Epidemiology

What is the main and critical difference between a "Ratio" and a "Proportion"?

In a ratio, the numerator is not part of the denominator, whereas in a proportion, the numerator is a part of the denominator

1. Which of the following indices best describes BMI (Body Mass Index)?

A. Ratio

1. A researcher found that some of the city's residents were sick with the flu and published that "the numerator is included in the denominator," but did not specify in which year or where the data were collected. Which quantitative measure is this?

C. Proportion

Ratio

The numerator is not part of the denominator: R = a/b
Compares two separate groups; example: sex ratio or BMI (kg/m²).

Proportion

The numerator is part of the denominator: P = a/(a+b)
Expresses a part of the whole; without a time element.

Rate

The numerator is part of the denominator: P = a/(a+b)
A rate requires specification of time and population (or location);

Incidence rate

: Measures the number of new cases that appear in a population during a specific time period.

Prevalence rate

: Measures the total number of cases (new and old) existing in a population at a specific point in time or over a specific period.

Static Proportion

A frozen, part-to-whole snapshot that calculates the fraction of an affected population (a/(a+b)) but completely lacks any context regarding when or where the data was collected.

Operational Rate

A dynamic metric that adds explicit time and geographic boundaries to a population comparison, allowing public health officials to actively measure the speed, flow, and frequency of a health condition.

Protective factor

A factor positively associated with disease occurrence. Exposure to or presence of this factor increases the risk of disease.

Protective Factor

A factor negatively associated with morbidity. As exposure to this factor increases, morbidity decreases and the likelihood of disease occurrence is lower.

The epidemiological Triangle

Agent, Host, Environment

The Agent

the direct cause of the disease and the necessary condition for its existence. It is the element without which the disease simply would not occur. (EX: COVID19)

The Host

The host is the ill individual (the human or another carrier). This side refers to the specific characteristics and variables of that person which influence their level of risk of becoming ill when exposed to the agent.

The Environment

The environment includes all the external conditions that surround the host and the agent, and influence the likelihood of infection or the severity of the disease.

Descriptive (Stage 1):

1.        Describes the extent, severity, and distribution of the disease and answers the basic questions (what, who, where, and when) in order to inform health policy.

Analytical (Stage 2):

1.        Investigates causes and risk factors of the disease. This stage is focused on answering the “why.” This stage relies on the information collected from the descriptive stage to identify the risk factors and compare groups (cohort studies and case-control studies).

Experimental

1.        Includes active interventions and experiments by the researcher to examine ways to improve disease outcomes or strengthen protective factors.

To correctly classify a measure, researchers ask three key questions:

• Is the numerator contained within the denominator? (No → Ratio; Yes → Proportion or Rate)

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• Does the denominator represent a population at risk in a specific time and place? (Yes → Rate; No → Proportion)

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• Is the comparison between two completely separate groups? (Yes → Ratio)

Crude Rate

●      Refers to the entire population. Calculated as the total number of cases(e.g., sick individuals)  divided by the total population.

Specific Rate

●      Refers to a specific subgroup within the population, such as a division by age, gender, or marital status. Calculated as the total number of cases(e.g., sick individuals)  in the sub-population divided by the number of persons belonging only to that sub-population (this is the denominator).

The "Disease Barrel" Metaphor

• Inflow (Incidence): The water pipe pouring new cases into the barrel.

• Water Level (Prevalence): The amount of water accumulated in the barrel at any given moment.

• Outflow (Recovery and Death): The drain holes at the bottom of the barrel through which cases leave the population.

 

Golden rule of incidence

subtract the ill population from the total population to isolate the true population

period prevalence

tracks the total accumulated burden, it would combine all instances of disease with the new cases for that year/time period

Point Prevalence (The Snapshot):

You freeze time for a fraction of a second. The numerator is strictly the number of existing cases at that exact, frozen moment.

Period Prevalence (The Window):

the total accumulated burden over a period of time so you must combine old cases with new cases to get the numorator and the denominator is the entire population

Crude Mortality Rate

An indicator referring to the general population. It can be calculated for all causes of death or for a specific disease:

All-cause mortality rate formula:

(Number of deaths from all causes during the year) ÷ (Size of the population at the beginning of the year).

Disease-specific mortality rate formula:

(Number of deaths from a specific disease during the year) ÷ (Size of the population at the beginning of the year).

Specific Mortality Rate:

indicator referring to a sub-population or a specific stratum in the population (e.g., age group, sex, marital status, etc.).

Proportional Mortality Rate:

An indicator that calculates the number of deaths from a specific cause out of the total number of deaths (and not out of the size of the living population).

Live birth

any sign of life at birth

still birth

baby wigs 500+ grams or 22+ weeks of pregnancy or both

miscarriage

no signs if life, weighs less than 500 grams and pregnancy was less than 22 weeks

IMR (infant mortality rate)

the number of infant deaths under one year of age relative to the number of live births in the same population and time period

IMR formula

(number of deaths under age 1/ number of live births) x 1000

neonatal mortality reate (NMR)

the number of deaths during the first 28 days of life (0-27)

Formula: (number of deaths age 0-27 days/ number of live births) x 1000

Early Neonatal Mortality Rate

Death of an infant during days 0-6 of life

Formula: (number of deaths age 0-6 days/ number of live births) x 1000

Late Neonatal Mortality Rate

Death of an infant at 7-27 days of life

Formula: (number of deaths age 7-27/ number of live births) x 1000

Post Neonatal Mortality Rate

Death of an infant from 28 days of age until less than one year of age

Formula: (number of deaths age 28 days- less than a year/ number of live births) x 1000

Neonatal Mortality

early neonatal mosrtality and late neonatal mortality

Infant mortality

Neonatal mortality and Post neonatal mortality

Infant mortality in developed countries

Most infant deaths occur during the neonatal period, mainly due to prematurity and congenital anomalies.

Infant mortality in developing countries

A larger proportion of deaths occur during the post-neonatal period, mainly due to infections and malnutrition.

Fertility

the number of live births in a population.

Fecundity

the biological ability of a woman to conceive and give birth.

Crude Birth Rate (CBR)

The crude birth rate describes the number of live births in a year relative to the total population.

Formula:

(Number of live births during the year / Mid-year population) × 1000

• General Fertility Rate (GFR)

This rate refers only to women of reproductive age

Number of live births during the year / Number of women aged 15–49) × 1000

Eradication

complete elimination of the disease from the world

Elimination

reduction to a level of sporadic cases while maintaining continuous prevention

Containment

reduction of the disease burden so that it does not constitute a significant public health problem

Sporadic

occurrence of isolated, irregular new cases in a population.

Endemic

constant and ongoing presence of a disease or infectious agent in a specific area.

Epidemic:

an increase in the number of new cases above the expected level in a population or area.

Pandemic

spread of a disease on a global scale across geographical boundaries

Carrier

an individual who harbors an infectious agent without symptoms but can transmit it.

Incubation period

the time between exposure to the disease agent and the appearance of clinical signs.

Herd immunity

a situation in which a high proportion of vaccinated individuals protects even those who are not vaccinated by preventing disease spread.

Healthy:

The individual is in a state of full health, free of disease.

Pre-clinical:

The disease is present in the body, but there are no outward signs or symptoms. This stage represents the "window of opportunity" for early detection. It is a critical window because the disease is present but silent; once symptoms emerge, the opportunity for secondary prevention ends and clinical treatment begins.

Clinical:

Symptoms appear, prompting the individual to seek medical intervention.

Outcome:

The disease progression results in recovery, complications, chronic disability, or death.

TP

true positive (correctly identified as sick)

FP

False positive (incorrectly identified as sick)

FN

false negative (incorrectly identified as healthy)

TN

true negative (correctly identified as healthy)

Sensitivity

in order to examine to what extent the test correctly diagnoses sick individuals. In other words, what percentage of the sick were correctly identified.

Specificity

in order to examine to what extent the test correctly diagnoses healthy individuals. In other words, what percentage of the healthy were correctly identified.

Positive Predictive Value (PPV)

in order to examine to what extent a positive test result correctly diagnoses. In other words, what percentage of those who received a positive result are truly sick.

Negative Predictive Value (NPV)

in order to examine to what extent a negative test result correctly diagnoses. In other words, what percentage of those who received a negative result are truly healthy.