To perform the matched case-control analysis using the SPSS Multinomial logistic procedure we compute differences between each pair of cases and controls on each predictor. So, for example, for the data on the previous page the first pair of observations consistis of a case with an IQ of 23 and a control with an IQ of 12 so the case-control IQ difference is 23-12=11. Similarly the case has the disorder (code=1) whereas the control does not (code=0) so the case-control difference on disorder is 1-0=1. These are entered on the first row of the table below. The differences in case-control disorders and IQs are worked out similarly for the other 5 pairs giving the table below which has six rows, one for each pair.
Disorder Difference |
IQ Difference |
Constant |
|||
-1 |
11 |
1 |
|||
-1 |
-28 |
1 |
|||
-1 |
11 |
1 |
|||
-1 |
51 |
1 |
|||
-1 |
-17 |
1 |
|||
1 |
18 |
1 |
|||
We also add a constant row which takes the value one for all observations.
We are now ready to do the matched pair analysis. We go to analyze:regression:multinomial logistic and enter the constant term as the dependent variable with disorder and iq as covariates. We also need to click on the model button and unclick the "include intercept in model" box. We then run the procedure and get the exactly the same results as using the Cox model method. We obtain an odds ratio of 0.55 for the odds of a disorder being in a control as opposed to a case.
Categorical predictors with 3 or more levels need to be converted into dummy variables before being differenced and entered as predictors into the multinomial logistic procedure. A Macro is available to do this. [PW TO ADD THIS IN]