Disease Prevalence, Incidence, and Risk Factors
Set 1: Disease Prevalence and Incidence
1. Population with a Stable Age Distribution:
How can the prevalence of a disease decrease despite a constant incidence rate?
Prevalence is a function of the incidence rate and the average duration of the disease. If the incidence rate is constant, a decrease in the average disease duration can cause prevalence to decrease. This can happen due to improved treatment (leading to quicker cures) or if the disease becomes more rapidly fatal.
2. Evaluating Preventive Programs:
Which is better for evaluating the success of a preventive program: comparing changes in incidence rates or comparing changes in prevalence of the disease? Explain.
Incidence rate is better. Prevalence is a function of both incidence and duration. Prevention should decrease new cases, which should decrease the incidence rate, but not necessarily the prevalence, as people with the existing disease will still be present.
3. Study of 1,000 Older Men:
In a study of 1,000 older men, 100 were found to have polyps of the colon. During the following 10 years, another 200 men were found with polyps. Calculate the measure(s) of disease occurrence.
- Prevalence: At the beginning of the study period, it is 100 / 1000 = 10%.
- Incidence rate: 200 / 1000 / 10 years = 0.02/year = 2/100 person-years.
- Cumulative incidence (risk): The proportion of newly affected individuals during this period is 200 / 1000 = 20% (10-year risk).
4. Children and Chronic Bronchitis:
Children develop chronic bronchitis in their first year of life in 3/20 homes where both parents are chronic bronchitics, compared to 5/100 homes nationally. What measure of disease occurrence is used above?
The number of people affected in the first year of life divided by the number at risk at the beginning of the first year represents risks (cumulative incidence).
5. Duodenal Ulcer Cases Over 5 Years:
During a 5-year period, 270 cases of duodenal ulcer occurred. The number of men was 18,500 at the beginning and 21,500 at the end. Calculate the measure of disease occurrence.
Since the number of people at risk increases during this time, indicating unequal lengths of follow-up, we should use the incidence rate (IR) instead of risk. Calculate person-time as (original population * years) + (additional population * ½ time). So, (18,500 * 5) + (21,500 – 18,500) * 2.5 = 100,000. Therefore, IR = 270 / 100,000 person-years = 27 / 10,000 person-years.
6. Breast Cancer Incidence in a Town:
In a town of 10,000 people (50% female), 100 women develop breast cancer in one year. The national incidence rate of breast cancer is 15 in 1000 person-years (0.015). Is the risk of obtaining cancer higher in your town than nationally?
With 5,000 women (10,000 / 2), the incidence rate is 100 cases / 5,000 person-years = 0.02 cases/person-year. Multiplying by 1000, we get 20 cases / 1,000 person-years. The risk is higher in this town (20/1,000 person-years) compared to the national incidence rate (15/1,000 person-years).
Set 2: Lung Cancer Deaths in Smokers and Ex-Smokers
Number and Rate (Per 1,000 Person-years) of Lung Cancer Deaths for Current Smokers and Ex-smokers by Years Since Quitting, Physician Cohort Study — Great Britain, 1951–1961
The following table presents data from the study:
| Status | Deaths | Rate/1000 person-years | Rate Ratio |
|---|---|---|---|
| Current Smokers | 133 | 1.3 | |
| Ex-Smokers <5 years | 5 | 0.67 | 9.6 |
| Ex-Smokers 5-9 years | 7 | 0.49 | 7.0 |
| Ex-Smokers 10-19 years | 3 | 0.18 | 2.6 |
| Ex-Smokers 20+ years | 2 | 0.19 | |
| Non-Smokers (Control) | 3 | 0.07 | 1.0 |
1. Rate Ratio Comparing Current Smokers with Nonsmokers:
Rate (smokers) / Rate (nonsmokers) = 1.3 / 0.07 = 18.57. Current smokers are 18.57 times more likely to die of lung cancer than nonsmokers.
2. Rate Ratio Comparing Ex-Smokers (Quit 20+ Years Ago) with Nonsmokers:
Rate (ex-smokers 20+ years) / Rate (nonsmokers) = 0.19 / 0.07 = 2.71. Ex-smokers who have not smoked in 20 years are 2.71 times more likely to die of lung cancer than nonsmokers.
3. Public Health Implications:
Smoking significantly increases the risk of dying from lung cancer. Even after quitting, ex-smokers remain at a higher risk compared to nonsmokers. However, the risk of dying from lung cancer decreases the longer an individual abstains from smoking.