Table 1 Characteristics of hospitals
From: Perioperative patient safety indicators and hospital surgical volumes
|  |  | Low-volume |  | Mid-volume |  | High-volume |  | Total | p value†|
|---|---|---|---|---|---|---|---|---|---|
| Â | Mean | S.D. | Mean | S.D. | Mean | S.D. | Mean | S.D. | Â |
Patient level characteristics | Â | Â | Â | Â | Â | Â | Â | Â | Â |
  Age (year) | 68.3 | 5.8 | 59.1 | 12.8 | 57.3 | 4.7 | 61.5 | 9.8 | p < 0.05 |
  Proportion of females (%) | 50.3 | 9.8 | 46.4 | 9.4 | 45.8 | 3.5 | 47.5 | 8.2 | p < 0.01 |
  Charlson comorbidity index | 1.10 | 0.66 | 1.04 | 0.46 | 1.08 | 0.32 | 1.07 | 0.49 | n.s. |
  Length of hospital stay (days) | 18.9 | 6.2 | 15.5 | 4.0 | 14.2 | 1.7 | 16.2 | 4.7 | p < 0.01 |
Hospital level characteristics | Â | Â | Â | Â | Â | Â | Â | Â | Â |
  Number of discharged patients per month | 141.4 | 66.8 | 336.7 | 86.6 | 693.7 | 219.1 | 393.8 | 270.3 | p < 0.01 |
  Number of surgeries per month | 45.1 | 24.5 | 135.2 | 30.2 | 313.3 | 114.5 | 166.1 | 132.3 | p < 0.01 |
  Percent of surgical patients (%) | 31.2 | 12.4 | 41.4 | 8.8 | 45.4 | 7.5 | 39.4 | 11.4 | p < 0.01 |
  Proportion of difficult surgeries (%) | 54.9 | 19.1 | 60.3 | 10.3 | 63.4 | 4.4 | 59.6 | 13.1 | p < 0.05 |
  Mortality rates (%) | 6.1 | 2.9 | 4.8 | 2.0 | 4.1 | 1.5 | 5.0 | 2.3 | p < 0.01 |
  Mortality rates of postoperative patients (%) | 5.7 | 6.8 | 2.9 | 1.4 | 2.7 | 0.9 | 3.7 | 3.7 | p < 0.01 |