Table 1.
Baseline Characteristics Stratified by ZIP Code–Based Socioeconomic Status (SES) Quartile
| Characteristics | SES Quartile 1 | SES Quartile 2 | SES Quartile 3 | SES Quartile 4 | P Value | Standardized Difference (Quartile 4−Quartile 1)* |
|---|---|---|---|---|---|---|
| N | 230 028 | 206 523 | 182 991 | 156 363 | ||
| Median household income, US$ | 1 to 38 999 | 39 000 to 47 999 | 48 000 to 62 999 | ≥63 000 | ||
| Mean (SE) age, y | 69.8 (14.6) | 71.5 (14.5) | 72.1 (14.4) | 73.3 (14.4) | <0.001 | 0.23* |
| Females, n (%) | 125 438 (54.6) | 111 562 (54.0) | 98 182 (53.7) | 83 176 (53.2) | <0.001 | 0.01 |
| Race, n (%)* | ||||||
| White | 104 038 (56.9) | 117 740 (74.9) | 110 071 (76.5) | 104 357 (79.8) | <0.001 | 0.24* |
| Black | 52 559 (28.8) | 21 517 (13.7) | 15 470 (10.8) | 10 332 (7.9) | <0.001 | 0.26* |
| Other | 26 246 (14.4) | 17 929 (11.4) | 18 271 (12.7) | 16 141 (12.3) | <0.001 | 0.03 |
| Weekend admission | 58 096 (25.3) | 52 703 (25.5) | 46 820 (25.6) | 40 046 (25.6) | 0.03 | 0.004 |
| Primary expected payer, n (%) | ||||||
| Medicare | 151 924 (66.2) | 140 437 (68.1) | 123 852 (67.8) | 106 726 (68.3) | <0.001 | 0.02 |
| Medicaid | 21 392 (9.3) | 12 666 (6.1) | 8840 (4.8) | 5336 (3.4) | <0.001 | 0.11* |
| Private insurance | 36 388 (15.9) | 38 445 (18.7) | 39 007 (21.4) | 37 230 (23.8) | <0.001 | 0.10* |
| Uninsured | 12 944 (5.6) | 9177 (4.5) | 6640 (3.6) | 4033 (2.6) | <0.001 | 0.07 |
| Other | 6866 (3.0) | 5404 (2.6) | 4378 (2.4) | 2871 (1.8) | <0.001 | 0.04 |
| Hospital characteristics, n (%) | ||||||
| Region | ||||||
| Northeast | 29 096 (12.7) | 29 338 (14.2) | 33 027 (18.1) | 44 209 (28.3) | <0.001 | 0.19* |
| Midwest | 43 290 (18.8) | 56 699 (27.5) | 47 942 (26.2) | 30 350 (19.4) | <0.001 | 0.01 |
| South | 131 341 (57.1) | 86 737 (42.0) | 61 923 (33.8) | 41 957 (26.8) | <0.001 | 0.31* |
| West | 26 301 (11.4) | 33 749 (16.3) | 40 099 (21.9) | 39 847 (25.5) | <0.001 | 0.18* |
| Bed size | ||||||
| Small | 25 059 (11.0) | 26 850 (13.1) | 23 299 (12.8) | 18 855 (12.1) | <0.001 | 0.02 |
| Medium | 53 779 (23.6) | 48 755 (23.8) | 44 339 (24.3) | 42 527 (27.2) | <0.001 | 0.04 |
| Large | 149 389 (65.5) | 129 520 (63.1) | 114 538 (62.9) | 94 732 (60.7) | <0.001 | 0.05 |
| Urban location | 167 993 (73.6) | 166 642 (81.2) | 171 706 (94.3) | 153 936 (98.6) | <0.001 | 0.33* |
| Teaching hospital | 96 652 (42.4) | 76 326 (37.2) | 77 757 (42.7) | 72 996 (46.8) | <0.001 | 0.04 |
| Comorbidities, n (%) | ||||||
| Diabetes | 81 756 (35.9) | 66 644 (32.5) | 56 263 (31.0) | 43 551 (28.0) | <0.001 | 0.08 |
| Hypertension | 176 920 (77.6) | 155 594 (75.8) | 138 665 (76.3) | 117 946 (75.7) | <0.001 | 0.02 |
| Smoking | 47 600 (20.7) | 42 132 (20.4) | 37 049 (20.3) | 27 813 (17.8) | <0.001 | 0.04 |
| Drug abuse | 6212 (2.7) | 3239 (1.6) | 2333 (1.3) | 1383 (0.9) | <0.001 | 0.06 |
| Alcohol abuse | 9486 (4.2) | 6989 (3.4) | 5875 (3.2) | 4290 (2.8) | <0.001 | 0.02 |
| Obesity | 14 711 (6.5) | 12 686 (6.2) | 11 272 (6.2) | 8247 (5.3) | <0.001 | 0.02 |
| Dyslipidemia | 85 402 (37.1) | 81 544 (39.5) | 76 700 (41.9) | 66 696 (42.7) | <0.001 | 0.06 |
| Congestive heart failure | 32 962 (14.5) | 28 319 (13.8) | 24 633 (13.6) | 20 869 (13.4) | <0.001 | 0.02 |
| Peripheral vascular disease | 17 601 (7.7) | 16 511 (8.0) | 15 495 (8.5) | 13 244 (8.5) | <0.001 | 0.01 |
| Chronic pulmonary disease | 35 338 (15.5) | 30 955 (15.1) | 25 882 (14.2) | 19 686 (12.6) | <0.001 | 0.04 |
| Chronic renal failure | 23 895 (10.5) | 19 471 (9.5) | 17 432 (9.6) | 14 441 (9.3) | <0.001 | 0.02 |
| Mean (SD) No. of Elixhauser comorbidities | 2.51 (1.49) | 2.43 (1.47) | 2.46 (1.51) | 2.41 (1.51) | <0.001 | 0.07 |
All quartiles were based on median household income of the respective ZIP code. Data are expressed as number (percentage) except where specified. Quartile 1, $1 to $38 999; quartile 2, $39 000 to $47 999; quartile 3, $48 000 to $63 000; quartile 4, ≥$63 000. Quartile 1 reflects the lowest‐income quartile, and quartile 4 reflects the highest‐income quartile.
Standardized differences were calculated between quartile 1 and quartile 4 by using Cohen's d statistic for continuous variables and Cramer's V statistic for categorical variables.
A small standardized difference (Cohen's d: 0.2 to 0.5, Cramer's V: 0.1 to 0.3).
Data on race were available on 614 671 discharges only.
A moderate standardized difference (Cohen's d: 0.5 to 0.8, Cramer's V: 0.3 to 0.5).