EPI10-2: CONFOUNDING: AN ILLUSTRATED EXAMPLE

CONFOUNDING: AN ILLUSTRATED EXAMPLE:

A prospective study of liver disease amongst bartenders found the following data:

Table 1
Alcohol Tobacco Person-years Cases Incidence Rate
No No 75 000 75 0.001
No Yes 25 000 50 0.002
Yes No 25 000 50 0.002
Yes Yes 75 000 300 0.004

Ignoring the information on tobacco use:

Table 2
Alcohol Incidence Rate Crude Incidence Rate Ratio
Yes 0.00350 2.8
No 0.00125  

If smoking is more common among those who use alcohol than among those who don't and if smoking is a cause of the disease then the observed results could occur without alcohol being a cause of the disease.

To examine this we stratify the data according to smoking in Tables 3 and 4.

Table 3: Smoking Stratum
Tobacco Alcohol Incidence Rate Stratum Specific relative Risk (RR)
Yes Yes 0.004 2
Yes No 0.002  
Table 4: Non-Smoking Stratum
Tobacco Alcohol Incidence Rate Stratum Specific relative Risk (RR)
No Yes 0.002 2
No No 0.001  

As you can see, a confounder will change the crude effect to an adjusted effect within each stratum. The stratum based effects will however be identical or very similar and combinable in some way to provide an effect adjusted for the confounder, if the extraneous variable is indeed a confounder..




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General Introduction to Occupational Health: Occupational Hygiene, Epidemiology & Biostatistics by Prof Jonny Myers is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License
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