Suggested answer to exercise: Readmission to hospital for asthmatic children, 7

Question 7: Does the adjustment for possible confounding variables change the estimate of the effect on readmission of the crisis plan?

Suggested answer

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
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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