CPH Focus: Evidence-Based Approaches to Public Health: Epidemiology – Measures of Disease Frequency: Mortality and Morbidity Rates
In this tutorial, we’ll cover another essential topic in public health: measures of disease frequency, specifically mortality and morbidity rates. These metrics are fundamental for understanding the health status of populations and are critical for the Certified in Public Health (CPH) exam.
By the end of this tutorial, you’ll understand what mortality and morbidity rates are, how they are calculated, and their relevance in public health. As always, we’ll include practice questions with answers to help reinforce your understanding.
Table of Contents:
- Introduction to Mortality and Morbidity
- Understanding Mortality Rates
- Crude Mortality Rate
- Cause-Specific Mortality Rate
- Age-Specific Mortality Rate
- Case Fatality Rate
- Understanding Morbidity Rates
- Incidence of Morbidity
- Prevalence of Morbidity
- Differences Between Mortality and Morbidity
- Importance of Mortality and Morbidity Rates in Public Health
- Practice Questions
- Conclusion
1. Introduction to Mortality and Morbidity
In epidemiology, the terms mortality and morbidity are used to describe the health impact of diseases and conditions in a population:
- Mortality refers to deaths caused by diseases or other health-related conditions.
- Morbidity refers to the state of being diseased or unhealthy within a population.
Both mortality and morbidity are essential measures in public health because they help us understand the burden of disease in populations and can guide interventions and policy decisions.
2. Understanding Mortality Rates
Mortality rates measure the frequency of death in a defined population over a specified period. Different types of mortality rates offer insights into various aspects of population health.
2.1 Crude Mortality Rate
The crude mortality rate is the total number of deaths from all causes in a population during a specific time period, usually expressed per 1,000 or 100,000 people.
Formula for Crude Mortality Rate:
[math] \text{Crude Mortality Rate} = \frac{\text{Total Number of Deaths}}{\text{Total Population}} \times 100,000 [/math]
Example:
If 500 people died in a population of 100,000 during a year, the crude mortality rate would be:
[math] \text{Crude Mortality Rate} = \frac{500}{100,000} \times 100,000 = 500 \text{ deaths per 100,000 people per year} [/math]
2.2 Cause-Specific Mortality Rate
The cause-specific mortality rate refers to the mortality rate from a specific cause, such as heart disease or cancer, in a population during a specific period.
Formula for Cause-Specific Mortality Rate:
[math] \text{Cause-Specific Mortality Rate} = \frac{\text{Number of Deaths from Specific Cause}}{\text{Total Population}} \times 100,000 [/math]
Example:
If 200 people die from heart disease in a population of 100,000 during a year, the cause-specific mortality rate for heart disease would be:
[math] \text{Cause-Specific Mortality Rate} = \frac{200}{100,000} \times 100,000 = 200 \text{ deaths per 100,000 people per year} [/math]
2.3 Age-Specific Mortality Rate
Age-specific mortality rate focuses on the mortality rate for a specific age group within a population.
Formula for Age-Specific Mortality Rate:
[math] \text{Age-Specific Mortality Rate} = \frac{\text{Number of Deaths in Age Group}}{\text{Population in Age Group}} \times 100,000 [/math]
Example:
If 30 children under the age of 5 die in a population of 10,000 children under 5, the age-specific mortality rate for this group would be:
[math] \text{Age-Specific Mortality Rate} = \frac{30}{10,000} \times 100,000 = 300 \text{ deaths per 100,000 children under 5 per year} [/math]
2.4 Case Fatality Rate
The case fatality rate (CFR) is the proportion of individuals diagnosed with a particular disease who die from that disease within a specific time period.
Formula for Case Fatality Rate:
[math] \text{Case Fatality Rate} = \frac{\text{Number of Deaths from Disease}}{\text{Number of Diagnosed Cases}} \times 100 [/math]
Example:
If 100 people are diagnosed with disease X, and 10 of them die, the case fatality rate would be:
[math] \text{Case Fatality Rate} = \frac{10}{100} \times 100 = 10\% [/math]
This means that 10% of those diagnosed with disease X died from the disease.
3. Understanding Morbidity Rates
Morbidity rates refer to the frequency of disease or illness in a population. Like mortality rates, there are different ways to measure morbidity, including incidence and prevalence.
3.1 Incidence of Morbidity
The incidence of morbidity refers to the number of new cases of a disease that develop in a specific population during a defined time period.
Formula for Incidence of Morbidity:
[math] \text{Incidence of Morbidity} = \frac{\text{Number of New Cases of Disease}}{\text{Total Population at Risk}} \times 100,000 [/math]
Example:
If 50 new cases of disease Y occur in a population of 100,000 during a year, the incidence of morbidity would be:
[math] \text{Incidence of Morbidity} = \frac{50}{100,000} \times 100,000 = 50 \text{ new cases per 100,000 people per year} [/math]
3.2 Prevalence of Morbidity
The prevalence of morbidity measures how widespread a disease is at a specific point in time or over a period, including both new and existing cases.
Formula for Prevalence of Morbidity:
[math] \text{Prevalence of Morbidity} = \frac{\text{Number of Cases (New and Existing)}}{\text{Total Population}} \times 100,000 [/math]
Example:
If there are 1,000 people with disease Z in a population of 100,000, the prevalence of morbidity would be:
[math] \text{Prevalence of Morbidity} = \frac{1,000}{100,000} \times 100,000 = 1,000 \text{ cases per 100,000 people} [/math]
4. Differences Between Mortality and Morbidity
While both mortality and morbidity measure disease frequency, they focus on different outcomes:
- Mortality focuses on deaths due to disease.
- Morbidity focuses on illness or disease (whether or not it results in death).
| Measure | Focus | Calculation Basis |
|---|---|---|
| Mortality | Death | Total Population or Specific Cause |
| Morbidity | Illness or Disease | Total Population or Incidence/Prevalence |
5. Importance of Mortality and Morbidity Rates in Public Health
Both mortality and morbidity rates are critical in public health because they provide a comprehensive picture of the burden of disease in a population:
- Mortality rates help identify leading causes of death and guide efforts to reduce preventable deaths.
- Morbidity rates highlight the prevalence and incidence of disease, which can guide public health interventions aimed at prevention and management of illnesses.
These metrics inform resource allocation, intervention planning, and health policies.
6. Practice Questions
Let’s solidify your understanding with some practice questions. Try to answer them before checking the solutions.
Question 1:
A population of 1,000,000 people experiences 5,000 deaths over the course of a year. What is the crude mortality rate?
Answer 1:
Answer, click to reveal
[math] \text{Crude Mortality Rate} = \frac{5,000}{1,000,000} \times 100,000 = 500 \text{ deaths per 100,000 people per year} [/math]
Question 2:
In a population of 500,000 people, 1,000 deaths occurred due to cancer during a year. What is the cause-specific mortality rate for cancer?
Answer 2:
Answer, click to reveal
[math] \text{Cause-Specific Mortality Rate} = \frac{1,000}{500,000} \times 100,000 = 200 \text{ deaths per 100,000 people per year} [/math] </details
Question 3:
If 200 cases of disease X occur in a population of 50,000 during a year, what is the incidence of morbidity?
Answer 3:
Answer, click to reveal
[math] \text{Incidence of Morbidity} = \frac{200}{50,000} \times 100,000 = 400 \text{ new cases per 100,000 people per year} [/math]
Question 4:
In a town of 100,000 people, 5,000 individuals currently have disease Y. What is the prevalence of morbidity?
Answer 4:
Answer Key, click to reveal
[math] \text{Prevalence of Morbidity} = \frac{5,000}{100,000} \times 100,000 = 5,000 \text{ cases per 100,000 people} [/math]
7. Conclusion
In public health, understanding mortality and morbidity rates is vital for assessing the health of populations, planning interventions, and addressing public health challenges. By mastering these measures, you will be well-prepared for the Certified in Public Health (CPH) exam and better equipped to make informed decisions that can improve population health outcomes.
Remember:
- Mortality measures death rates.
- Morbidity measures illness rates.
Final Tip for the CPH Exam:
Practice calculating both mortality and morbidity rates using real-world scenarios. Understand the different types of mortality rates (crude, cause-specific, age-specific, case fatality) and how they are used in public health assessments. The more you practice, the more confident you’ll feel in tackling questions on these topics in the exam.
Humanities Moment
The featured artwork for this article is Roman Charity (about 1767) by Jean-Baptiste Greuze (French, 1725-1805). Jean-Baptiste Greuze, born in Tournus in 1725, was a French painter known for bringing Dutch-inspired realism into French genre painting and portraiture. After early training in Lyon, he moved to Paris, where his successful debut at the 1755 Salon brought acclaim but also led to conflicts with peers. His moralizing paintings used vivid expressions to educate viewers and evoke emotions. Despite his initial success, Greuze faced rejection from the Académie and eventually fell into obscurity, his popularity waning with the rise of Neoclassicism. He endured financial difficulties and died in poverty, largely forgotten.