Poisson regression for the effect of month on dSSI rates over time (laminectomy).
| Period | Patients (n = 7931) | No. of dSSI (n = 128) | dSSI Rate (%) | Rate Ratio (95% CI)a | P Value |
| Overall | 7931 | 128 | 1.64 | 1.02 (0.90, 1.15) | >0.05 |
| Month | |||||
| July (reference variable) | 766 | 7 | 0.91 | --- | --- |
| January | 590 | 1 | 0.17 | 0.15 (0.02, 1.26) | >0.05 |
| February | 674 | 3 | 0.45 | 0.46 (0.12, 1.76) | >0.05 |
| March | 784 | 1 | 0.13 | 0.15 (0.02, 1.21) | >0.05 |
| April | 678 | 2 | 0.29 | 0.39 (0.08, 1.90) | >0.05 |
| May | 704 | 4 | 0.57 | 0.63 (0.18, 2.15) | >0.05 |
| June | 734 | 4 | 0.54 | 0.53 (0.16, 1.82) | >0.05 |
| August | 672 | 4 | 0.59 | 0.68 (0.20, 2.31) | >0.05 |
| September | 550 | 4 | 0.73 | 0.78 (0.23, 2.65) | >0.05 |
| October | 625 | 1 | 0.16 | 0.17 (0.02, 1.38) | >0.05 |
| November | 550 | 3 | 0.55 | 0.83 (0.21, 3.20) | >0.05 |
| December | 604 | 4 | 0.66 | 0.71 (0.21, 2.45) | >0.05 |
Abbreviation: dSSI, deep surgical site infection.
↵a Rate ratio obtained from Poisson regression.