Poisson regression for the effect of month on dSSI rates over time (spinal fusion).
| Period | Patients (n = 15,376) | No. of dSSI (n = 176) | dSSI Rate (%) | Rate Ratio (95% CI)a | P Value |
| Overall | 15,376 | 176 | 1.14 | 0.88 (0.84, 0.93) | <0.01 |
| Month | |||||
| July (reference variable) | 1200 | 14 | 1.17 | --- | --- |
| January | 1150 | 22 | 1.91 | 1.49 (0.76, 2.94) | >0.05 |
| February | 1264 | 14 | 1.11 | 1.13 (0.54, 2.38) | >0.05 |
| March | 1323 | 15 | 1.13 | 1.16 (0.56, 2.41) | >0.05 |
| April | 1266 | 20 | 1.56 | 1.86 (0.94, 3.68) | 0.04 |
| May | 1312 | 19 | 1.40 | 1.35 (0.68, 2.70) | >0.05 |
| June | 1355 | 19 | 1.36 | 1.34 (0.67, 2.66) | >0.05 |
| August | 1225 | 17 | 1.39 | 1.48 (0.73, 3.01) | >0.05 |
| September | 1092 | 12 | 1.10 | 1.17 (0.54, 2.53) | >0.05 |
| October | 1139 | 11 | 0.97 | 0.91 (0.41, 2.00) | >0.05 |
| November | 1125 | 6 | 0.53 | 0.76 (0.29, 1.98) | >0.05 |
| December | 1014 | 12 | 1.18 | 1.05 (0.49, 2.28) | >0.05 |
Abbreviation: dSSI, deep surgical site infection.
Note: Boldface indicates statistically significant findings.
↵a Rate ratio obtained from Poisson regression.