Odds Ratios- By-hand and SPSS
Hello, this is partly a revision exercise for me, and I though it might be useful for other students of psychiatry/biomed/psychology to have another resource from which to learn if they are struggling. I'm going to go over an example of how to calculate an odds ratio, both by hand and using SPSS. The reason why I'm showing both ways is a) so you can see and understand how SPSS calculates it and b) some courses (including mine) may require you to calculate by hand in an exam.
The example I will be using is the rates of schizophrenia (S1) in people who were exposed to asphyxia during birth. The numbers have been changed slightly to allow me to input into SPSS quicker.
We need to work out what the odds are of someone with S1,also suffering from asphyxia at their time of birth. To do this we need to divide the odds of the cases by the odds of the non cases.
The odds of the cases having birth asphyxia is 28/40=0.7
The odds of non-cases having birth asphyxia is 22/65=0.34
0.7/0.34=2.06
This means that if you have S1, the odds are 2.06 times higher that you had asphyxia when you were born. This suggests there is a strong association between the two variables. Bit it important to remember that it does not necessarily mean that birth asphyxia causes S1.
SPSS
Now it's time to do it the shorter way. Input all your variables into SPSS, I just assigned values to S1/no S1, Asphyxia/no Asphyxia and did it that say.
The next bit is simple,go to- Analyze ->Descriptive Statistics -> Crosstabs
Stick S1 in the columns box, and Asphyxia into Rows.
Go into Statistics and select Risk, this is important!
If you wish in the Cells options menu, you can choose row or column percentages, this is particularly useful if you're going to write the results up later. You can now click Ok and the analysis will be done. The 3rd table (shown below) gives the odds ratio and 95% confidence intervals.
As you can see the SPSS output is very similar to my calculation. How close they are depends on how you prefer to round the values. That's pretty much all that you have to do. I hope it's helped you all get your heads around odds ratios. Has it been helpful? Let me know if there are any other topics you want me to cover, in terms of hand calculation, SPSS or study design. Have fun!