Case Control Study and Odds Ratio:

Learning Objectives

• Analyze data from a Case Control study using the 2x2 table and logistic regression
• Calculate & interpret the odds ratio and confidence intervals
• Consider control sampling methods

You conduct a case-control study looking at the association between owning a turtle as a pet and Salmonella.  In your case-control study, you recruit 100% of the new onset cases of salmonella that are reported to the NYC Department of Health over a period of 6 months (cases).   At the same time, you select a random sample of New Yorkers who do not have salmonella using density sampling (you select two controls per).  You ask the cases and the controls if they have owned a pet turtle in the past month.  You also ask about consumption of spinach in the past month, as this food item has been linked to salmonella.  You end up with a total sample of 570 (188 cases and 376 controls).  The case control study data, called "Homework3.sav," should be used for this assignment.

I. Looking at the association between turtle ownership and salmonella

1. What are the null and alternate hypotheses you are testing in your case-control study regarding turtle ownership and salmonella?

2.  Calculate the odds ratio for the turtle ownership and salmonella by hand from the 2x2 table.

3. What are the odds ratio and 95% confidence interval calculated by SPSS with the crosstab function?

4. What are the odds ratio, 95% confidence interval and P-value calculated with logistic regression (note, first you need to re-code exposure & disease so E - =0, E+=1, D- =0, D+=1)

5. Interpret the odds ratio and 95% confidence interval.

6. Calculate the confidence intervals around the odds ratio by hand (show your work).

II. Repeat for the association between spinach consumption and salmonella

7. What are the null and alternate hypotheses you are testing in your case-control study regarding spinach consumption and salmonella?

8.  Calculate the odds ratio for the spinach consumption and salmonella by hand from the 2x2 table.

9. What are the odds ratio and 95% confidence interval calculated by SPSS with the crosstab function?

10. What are the odds ratio, 95% confidence interval and P-value calculated with logistic regression (note first you need to re-code exposure & disease so E-=0, E+=1, D-=0, D+=1)

11. Interpret the odds ratio and 95% confidence interval.

12. Calculate the confidence intervals around the odds ratio by hand.

III. Conceptual questions

13. What kind of sampling did this case control study use?

14. If you had done a cohort study in the same population looking at this exposure disease association, what measure of effect from that cohort study would the odd ratio you got in this case control study approximate?

15. Thinking about the results of your analyses regarding turtle ownership-salmonella and spinach consumption-salmonella, do you think that confounding might be a problem?  Why or why not?

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