI Slope	7,427	−0.015	0.015	(−0.048, 0.014)
WHI
BMI Slope	82,423	−0.009	0.002	(−0.013, −0.005)

NHANES I, First National Health and Nutrition Examination Survey; NHIS, National Health Interview Survey; TCHS, Tecumseh Community Health Study; WHI, Women’s Health Initiative.

Adjusted for age, race, smoking status, alcohol consumption, hypertension, and SES variables;

BMI was divided into 1, 2, or 3 contiguous segments (depending on the number of knots in the model—0, 1, or 2, respectively) and will have an estimate of the linear rate of change in log odds of headache per segment: low, mid, high;

Extended models were fit with the same number of knots as the primary model;

100% whites;

Employment status for men only and not adjusted for smoking as ~60% of records are missing that data.