Question

1. An intrepid epidemiology student was examining statistics from the National Highway Traffic Safety Administration when she made a startling discovery: smoking among drivers appeared to be associated with fatal car accidents. She gathered 2009 data from Florida and arranged it in the 2x2 table below:

	Fatal Accident	Nonfatal Accident	Total
Smoking	1289	164723	166012
No Smoking	1269	688421	689690
Total	2558	853144	855702

1. Calculate and interpret the risk ratio and risk difference based on the above table.

2. With such compelling results, the student immediately began thinking of reasons for the observed association. Could it be that people who smoked were more distracted? That they tended to drive with only one hand on the wheel and were at greater risk of losing control? That they had reduced visibility because of the smoky car interior? Her mentor had another idea, however, and suggested that she consider whether a third variable, such as alcohol consumption, might be driving the results. The student first considered whether the driver’s testing positive for blood alcohol at the scene of the accident was associated with smoking in her data:

	Smoking	No Smoking	Total
Alcohol	124337	335475	459812
No Alcohol	41675	354215	395890
Total	166012	689690	855702

3. Calculate and interpret the risk ratio and risk difference based on the above table.

She then considered whether testing positive for alcohol was associated with being in a fatal car crash in her data:

	Fatal Accident	Nonfatal Accident	Total
Alcohol	1639	458173	459812
No Alcohol	919	394971	395890
Total	2558	853144	855702

4. Calculate and interpret the risk ratio and risk difference based on the above table.

5. Do you think that the association between smoking and fatal car accidents originally observed by the student may have been confounded by alcohol? Explain.

Answer 1