Question

In a population based study a total of 245 prevalent heart failure cases were identified on January 1, 2000. In the remaining study population (n=7734), researchers identified 725 incident cases of heart failure. The study period was 7 years with a total of 50268 person-years of observation in total. Here is an additional table showing number of cases by age categories:

Age Number of incidents Person-yrs
                       
55–59            4                    2888.6
60–64            27                  8713.6
65–69           56                  10392.1

2. Calculate point prevalence on December 31, 2006. Summarize your answer and what it means in a sentence.

3. Calculate the total cumulative incidence for this study. Summarize your answer and what it means in a sentence.

4. Calculate the total incidence rate over the study period. Summarize your answer and what it means in a sentence.

5. Calculate the incidence rate in the 55-59 age group. Summarize your answer and what it means in a sentence.

6. Calculate the incidence rate in the 60-64 age group. Summarize your answer and what it means in a sentence

7. Calculate the incidence rate in the 65-69 age group. Summarize your answer and what it means in a sentence.

8. What is the incidence rate difference for the 60-64 age group as compared to the 55-59 year olds. Explain the meaning in a sentence.

9. What is the incidence rate difference for the 65-69 age group as compared to the 55-59 year olds. Explain the meaning in a sentence.

10. What is the incidence rate ratio for the 60-64 age group as compared to the 55-59 year olds. Explain the meaning in a sentence.

11. What is the incidence rate ratio for the 65-69 age group as compared to the 55-59 year olds. Explain the meaning in a sentence.

Answer 1

2.

Point prevalence = Total number of cases/ Total population = (245+725) / 7734 = 0.1254

Point prevalence of heart failure on December 31, 2006 is 0.1254

Interpretation:

For every 100 person in the population, 12.54 person is affected by heart failure on December 31, 2006.

3.

Total cumulative incidence = Number of new cases of disease/Size of population

=725/7734 = 0.0937.

Total cumulative incidence of heart failure for the given study is 0.0937

Interpretation: For every 100 person in the population, 9.37 new cases of heart failure is recorded during the study.

4.

total incidence rate = Number of new cases of disease/Total person hours under study

= 725/ 50268 = 0.0144 = 14.4 cases per 1000 person years.

Total incidence rate of heart failure for the given study is 0.0144 per person hour.

Interpretation: For every 1000 person years recorded in the study, 14.4 new cases of heart failure is recorded during the study.

5.

Incidence rate in the 55-59 age group = Number of new cases in the group /Total person hours in the group

= 4/2888.6 = 0.0014

Total incidence rate of heart failure for the 55-59 age group is 0.0014 per person hour.

Interpretation: For every 1000 person years observed in the 55-59 age group, 1.4 new cases of heart failure is recorded during the study.

6.

Incidence rate in the 60-64 age group = Number of new cases in the group /Total person hours in the group

= 27/8713.6 = 0.0031

Total incidence rate of heart failure for the 60-64 age group is 0.0031 per person hour.

Interpretation: For every 1000 person years observed in the 60-64 age group, 3.1 new cases of heart failure is recorded during the study.

7.

Incidence rate in the 65-69 age group = Number of new cases in the group /Total person hours in the group

= 56/10392.1 = 0.0054

Total incidence rate of heart failure for the 65-69 age group is 0.0054 per person hour.

Interpretation: For every 1000 person years observed in the 65-69 age group, 5.4 new cases of heart failure is recorded during the study.

8.

Incidence rate difference = Incidence rate for the 60-64 age group - incidence rate of 55-59 year olds

= 0.0031-0.0014

= 0.0017

Incidence rate difference between the 60-64 age group and 55-59 year age group is 0.0017 per person hour.

Interpretation: For every 1000 person years observed in the study, 1.7 more new cases of heart failure is recorded in the 60-64 age group than 55-59 age group.

9.

Incidence rate difference = Incidence rate for the 65-69 age group - incidence rate of 55-59 year olds

= 0.0054-0.0014

= 0.0040

Incidence rate difference between the 65-69 age group and 55-59 year age group is 0.0040 per person hour.

Interpretation: For every 1000 person years observed in the study, 4 more new cases of heart failure is recorded in the 64-69 age group than 55-59 age group.

10.

Incidence rate raio = Incidence rate for the 60-64 age group /incidence rate of 55-59 year olds

= 0.0031/ 0.0014

= 2.2143

Incidence rate difference of the 60-64 age group compared to the 55-59 year age group is 2.2143.

Interpretation: Incidence rate of heart failure in the 60-64 age group is 2.2143  times the rate of the 55-59 age group.

11.

Incidence rate raio = Incidence rate for the 65-69 age group /incidence rate of 55-59 year olds

= 0.0054/ 0.0014

= 3.8571

Incidence rate difference of the 65-69 age group compared to the 55-59 year age group is 3.8571.

Interpretation: Incidence rate of heart failure in the 65-69 age group is 3.8571 times the rate of the 55-59 age group.