Basic Measurements in Epidemiology

Epidemiological Tools

1- rates

2-ratios

3-proportion

Rate

A rate measures the occurrence of some particular event in a population during a given time period

Note

A rate measures the occurrence of some particular event in a population during a given time period

Rate Comprises 4 Elements {Components}

1. Numerator

2. Denominator

3. Time Specification

4. Multiplier

Numerator

It is the number of times an event has occurred in a population , during a specified period

Denominator

this can be :

A - related to the population ,and this include

B- related to total event

related to the population ,and this include

1. Mid-year population

2. Population at risk

3. sub-groups of the population

Types of the rates

1. crude {unstandardized} rates

2. specific rates

3. standardized rates

crude {unstandardized} rates

These are the actual observed rates ,e.g. birth and death rates

specific rates

These are the actual observed rates :

1- due to specific causes ,e.g. TB

2- occurring in specific groups , e.g. age

3- during specific time period , e.g. annual ,monthly rates.

Ratios

This expresses a relation in size between two random quantities

Note

The numerator is not a component of the denominator

In The Ratio

Proportion

This refers to the ratio which indicates the relation in magnitude of part from the whole.

Note

Here the numerator is always included in the denominator

In The Proportion

Morbidly

Any departure , subjective or objective , from a state of physiological well-being.

Importance of morbidity data

• Description of the extent and nature of the disease load in the community ,and so assist in the establishment of priorities .

• Provision of more comprehensive and accurate information about patient characteristics than those obtained from mortality data , and thus it is essential for basic research.

• The data serve as starting point for etiological studies , and thus play an essential role in disease prevention

• These data are needed for monitoring and evaluation of disease control activity.

Importance of morbidity data

• The data provide insight into the community's disease burden to help set health priorities.

• offer detailed patient information crucial for research.

• support the identification of disease causes for prevention

• essential for monitoring and evaluating disease control efforts.

The morbidity ratios measure 3 aspects of morbidity , which are

1. Frequency :

this is measured by the incidence and prevalence rates.

2. Duration :

this is measured disability rate

3. Severity :

this is measured by case fatality rate

Incidence

It is defined as the number of NEW cases occurring in a defined population during a specified period of time.

Special incidence rates

1. Attack rate

2. Secondary attack rate

3. Incidence density

Attack rate

It is the incidence rate used only when the population is exposed to risk for a limited period of time .e.g. ,during an epidemic

Secondary attack rate

This is the number of exposed persons

developing the disease within the range of the incubation period following exposure to a primary case .

• In other words , it is the incidence of the disease among the contacts

Incidence density

Here the denominator is the total person — time of follow up

Incidence rate is important (Uses of incidence)

1. Controlling the disease .

2. For research into etiology and pathogenesis ,distribution of disease ,and efficacy of preventive and therapeutic measures

Prevalence

It refers to all current cases {old and new} existing at a given point in time, or over a period of time in a given population

Types of prevalence ratios

1. Point prevalence .

2. Period prevalence .

Uses of prevalence

1. It helps to estimate the magnitude of health/disease problems in the community, and identify potential high risk population

2. It is useful for administrative and planning purposes ,e.g., hospital beds, manpower needs, facilities …etc.

Relationship between prevalence and incidence

Prevalence depends on two factors:

1- incidence 2- duration

Measurements of Mortality

1. Easily to obtain & reasonably accurate.

2. Crude death rate

Specific death rates

A/ Disease specific

B/ Age specific

C/ Gender Specific

D/ Combination.

Case Fatality Rate

• It is the killing power of disease.

• It is ratio of deaths to cases.

• Used in acute infectious diseases.

Maternal mortality rat

The death of a woman while pregnant or within 42 days of termination of pregnancy, irrespective of duration & site of pregnancy.

Child Mortality Rates

1. Infant mortality rate : Number of deaths of children below one year of age per 1000 live births during the same year and in the same area.

2. Neonatal mortality rate : Newborns 1- 28 days.

3. Under 5 mortality rate : 1 day up to less 5 years.

Disease reporting

1. Immediate Notification.

2. Weekly reporting.

3. Monthly reports.

4. Annual reports