Week 5 Assignment-Case Study: Cigarette Smoking and Lung Cancer

Week 5 Assignment-Case Study: Cigarette Smoking and Lung Cancer

Cindy Earle, Nov. 2014

1. What makes the first study a case-control study?

It is a case-control study because it has the retrospective component to it. Researchers knew the outcome (patients newly diagnosed with lung cancer) and then were able to trace backwards to investigate exposures. Case control studies help to determine if an exposure (i.e.: smoking) is associated with a specific outcome (i.e.: lung cancer). A similar group without the problem (lung cancer) was also studied, as the control group.

1. What makes the second study a cohort study?

In the cohort study, a population of patients of certain physicians in a certain area was examined to study their past and present smoking habits. It was a longitudinal study, as participants were followed until their death years later. The group was identified during a particular period (Oct. 1951). The risk factor of smoking was identified, and followed over a period of time, until the health outcome was identified.

1. Why might hospitals have been chosen as a setting for this study?

The hospital setting was helpful in providing validity to the case-control study, due to the accessibility of valid health information.

1. What other sources of cases and controls might have been used?

Population controls, such as household registries within a certain geographic area could have been used. Other sources would be clinics.The important factor is to have the cases and the control groups come from the same thing: i.e.: hospital, versus population.

1. What are the advantages of selecting controls from the same hospitals as cases?

An advantage of selecting controls from the same hospital is that the controls are in some way matched to the hospital cases, so they are comparable to cases in regards to recall of exposure; they are easy to access and information is gathered in a similar way to the cases; and they may be more willing to take part than healthy controls from the general population.

1. How representative of all persons with lung cancer are hospitalized patients with lung cancer?

There could be a bias with hospital patients as they can have different characteristics than the non-hospitalized patients, as they are acutely ill. For example, they might have been more chronic smokers.

1. How representative of the general population without lung cancer are hospitalized patients without lung cancer?

Acutely ill hospitalized patients are not representative of the general population, for same reasons as in #6.They are not typical of healthy population controls in regard to a variety of exposures.-it will be important to exclude diseases that are known to be associated with the exposure of interest, such as emphysema in a smoker, versus lung cancer in a smoker.

1. How may these representativeness issues affect interpretation of the study’s results?

Since smoking is related to the exposure under investigation, the estimates of the exposure (chronic smokers are more likely to be hospitalized for an acute condition, and are thus more ill than non-hospitalized smoker) among controls may be different from that in the reference population, which could result in a biased estimate of the association between exposure and disease.

1. From this table, calculate the proportion of cases and controls who smoked.

Proportion smoked, cases: 1350/1357: .99 or 99%

Proportion smoked, controls: 1296/1357=.95 or 95%

1. What do you infer from these proportions?

Since the smoked cases have a larger proportion than the control, it suggests that the hypothesis is valid.

11 a. Calculate the odds of smoking among the cases.

1350/7=192.8

11 b. Calculate the odds of smoking among the controls.

1296/61=21.2

12. Calculate the ratio of these odds. How does this compare with the cross-product ratio?

192.8/21.2=9.09 =odds ratio.

1350x61=82,350 7x1296= 9,072 82,350/9072= 9.08

The odds ratio is basically the same as the cross-product ratio.

13. What do you infer from the odds ratio about the relationship between smoking and lung cancer?

The risk of a smoker getting lung cancer is about 9 times the risk of a nonsmoker getting lung cancer.

14. Compute the odds ratio by category of daily cigarette consumption, comparing each smoking category to nonsmokers.

a. (1-14 cigs) : 565x61/7x706= 34,465/4,942 = 6.97

b. (15-24 cigs): 445x61/7x408= 27,145/2,856= 9.50

c. (25+ cigs): 340x61/7x182=20,740/1,274= 16.27

d. (all smokers): 1350x61/1296x7=82350/9,076= 9.07

15. Interpret these results:

a. The risk of getting lung cancer smoking 1-14 cigs/day is about 7 times the risk of a nonsmoker.

b. The risk of getting lung cancer smoking 15-24 cigs/day is almost 10 times that of a nonsmoker.

c. The risk of getting lung cancer smoking 25+ cigs/day is 16 times that of a nonsmoker.

d. The risk of a smoker getting lung cancer is 9 times the risk of a non-smoker.

16. What are the other possible explanations for the apparent association?

*When smoking is not present, the chance of getting lung cancer diminishes.

*Persons that smoke 25+ cigs/day are more likely to experience lung cancer than others.

*Men might have other unidentified susceptibility factors that increase their risk for lung cancer.

17. How might the response rate of 68% affect the study’s results?

Since the response rate is >50% the study population adequately represents the target population, decreasing the risk for bias.

18. Compute lung cancer mortality rates, rate ratios, and rate differences for each smoking category. What does each of these measures mean?

*Mortality rate is the number of deaths in a particular period from a particular cause.

*Rate ratio is a relative difference measure used to compare incidence rates of events occurring at any given point in time.

*Rate difference is how fast the disease is occurring in a population.

19. What proportion of lung cancer deaths among all smokers can be attributed to smoking? What is this proportion called?

The proportion or cumulative incidence is: 133:102,600.

20. If no one had smoked, how many deaths from lung cancer would have been averted?

1.22

21. Which cause of death has a stronger association with smoking? Why?

Attributable risk is the amount of risk that is due to that specific factor. Lung Cancer has a stronger association with smoking as demonstrated by the 95% attributable risk compared to 23% for CVD.

22. Calculate the population attributable risk percent for lung cancer mortality and for cardiovascular disease (CVD) mortality. How do they compare? How do they differ from the attributable risk percent?

Lung CA: PAR= (.94-.07)/.94 .87/.94=.92 = 92% PAR

CVD: PAR= (8.87-7.32)/8.87 1/55/8.87 = .174= 17% PAR

*92% of lives would be saved if people no longer smoked, and 17% of lives would not experience CVD.

*Attributable risk indicates the number of cases among exposed individuals that can be attributed to that exposure (smoking)- in this case, 95% for lung cancer, and 23% for CVD.

23. How many lung cancer deaths per 1000 persons per year are attributable to smoking among the entire population? How many cardiovascular disease deaths?

Lung Cancer = 95% of the deaths per 1000 persons per year among those who were exposed is directly attributable to the exposure.

CVD= 23% of the deaths per 1000 persons per year among those who were exposed is directly attributable to the exposure.

24. What do these data imply for the practice of public health and preventative medicine?

The data supports the fact that people who stop smoking greatly reduce their risk of premature death via lung cancer. The data supports the importance of public health education and research initiatives to prevent tobacco use.

25. Compare the results of the two studies. Comment on the similarities and differences in the computed measures of association.

The odds ratios indicate the strong association of the hazards of smoking especially as it increases according to the number of cigarettes smoked daily.

26. What are the advantages and disadvantages of case-control vs. cohort studies?

Cohort Disadvantages:

• Costly and time consuming
• Prone to bias due to loss to follow-up
• Prone to confounding
• Participants may move between one exposure category
• Knowledge of exposure status may bias classification of the outcome
• Being in the study may alter a participant’s behavior
• Poor choice for the study of a rare disease
• Classification of individuals (exposure or outcome status) can be affected by changes in diagnostic procedures.
• Follow-up difficult

Cohort Advantages:

• Multiple outcomes can be measured for any one exposure
• Can look at multiple exposures
• In prospective cohort studies exposure is measured before the onset of disease
• Good for measuring rare exposures
• Demonstrate direction of causality
• Can measure incidence and prevalence
• Allows for studying multiple outcomes
• Large sample size
• Allows studying natural history of disease-easier to show that cause precedes effect

Case-Control Disadvantages:

• Subject to selection bias
• Inefficient for rare exposures
• Information on exposure is subject to observation bias
• Generally do not allow calculation of incidence (absolute risk)
• Does not allow studying multiple outcomes
• More difficult to follow natural progression of disease

Case-Control Advantages:

• Efficient for rare diseases or diseases with a long latency period between exposure and disease manifestation
• Less costly and less time-consuming; they are advantageous when exposure date is expensive or hard to obtain
• Advantageous when studying dynamic populations when follow-up is difficult
• Small sample size
• No follow up

27. Which type of study (cohort or case-control) would you have done first? Why? Why do a second study? Why do the other type of study?

Both studies are observational studies. The case-control study’s sampling is done in regard to disease or effect. In co-hort studies, the sampling is done in regard to exposure, characteristic, or suspected cause. I would have done the case control study first in order to justify the more costly and time-consuming longitudinal study later. The advantage of doing the co-hort study second is that it can show that cause precedes the effect.

28. Which of the following criteria are met by the evidence presented from these two studies?

Strong association: yes

Consistency among studies: yes

Exposure precedes disease: yes

Dose-response effect: yes

Biologic plausibility: yes