Question 7: Does the adjustment for possible confounding variables change the estimate of the effect on readmission of the crisis plan?
We need the hazard ratio for the crisis plan without any adjustment. In SPSS, the way to do this is to run the Cox regression with crisis plan as the only predictor. We do this exactly as in the last question, except for the covariate list, which contains only "Crisis plan".
We get the following output:
| Omnibus Tests of Model Coefficientsa,b | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| -2 Log Likelihood | Overall (score) | Change From Previous Step | Change From Previous Block | ||||||
| Chi-square | df | Sig. | Chi-square | df | Sig. | Chi-square | df | Sig. | |
| 7088.024 | 32.412 | 1 | .000 | 27.878 | 1 | .000 | 27.878 | 1 | .000 |
| a. Beginning Block Number 0, initial Log Likelihood function: -2 Log likelihood: 7115.902 | |||||||||
| b. Beginning Block Number 1. Method = Enter | |||||||||
| Variables in the Equation | ||||||||
|---|---|---|---|---|---|---|---|---|
| B | SE | Wald | df | Sig. | Exp(B) | 95.0% CI for Exp(B) | ||
| Lower | Upper | |||||||
| crisis | .599 | .107 | 31.466 | 1 | .000 | 1.821 | 1.477 | 2.244 |
Without any adjustment, we estimate that the effect of the crisis plan is to increase readmission by a factor of 1.82 (95% confidence interval 1.48 to 2.24).
The adjusted estimate of the effect of the crisis plan was an increased risk of readmission by a factor estimated to be 1.76 (95% confidence interval 1.41 to 2.19). Hence the adjustment has made very little difference to the estimate.
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Last updated: 6 February, 2008.