Lung Cancer Case Control Study:



Tanya Staton

Lung Cancer Case Control Study:

Question 1: What makes the first study a case-control study?

The first study is considered a case-study because the population in question are picked on the absent or present of a disease, and the study is not distinct.

Question 2: What makes the second study a cohort study?

The study is a specific population, and they were separated based on specific exposures. Rate of lung cancer death for smokers versus non-smokers.

Question 3: Why might hospitals have been chosen as the setting for this study?

Hospitals were probably chosen because there is a higher probability that lung cancer patients are admitted. Hospitals will provide many controls, and hospital records are available at ease.

Question 4: What other sources of cases and controls might have been used?

I think a great source for data would be a cancer registry or death certificates. I am sure there were not many cancer registries available at the time of the study though. Controls could be picked in the general population that are likely to be well.

Question 5: What are the advantages of selecting controls from the same hospitals as cases?

An advantage of selecting controls from the same hospital is they probably come from similar a similar population area, and may have access to the same healthcare if both the cases and controls are at the same hospital.

Question 6: How representative of all persons with lung cancer are hospitalized patients with lung cancer?

For the most part I feel they are represented correctly. Most cancer patients have a definite diagnosis and doctors are not trying to determine what disease or illness they have. As always a misdiagnosis is always possible.

Question 7: How representative of the general population without lung cancer are hospitalized

patients without lung cancer?

I also feel they are represented very well. When a patient is admitted a complete blood count is done if the patient’s white blood count is low or high further tests are done to rule out cancer.

Question 8: How may these representativeness issues affect interpretation of the study's results?

Study participants issues could be misinterpreted if co-founders exist with lung cancer. Some people could have smoked previously then stopped.

Question 9: From this table, calculate the proportion of cases and controls who smoked.

Cases Controls

|Cigarette Smokers |1,350 |1,296 |

|Non-smokers |7 |61 |

|Total |1,357 |1,357 |

Proportion smoked, cases: 1,350/ 1,357 x 100 = 99.5%

Proportion smoked, controls: 1,296/ 1,357 x 100 = 95.5%

Question 10: What do you infer from these proportions?

The prevalence of smoking is high in both the case and control groups.

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

1,357/ 7 = 192.9 to 1

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

1,296/ 61 = 21.2 to 1

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

ratio?

Odds Ratio = (1,350/ 7) / (1,296/ 61) = 192.9/ 21.2 = 9.1

Question 13: What do you infer from the odds ratio about the relationship between smoking and

lung cancer?

The odds of getting lung cancer from smoking is 9 times higher than for non-smokers.

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

|Daily # of Cigarettes |# Cases |# Controls |Odds Ratio |

|0 |7 |61 |Referent |

|1-14 |565 |706 |7.0 |

|15-24 |445 |408 |9.5 |

|25 + |340 |182 |16.3 |

|All Smokers |1,350 |1,296 |9.1 |

|Total |1,357 |1,357 | |

1-14: (565 x 61) / (706 x 7) = 7.0

15-24: (445 x 61) / (408 x 7) = 9.5

25 +: (340 x 61) / 182 x 7) = 16.3

All Smokers: (1,350 x 61) / (1,296 x 7) = 9.1

Question 15: Interpret these results.

The odds of getting lung cancer is directly proportional to the number of cigarettes smoked per day.

Question 16: What are the other possible explanations for the apparent association?

Probably unlikely but it could just be chance. Another explanation could be data collection error.

Question 17: How might the response rate of 68% affect the study's results?

A low response rate could alter the odds ratios therefore altering the results.

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

|Daily # of Cigarettes|Deaths from lung |Person years at risk |Mortatility rate per |Rate Ratio |Rate differences per |

| |cancer | |1000 person-years | |1000 person-years |

|0 |3 |42,800 |0.07 |referent |referent |

|1-14 |22 |38,600 |0.57 |8.1 |0.5 |

|15-24 |54 |38,900 |1.39 |19.8 |1.32 |

|25 + |57 |25,100 |2.27 |32.4 |2.2 |

|All smokers |133 |102,600 |1.3 |18.6 |1.23 |

|Total |136 |145,400 |0.94 | | |

See calculations below

Question 19: What proportion of lung cancer deaths among all smokers can be attributed to

smoking? What is this proportion called?

AR% = (1.3 - 0.07) / 1.3 x 100 = 94.6%

This proportion is called attributable risk

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

If no one had smoked 95% of lung cancer deaths could have been avoided. So out of the 133 deaths about 126 would have been avoided.

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

There is a higher chance of getting lung cancer from smoking then cardiovascular disease. The rate ratio= 18.5/ 1.3 = 14.2. The chance of getting lung cancer is about 14 times greater from smoking.

Question 22: Calculate the population attributable risk percent for lung cancer mortality and for

cardiovascular disease mortality. How do they compare? How do they differ from the

attributable risk percent?

PAR% lung cancer = (0.94-0.07) / 0.94 = 0.925 x 100 = 92.5%

PAR% cardiovascular disease = (8.87- 7.32) / 8.87 = 0.174 x 100 = 17.4%

Question 23: How many lung cancer deaths per 1,000 persons per year are attributable to smoking among the entire population? How many cardiovascular disease deaths?

Lung cancer: 0.925 x 0.94 = 0.87 deaths per year

Cardiovascular disease: 0.17 x 8.87 = 1.51 deaths per year

Question 24: What do these data imply for the practice of public health and preventive medicine?

Although, never smoking has the lowest risk of dying from lung cancer quitting smoking will greatly reduce your chances of developing lung cancer. The best is to focus on preventive practices to never start smoking.

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

Both studies show a strong relationship between smoking and lung cancer, and prove the chances of developing lung cancer increase with number of cigarettes smoked per day. The odds ratio in the case-control study is lower than the cohort study.

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

Answer 26

Case-control Cohort

Sample size smaller larger

Costs cheap expensive

Study time short long

Rare disease good for disadvantage

Rare exposure disadvantage advantage

Multiple exposures advantage disadvantage

Multiple outcomes disadvantage advantage

Progression, spectrum of illness disadvantage advantage

Disease rates advantage

Recall bias can cause problems less problems

Loss to follow-up advantage cause problems

Selection bias cause problems less problems

Question 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?

I probably would have done the case-control first because it is easier and costs less. If a relationship was discovered then I would have done the cohort study after to verify my findings.

Question 28: Which of the following criteria for causality are met by the evidence presented from

these two studies?

Answer 28

YES NO

Strong association yes

Consistency among studies yes

Exposure precedes disease yes

Dose-response effect yes

Biologic plausibility no

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