Table 1. Cohort Characteristics.

Characteristic	Back Pain Visits	Headache Visits	Headache Visits Seen by Clinicians Who Also Saw Back Pain
Primary care physicians, No.	57 859	52 876	34 190
Visits per primary care physician, median (IQR)	7 (4-13)	7 (4-13)	9 (5-17)
Ownership of imaging (% of primary care physicians)	26.6	4.4	5.4
Uncomplicated clinical visits, No.	534 559	492 804	405 721
Unique patients seen, No.	478 640	417 010	344 991
Images ordered (% of visits)	13.3	7.5	7.6
Chiropractors, No.	27 583	NA	NA
Visits per chiropractor, median (IQR)	10 (5-19)	NA	NA
Ownership of imaging (% of chiropractors)	53.1	NA	NA
Uncomplicated clinical visits, No.	366 529	NA	NA
Unique patients seen, No.	306 507	NA	NA
Images ordered (% of visits)	17.0	NA	NA
Specialist physicians, No.a	15 535	NA	NA
Visits per specialist, median (IQR)	5 (3-9)	NA	NA
Ownership of imaging (% of specialists)	41.3	NA	NA
Uncomplicated clinical visits, No.	106 304	NA	NA
Unique patients seen, No.	93 573	NA	NA
Images ordered (% of visits)	36.5	NA	NA
Patient age distribution (% of visits), y
18-25	7.7	10.0	10.1
26-35	19.5	21.4	22.1
36-45	25.8	26.8	27.2
46-55	27.8	25.9	25.6
56-64	19.2	15.9	15.1
Female, %	53.9	70.2	69.3
Low neighborhood education level, %b	17.2	19.1	20.0
High neighborhood poverty level, %c	27.6	28.9	29.6
White race/ethnicity, %	72.8	67.8	66.2
Census region, %
West	17.8	15.8	15.6
South	45.2	51.3	54.3
Midwest	29.0	23.5	23.7
Northeast	8.0	9.3	6.5

Abbreviations: IQR, interquartile range; NA, not applicable.

^aThe most common specialties (accounting for two-thirds of all specialist visits) were orthopedic surgery, neurosurgery, back and spine surgery, physical medicine, and rheumatology.

^bLow neighborhood education was defined as census block groups in which more than 25% of residents had education levels below high school.

^cHigh neighborhood poverty was defined as census block groups in which more than 10% of residents had incomes below the poverty level.