Table 4. Time Series Models for Staphylococcus aureus Acquisition Rates in the Intervention Intensive Care Unit (ICU) (2002-2007) and Control ICU (2005-2007).
| Intervention (Surgical) ICUa | Control (Medical) ICU | |||
|---|---|---|---|---|
| Crude rates | Rate (per 1,000 pt-days at riskb) | Rate (per 1,000 pt-days at riskb) | ||
| Pre-intervention | 19.73 | 19.21 | ||
| Post-intervention | 15.63 | 15.33 | ||
| Percent change | −20.78% | −20.18% | ||
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| ||||
| Time Series Modelb | Beta [95% CI] | P | Beta [95% CI] | p |
| Intervention | −4.10[−7.25, −0.95] | 0.012 | −6.75[−20.75, 7.26] | 0.334 |
| Adjusted S. aureus colonization pressure |
49.13 [26.21, 72.04] | 0.000 | 25.96 [−15.38, 67.31] | 0.210 |
| AR(1) | 0.24 [−0.13, 0.61] | 0.191 | ||
| MA(4) | −0.38 [−0.74, −0.02] | 0.039 | ||
| Constant | 2.40 [−5.99, 10.79] | 0.569 | 12.50 [−9.77, 34.76] | 0.261 |
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| Model parameters | ||||
| Adjusted R2 | 0.28 | 0.10 | ||
| Durbin-Watson statistic | 1.90 | 1.99 | ||
| Q(4) p-value | 0.83 | 0.81 | ||
| Q(8) p-value | 0.87 | 0.87 | ||
| Q(12) p-value | 0.87 | 0.88 | ||
Note: AR(1)= auto-regressive variable, 1; MA(4)= moving-average variable, 4.
The inclusion of AR and MA terms in the model addressed serial correlation seen, as evidenced by the Durbin-Watson statistic and the Box-Ljung Q(k) tests of the unadjusted model.
a
Models are based on all available data for the surgical ICU (61months, 2002-2007) and medical ICU (24 months, 2005-2007).
b
See study methods for definition of patient-days at risk.
c
In the multivariate time series model, ‘beta’ indicates the magnitude and the direction of the variable in the model, while the p value and 95% CI indicate the precision and significance of the variable within the model.