# Status-Specific Rates, Incidence Rates

Part C: Smoking Status-Specific Rates and More on Cumulative Incidence and Incidence Density

Table 3: 22 Years of Data from the Chicago Western Electric Study of Heart Disease: Follow-up (Cohort) Study of Heart Disease Risk among Men

Risk of Coronary Death, by Number of Cigarettes Smoked Per Day for 1,416 employed middle-aged men: Chicago Western Electric Study, 22 Year Follow-Up

1) Using information provided in Table 3 above, calculate the crude cumulative incidence (CI) of coronary death for the entire 22 years of the study Convert your result to "per 1,000 men". In other words, use 1,000 as a multiplier. Report rates to the nearest tenth. Show all work.

2) Using information provided in Table 3 above, calculate the smoking status specific cumulative incidence (CI) of coronary death for the 22 years of the study. In other words: calculate separate rates for nonsmokers, those who smoked 1-14 cigarettes per day, and each of the other groups. Convert your results to "per 1,000 men", as shown in the table. Report rates to the nearest tenth. Show all work. There will be 4 rates.

3) Now convert all of your answers in #1 and #2 (5 rates) to average annual (yearly) cumulative incidence per 1,000 men. In other words, assuming that the cumulative incidence of all groups remained constant for each of the 22 years, what would it be for one year? This is what is asked for in the table.

4) Also calculate the crude and smoking status-specific incidence density (ID) expressed as deaths per 1,000 person-years. Use 1,000 as a multiplier. Report rates to the nearest tenth. Show all work. There is no need to adjust this for annual rates, as it is already taken into consideration when you divide from the table. After doing this, you will have calculated 5 more rates.

5) In a few sentences, compare the smoking status-specific rates of either the cumulative incidence or the incidence density with each other and with the crude rate. In other words, how does the incidence of coronary death vary by smoking group? Describe the pattern.

6) Now compare the cumulative incidence and incidence density of coronary death for the same smoking status groups (for instance, compare the cumulative incidence and incidence density for nonsmokers). Which one is always lower?

7) Why is one of the ways of calculating incidence always lower than the other?

8) Although these rates are called "incidence" in this exercise, they are really the incidence of what? So they can also be called _______ rates.

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#### Solution Preview

1) Using information provided in Table 3 above, calculate the crude cumulative incidence (CI) of coronary death for the entire 22 years of the study Convert your result to "per 1,000 men". In other words, use 1,000 as a multiplier. Report rates to the nearest tenth. Show all work.

Cumulative incidence (incidence proportion, attack rate) measures the disease frequency over a period of time. It is calculated as the number of new cases within a specified time period divided by the size of the population at risk.

cumulative incidence for coronary deaths = (new cases of coronary deaths / total population size) x 1000

cumulative incidence for coronary deaths = (232 / 1416) x 1000 = 163.8

2) Using information provided in Table 3 above, calculate the smoking status specific cumulative incidence (CI) of coronary death for the 22 years of the study. In other words: calculate separate rates for nonsmokers, those who smoked 1-14 cigarettes per day, and each of the other groups. Convert your results to "per 1,000 men", as shown in the table. Report rates to the nearest tenth. Show all work. There will be 4 rates.

Do the same thing as in question 1, only divide the ...