Measures of mortality are a cornerstone in population health and public health practice. You have likely encountered discussions about measures of mortality in your epidemiology courses, whether during your schooling years or in your current studies. Whether you recall those lectures vividly or you have forgotten much of the details, this blog will provide you with clear insights into what mortality measures are and why they matter in public health.

What is Mortality?

Before we get into the details, let’s be clear on what we are talking about. There are different measures of mortality, and each offers a unique perspective on death rates and their implications, providing a more detailed understanding of public health challenges. Let’s define some key terms before we get technical:

• Death: Refers to the permanent disappearance of all evidence of life at any time after birth.
• Mortality: The average risk of a person dying during a time span
• Mortality Rate: The measure of the frequency of occurrence of death in a defined population during a specified interval.

Importance of Measuring Mortality in Public Health

As crucial metrics in public health, mortality measures gauge the overall health status of populations, assess the impact of diseases, and help in designing effective health policies. Specifically, they are:

• Useful in projecting future population size as demographers can predict future population changes, such as whether a population is likely to grow, shrink, or age. This information is vital for planning future healthcare needs, infrastructure, and social services.
• Some mortality measures like infant mortality rates and age-specific death rates, help identify population groups at high risks and in need of health services. This can tailor services and interventions to meet their specific needs, ensuring that those who need help the most receive it.
• It indicates the quality of life and life expectation at birth helping to gauge the effectiveness of public health efforts. For instance, a lower life expectancy at birth might indicate poor living conditions, inadequate healthcare, or high disease prevalence.
• Indicates priority areas for health actions, including the allocation of resources and health interventions. Making room for more strategic allocation of resources, directing funds, and interventions to where they will have the most significant impact.

Determinants or Factors Influencing Death Rates

Mortality is influenced by a complex mix of socioeconomic, environmental, healthcare-related, behavioural, biological, and social factors. Addressing these determinants through targeted public health interventions and policies is important for public health efforts aimed at reducing mortality rates and improving overall population health. The table below provides a summary of some factors that determine death rates.

Table 1: Summary of factors influencing death rates

Factor	Summary
Socioeconomic Factors
– Income and Poverty	Higher income provides better access to healthcare, nutritious food, and safe living conditions.
– Education	Higher educational attainment improves health literacy, leading to healthier lifestyle choices and better use of healthcare services.
– Employment	Employment impacts mortality through income stability, access to health insurance, and workplace safety.
Environmental Factors
– Air and Water Quality	Exposure to air and water pollutants increases the risk of respiratory and cardiovascular diseases.
– Living Condition	Quality of housing, sanitation, and access to clean water and nutritious food directly affect health outcomes.
Healthcare Access and Quality
– Availability of Healthcare Services	Access to preventive care, emergency services, and ongoing treatment is crucial in reducing mortality rates.
– Quality of Healthcare	Competent providers and advanced medical technologies improve diagnosis, treatment, and disease management, reducing mortality.
Behavioural and Lifestyle Factors
– Diet and Nutrition	Poor nutrition increases the risk of obesity, diabetes, and cardiovascular diseases, all of which elevate mortality risk.
– Physical Activity	Regular exercise lowers the risk of chronic diseases, such as heart disease, stroke, and diabetes.
Mental Health
– Mental Health Conditions	Mental health conditions can lead to behaviors that increase mortality risk, such as substance abuse and suicide.
Social and Cultural Factors
– Social Support	Strong social networks, including family and community, enhance health and well-being, reducing mortality risk.
– Cultural Practices	Cultural beliefs and practices influence health behaviors and access to healthcare, with some practices promoting better health outcomes.
Biological Factors
– Age	Age is a primary determinant of mortality. Older individuals have higher mortality rates due to the increased prevalence of chronic diseases and age-related conditions.
– Sex	Mortality rates often differ between males and females due to biological, behavioral, and social factors. For instance, men typically have higher mortality rates from accidents and violence.
– Genetics	Hereditary conditions, such as certain cancers and genetic disorders, can lead to higher mortality rates in affected individuals.

Key Mortality Indicators

1. Crude Death Rate (CDR)

This is the total number of deaths in a calendar year per 1,000 people in a population. i.e The crude mortality rate is the mortality rate from all causes of death for a population. CDR provides a general overview of the mortality level within a population, making it useful for assessing overall health and comparing mortality rates over time within the same population. It can be calculated as follows:

                

• CDR is easy to calculate and provides a quick snapshot of mortality levels.
• One problem with CDR is that it does not consider the age, sex, or other demographic factors, due to which it is called a crude measure.
• Weakness for international comparisons as it makes no allowance for differential age and sex composition.

Worked Example

 The CDR for Cameroon for year 2019 may be computed as follows: Total number of deaths during the year 2019 = 70,000

Total population at the middle of the year i.e., midyear population during 2019 = 12,500,000 Therefore:

2. Cause Specific Mortality Rate

The cause-specific mortality rate is the mortality rate from a specified cause for a population (eg HIV, Cancer, accidents, NCDs etc). CSMR focuses on mortality from specific causes, such as diseases or accidents, which helps in identifying public health priorities and targeting interventions. Thus calculated as:

• Provides insights into specific health issues, allowing for targeted public health strategies.
• However, it does not provide a comprehensive view of overall mortality and may require detailed cause-of-death data, which is not always available.

Worked example:

10,000 deaths due to Disease A occurred in Town R (midyear population 100,000) in 2004. Therefore:

 3. Age-Specific Death Rate (ASDR)

Age-Specific Death Rates are mortality rates within specific age groups. ASDR provides insights into the mortality risk for different age groups, crucial for identifying vulnerable populations and targeting public health interventions where needed the most. It can be calculated as follows:

• It plays a vital role in understanding the health risks faced by different age groups, allowing for more effective age-targeted interventions.
• Though important in understanding health risks, it requires detailed age-specific data, which may not always be readily available.

Some specific types of age-specific mortality rates are neonatal, postneonatal, and infant mortality rate.

Worked Example

For the age group 0-4 years in 2019, there were 12,000 deaths following a cholera outbreak in the Littoral Region from a total population of about 2,000,000 people. The ASDR can be calculated as follows:

4. Infant Mortality Rate (IMR)

The Infant Mortality Rate (IMR) Defined as the number of infant deaths (under 1 year of age) occurring in an area within a specified calendar year per 1000 live births in the same community during the same calendar year

IMR is a key indicator of the overall health status and well-being of a population, reflecting the quality of healthcare, maternal health, and socioeconomic conditions.

Advantage: IMR is a sensitive indicator of both current and long-term public health conditions, guiding policy and resource allocation.

 Disadvantage: It focuses only on infant mortality, so it does not provide information about the health of other age groups or the population as a whole.

It can be calculated as follows:

Worked Example

200,000 live births were recorded in 2020 and 1,500 infant deaths reported in the same year. By using this formula, the IMR can be calculated as follows:

5. Neonatal Mortality Rate(NMR)

 NMR reflects the number of deaths among newborns (within the first 28 days of life) and is a crucial indicator of the quality of care provided during pregnancy, childbirth, and the immediate postnatal period. It helps in assessing the overall healthcare system’s capacity to care for newborns.

It is given by: The number of neonatal deaths (infants under 28 days of age) in a given year divided by the total number of live births in the same year, then multiplied by 1,000.

• NMR provides specific insights into early life mortality, which can guide targeted interventions to improve neonatal care and reduce preventable deaths.
• However, NMR only covers a narrow timeframe (the first 28 days), so it doesn’t capture mortality risks that occur later in infancy or childhood.

6. Post-Neonatal Mortality Rate (PNMR)

Measures the number of deaths among infants aged 28 days to under one year. This indicator helps in understanding the health risks that affect infants after the neonatal period, such as infections, malnutrition, or environmental hazards.

It is calculated as follows: The number of post-neonatal deaths (infants aged 28 days to under one year) each year is divided by the total number of live births in the same year, then multiplied by 1,000.

• PNMR allows for a focus on health risks and interventions needed after the neonatal period, contributing to better infant survival rates.
• Like NMR, it only focuses on a specific age group, missing broader health trends that might affect other age groups.

 7. Peri-Natal Death Rate (PNDR)

 PNDR encompasses both late fetal deaths (stillbirths) and early neonatal deaths (within the first seven days of life). This measure is critical for evaluating the effectiveness of maternal and newborn healthcare during late pregnancy and the immediate post-birth period.

It is given by the number of peri-natal deaths (late fetal deaths plus early neonatal deaths) in a given year divided by the total number of live births plus late fetal deaths, then multiplied by 1,000.

• PNDR provides a comprehensive view of mortality around the time of birth, highlighting the need for improved maternal and newborn care.
• PNDR can be difficult to calculate accurately in settings where data on stillbirths and early neonatal deaths are incomplete or unreliable.

Below is a flow diagram with time reference for mortality through infancy to help you understand the different time durations.

Figure 1: Time reference for mortality in Childhood and infancy

8. Maternal Mortality Ratio/Rate (MMR)

Maternal death is defined as, “the death of a woman while pregnant or within 42 days of termination of pregnancy irrespective of the duration and site of the pregnancy, from any causes related to, or aggravated by the pregnancy or its management, but not from accidental or incidental causes.”

• It is a primary indicator of the health status or QOL of a country or geographic area.
• MMR highlights the risk of death due to pregnancy or childbirth-related causes, indicating the quality of maternal healthcare and access to medical services.

Advantages: Provides critical data for assessing and improving maternal health services.

 Disadvantages: The data collection can be challenging, especially in areas with weak health information systems, and it only focuses on maternal deaths, not on other aspects of maternal health.

Note: For Maternal Mortality Rate, the numerator is a part of the denominator, Whereas, in Maternal Mortality Ratio, the numerator is not a part of the denominator.

9. Death-to-case ratio

 The death-to-case ratio is the number of deaths attributed to a particular disease during a specified time period divided by the number of new cases of that disease identified during the same time period.

It is important in its ability to help in understanding the lethality of a particular disease, which is crucial for assessing the severity of an outbreak or the effectiveness of treatment measures. It can be derived as follows:

Worked Example

 Between 1940 and 1949, a total of 143,497 incident cases of diphtheria were reported. During the same decade, 11,228 deaths were attributed to diphtheria. Calculate the death-to-case ratio.

• It is useful for tracking disease outbreaks and evaluating the effectiveness of healthcare responses.
• However, it can be affected by the accuracy of case reporting and does not account for differences in population sizes or characteristics thus one of its limitations.

10. Case-fatality rate(CFR)

Refers to the proportion of persons with a particular condition (cases) who die from that condition. CFR is a measure of the severity of a disease, showing the proportion of individuals diagnosed with a disease who die from it, which is vital for understanding the virulence and impact of an infectious disease.

It is given by the number of deaths due to a particular disease  divided by the number of incident cases of the disease, then multiplied by 100. As follows:

• It represents the ratio of death to cases
• Establish the virulence and killing power of a disease
• Useful in acute infectious diseases, like cholera, for quickly assessing the deadliness of an outbreak.
• It’s major disadvantage is the fact that the CFR can vary over time and may not reflect the long-term mortality risk if the disease has a long duration.

Worked Example

 In an epidemic of a cholera outbreak in Yaounde, 555 cases were identified and confirmed. Three of the case patients died as a result of their infections. Calculate the case-fatality rate. 

References

• Centres for Disease Control and Prevention (CDC) 2019. Age-Specific Death Rates for Selected Age Groups, by Race and Hispanic Origin and Sex: United States, 2019. Retrieved from [CDC](https://www.cdc.gov/nchs/data/nvsr/nvsr70/nvsr70-08-508.pdf).
• WHO. 2017. Mortality, Health Topics. Available online at: http://www.who.int/topics/mortality/en/
• United Nations. 2017. Death, Mortality, Demographic and Social Statistics. Available online at: https://unstats.un.org/unsd/demographic/sconcerns/mortality/mortmethods.htm
• Principles of Epidemiology in Public Health Practice; 3rd Edition An Introduction to Applied Epidemiology and Biostatistics, center for disease control, 2012.
• World Health Organisation. 2018. Maternal Mortality Ratio (per 100,000 live births), Health Statistics and Information Systems. Available online at: http://www.who.int/healthinfo/statistics/indmaternalmortality/en/

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