CDC Measure of Associations

Measure of associations
Exercise 3.7
1. Rate ratio comparing current smokers with nonsmokers
Rate ration = rate of current smokers/rate of nonsmokers
(Rate of current smokers = rate of smokers per 1000 persons-years = 1.3)
(Rate of nonsmokers = rate of nonsmokers per 1000 persons-years = 0.07)
Rate ration = 1.3/0.07
=18.57
2. Rate ratio comparing ex-smokers who quit at least 20 years ago with nonsmokers
Rate ration = rate of ex-smokers quitting at 20 years/rate of nonsmokers
(Rate of ex-smokers quitting at 20 years = rate per 1000 persons-years = 0.19)
(Rate of nonsmokers = rate per 1000 persons-years = 0.07)
Rate ration = 0.19/0.07
= 2.71
3. What are the public health implications of these findings?
Based on the calculation above, it is evident that the rate of lung cancer incidences among smokers is way too high, 18 times, as compared to nonsmokers. This leads to the conclusion that, smoking is a factor contributing to the risk of lung cancer. This conclusion is further evidenced by the fact that, rate ration of lung cancer among those who quit smoking after 20 years as compared to nonsmokers is significantly less, approximately 3 times (CDC, 2006). This finding points to the fact that, smoking is a risk factor for lung cancer. Public health implications for these findings are that, those who are smoking should be encouraged to stop forthwith as every puff is a step towards developing lung cancer. More importantly, those who haven’t smoked should be encouraged not to start.
Exercise 3.8
Odds ration of tuberculosis
Odds ration = (exposed ill × unexposed well)/ (exposed well × unexposed well)
= (28 × 133)/ (129 × 4)
= 3724/516
= 7.22
Odds ration as compared to rate ration
Rate ration = rate ration of exposed/rate ration of unexposed
(Rate ratio of exposed = ill in East wing/total number of inmates in East wing)
= 28/157
= 0.19
(Rate ratio of unexposed = ill in West wing/total number of inmates in West wing)
= 4/137
= 0.03
Rate ration = 0.19/0.03
= 6.33
The odds ratio for developing tuberculosis in the prison is 7.22, while the rate ration is 6.33. The difference between the two is 0.89, which translates to 14 percent of the rate ration. While the odds ratio is a measure for the association between exposure and outcome, rate ratio is a measure of the relative risk. As a result, it would be expected that for a disease with high outcome, the odds ratio and rate ratio would be on opposite extremes. However, popularity of a disease is not quantifiable on a standard figure for example; a few incidences of polio are considered popular, while thousand cases of common cold are considered a non-issue.
Therefore, the difference between the odds ratio and the rate ratio in this case is considered to be a largely a personal-judgment case. Nevertheless, and given there are individuals who are immune to tuberculosis, while other are not, the 28 cases of illness among the exposed and 4 cases the unexposed is considered to be a practical real-life scenario. As a result it would be considered that, tuberculosis wouldn’t be categorized as a popular disease, thus the difference between the odd ratio and the odd ratio wouldn’t be significantly divergent hence, 14% difference is considered practical.


References
Centers for Disease Control and Prevention. (2006). Principles of epidemiology in public health practice: an introduction to applied epidemiology and biostatistics. Retrieved from https://www.cdc.gov/ophss/csels/dsepd/ss1978/lesson3/section5.html on 29 February 2018