Measure of mortality or morbidity

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Measures of Mortality and Morbidity

These are quantitative tools used in epidemiology and public health to describe the health status of a population. They underpin priority-setting, resource allocation, and monitoring of disease control programs.

A. MEASURES OF MORTALITY

Mortality data measure the frequency of death in a population. They are essential for understanding disease burden and trends.

1. Crude Death Rate (CDR)

The simplest measure of mortality.

Definition: Number of deaths from all causes per 1,000 mid-year population in one year, in a given place.

Formula:

CDR = (Number of deaths during the year / Mid-year population) × 1,000

Limitation: CDR is affected by population composition (age structure). A young developing-country population may show a lower CDR than an older developed-country population, even if the developing country has higher age-specific death rates. This is the key limitation that necessitates age standardization.

2. Specific Death Rates

Used when analysis needs to throw light on aetiology. Can be:

Cause-specific (e.g., tuberculosis, cancer, accidents)
Age-specific (deaths in age group 15-20 per 1,000 in that group)
Sex-specific (deaths among males per 1,000 male population)
Can also be specific for income, race, housing, religion, etc.

Examples:

Rate	Formula
TB-specific death rate	(Deaths from TB / Mid-year population) × 1,000
Sex-specific death rate	(Deaths in males / Mid-year male population) × 1,000
Age-specific death rate	(Deaths aged 15-20 / Mid-year population aged 15-20) × 1,000

Specific death rates identify "at-risk" groups and permit comparisons between causes within the same population.

3. Case Fatality Rate (CFR)

Represents the killing power of a disease.

Formula:

CFR = (Total deaths due to a disease / Total cases of the same disease) × 100

Expressed as a percentage (ratio, not a true rate - no time interval)
Used mainly in acute infectious diseases (cholera, food poisoning, measles)
Limited use in chronic diseases (long, variable onset-to-death interval)
Closely related to virulence of the pathogen
May vary between epidemics due to changes in agent, host, and environment

4. Proportional Mortality Rate (PMR)

Expresses what proportion of all deaths are due to a specific cause or in a specific age group.

Formulas:

(a) Disease-specific PMR:

PMR = (Deaths from specific disease in a year / Total deaths from all causes) × 100

(b) Under-5 Proportional Mortality Rate:

= (Deaths under 5 years / Total deaths during same period) × 100

(c) PMR for aged 50+ years:

= (Deaths aged ≥50 / Total deaths of all age groups) × 100

Useful when population data are not available
Indicates relative importance of a disease as a cause of death
Limited for comparisons between populations because both numerator and denominator may differ

5. Survival Rate

Describes prognosis in a disease condition.

Survival rate = (Patients alive after 5 years / Total patients diagnosed or treated) × 100

Reckoned from date of diagnosis or start of treatment
Widely used as a yardstick for standards of therapy in cancer studies

6. Standardized (Adjusted) Death Rates

Used to remove the confounding effect of different age structures when comparing two populations.

Two methods:

Direct standardization: Apply the age-specific rates of each study population to a common "standard population" to get expected deaths, then calculate a standardized rate.
Indirect standardization: Yields the Standardized Mortality Ratio (SMR):
```
SMR = (Observed deaths / Expected deaths) × 100
```
SMR > 100 = more deaths than expected; SMR < 100 = fewer deaths.

Can also be standardized for sex, race, and parity.

Key Mortality Indicators (WHO / Millennium Development Goals)

Indicator	Description
Infant Mortality Rate (IMR)	Deaths < 1 year per 1,000 live births
Child Mortality Rate	Deaths < 5 years per 1,000 live births
Maternal Mortality Ratio	Maternal deaths per 100,000 live births
Life Expectancy at Birth	Average years a newborn is expected to live
Disease-specific Mortality	Deaths from a specific disease per population

B. MEASURES OF MORBIDITY

Morbidity is defined as "any departure, subjective or objective, from a state of physiological well-being" (WHO). It is equivalent to sickness, illness, or disability.

The WHO Expert Committee on Health Statistics noted morbidity can be measured in terms of:

Persons who were ill
Illnesses (periods/spells) those persons experienced
Duration (days, weeks) of those illnesses

Three aspects measured by morbidity rates:

Aspect	Measured by
Frequency	Incidence & Prevalence rates
Duration	Disability rate (avg. days of disability per person)
Severity	Case fatality rate

1. Incidence Rate

Definition: Number of new cases of a disease occurring in a defined population during a specified period.

Incidence Rate = (New cases of disease during period / Population at-risk during that period) × 1,000

Example: 500 new cases in a population of 30,000 in a year: = 500/30,000 × 1,000 = 16.7 per 1,000 per year

Key points:

Refers only to new cases
Must always include the unit of time (e.g., "per year")
Denominator = population at risk (those who can develop the disease)
Not influenced by duration of disease
Most useful for acute conditions

Special Incidence Rates

a. Attack Rate An incidence rate used when population is exposed for a limited time (epidemic setting):

Attack Rate = (New cases during time interval / Total population at risk during interval) × 100

Expressed as a percentage; reflects the extent of an epidemic.

b. Secondary Attack Rate (SAR) Number of exposed persons developing disease within the incubation period following exposure to a primary case. Measures transmissibility of an agent within households/contacts.

c. Hospital Admission Rate Number of admissions per defined population in a given period.

2. Prevalence Rate

Definition: All current cases (old AND new) existing at a given point in time or during a period.

Although called a "rate," prevalence is technically a ratio (no time denominator in the strict sense).

Two types:

(a) Point Prevalence (more commonly used):

Point Prevalence = (All current cases at a given point / Estimated population at same point) × 100

The "point" may span several days in practice (time to survey the population).

(b) Period Prevalence:

Period Prevalence = (All cases existing during a period / Mid-interval population at risk) × 100

Includes cases beginning before the period but continuing into it.

Relationship Between Incidence and Prevalence

For a disease in a stable (steady-state) situation:

Prevalence ≈ Incidence Rate × Mean Duration of Disease

Measure	Influenced by
Incidence	Rate of new disease occurrence
Prevalence	Incidence + duration (and recovery/death)

High prevalence relative to incidence = long-lasting disease (e.g., diabetes, HIV)
Low prevalence relative to incidence = short illness (recovery or death is rapid, e.g., influenza)

Uses of Morbidity Data

Describe the extent and nature of disease burden - assists in establishing priorities
Provide more clinically relevant information than mortality data - essential for basic research
Serve as starting point for aetiological studies and disease prevention
Needed for monitoring and evaluation of disease control activities

Summary Table

Measure	Numerator	Denominator	Multiplier	Use
Crude Death Rate	All deaths	Mid-year population	× 1,000	Overall mortality
Age-specific Death Rate	Deaths in age group	Mid-year population in that group	× 1,000	Mortality by age
Case Fatality Rate	Deaths from disease	Cases of same disease	× 100	Killing power of disease
Proportional Mortality	Deaths from specific cause	Total deaths	× 100	Relative importance of cause
Survival Rate	Alive at 5 years	Total diagnosed	× 100	Prognosis
Incidence Rate	New cases	Population at risk	× 1,000	Rate of new disease
Attack Rate	New cases (epidemic)	Population at risk	× 100	Extent of epidemic
Point Prevalence	All current cases	Population at same time	× 100	Disease burden at a point
Period Prevalence	All cases in period	Mid-period population	× 100	Disease burden over time
SMR	Observed/Expected deaths	-	× 100	Comparative mortality

Source: Park's Textbook of Preventive and Social Medicine

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