Table 1: Early-stage breast cancer patient characteristics by surgery choice and instrumental variable-based patient quintile groups.

CovariatesSurgery choice Instrument quintile groups
(higher area treatment ratio (ATR)→)
VariableCategoryTotalBCSRMastectomy value 12345 value
Column %Column %

SurgeryMastectomy16,103 (56%)0100% na37.950.259.765.768.6<0.0001*
BCSR12,572 (44%)100%0%62.149.840.334.331.4<0.0001*

Age66–7516,442 (57%)63.952.2<0.0001*57.858.056.257.457.30.2251
76+12,233 (43%)36.147.842.242.043.842.642.70.2251

Stage at diagnosisStage I17,908 (62%)75.152.6<0.0001*63.862.062.460.863.20.0908
Stage IIa7,700 (27%)20.132.125.727.526.727.826.60.1261
Stage IIb2,827 (10%)4.414.19.79.610.110.69.30.4096
Stage IInos240 (1%)0.41.20.80.90.80.80.90.2169

Tumor size 108,797 (31%)4023.4<0.0001*31.330.330.029.931.90.3633
11–2011,958 (42%)43.440.342.141.842.540.841.40.1152
>217,680 (27%)16.235.125.827.126.728.525.80.2090
unknown240 (1%)0.41.20.80.90.80.80.90.2169

GradeWell differentiated4,751 (17%)20.713.4<0.0001*16.417.016.016.117.40.2426
Moderately differentiated10,804 (38%)39.935.939.338.437.336.636.60.0002*
Poorly differentiated6,718 (23%)20.325.923.122.424.424.622.60.2138
Undifferentiated/unknown6,402 (22%)19.124.921.222.122.322.723.30.0031*

ComorbidityBelow median15,784 (55%)58.152.7<0.0001*54.655.754.756.054.10.3759
Above median12,891 (45%)41.947.345.444.345.344.045.90.3759

Physician surgical volumeLow volume8,510 (30%)26.032.6<0.0001*27.533.230.329.228.40.1447
High volume20,165 (70%)74.067.472.566.869.770.971.60.1447

Hospital bed size 35015,461 (54%)52.654.9<0.0001*54.857.756.852.147.9<0.0001*
351+13,214 (46%)47.445.145.242.343.247.952.1<0.0001*

Residence area sizeMetro24,530 (86%)90.781.5<0.0001*91.087.878.877.492.7<0.0001*
Nonmetro4,145 (14%)9.318.59.012.221.222.67.3<0.0001*

Area median incomeAbove median14,483 (51%)53.947.8<0.0001*52.451.941.850.256.30.0032*
Below median13,730 (48%)44.650.446.946.357.346.642.20.0001*
Missing462 (2%)1.51.70.71.80.93.21.5<0.0001*

aTest of difference in characteristic distribution between surgery choices.
bCochran-Armitage test of trend in characteristic value across patients grouped into quintiles based on local area mastectomy practice style measure. For example, the value for stage I tests whether a linear trend in stage I diagnoses exists across the instrument-based patient groups.
* .