measurement

Epidemiological measureas

How do we determine disease frequency for a population?

Measures of Disease Frequency

The most basic measure of disease frequency is a simple count of affected individuals.

However, counting is not enough!

Why is a simple count not enough?

3 cases of cancer per year from a city of 1,000 people is very different than 3 cases per year from a city of 100,000 people

So, in epidemiology we must know:

the size of the population from which the affected individuals come, and
the time period the information was collected.


• Counts

• Proportions

• Ratios

• Rates

Tools of Measurement
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Case Counts

Measuring disease or health or health care frequency starts with counting cases

Simplest and most frequently gathered measure in epidemiology

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• Refers to the number of cases of a disease or

other health phenomenon being studied


i.e. Number of cases of influenza in Astana in January 2012

• Can be useful for allocation of health resources

• Limited usefulness for epidemiologic purposes without knowing size of the source population
Counts
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Persons included in the numerator are always included in the denominator:

A
Proportion: --------
A + B

Indicates the magnitude of a part, related to the total. In epidemiology, tells us the fraction of the population that is affected.
Proportions
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Proportions - Example

• A
• B
• Total (A + B)
• # persons with hypertension
• # persons without hypertension
• Total study population
• 1,400
• 9,650
• 11,050
P = A / (A + B) = (1,400 / 11,050) = 0.127
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Like a proportion, is a fraction, BUT without a specified relationship between the numerator and denominator

Example: Occurrence of Major Depression

Female cases = 240 240
------------------------ = ---- 2:1 female to male
Male cases = 120 120

Ratios

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A ratio in which TIME forms part of the denominator

Epidemiologic rates contain the following elements:

• health issue frequency (in the numerator)

• unit size of population
• time period during which an event occurs

Rates

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Rate: a measure of the occurrence of a health event in a population group at a specified time period

Number of events

in time period
Number at risk
for the event in period

numerator

denominator
:
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Rates are the basic tool of epidemiologic practice

Why are rates important?

because they provide more complete information to describe or assess the impact of a health issue in a community or population
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Rates allow comparisons between two or more populations of different sizes or of a population over time


Compute Disease Rate
Number of persons at risk = 5,595,211

Number of persons with disease = 17,382

Rate = 17,382 persons with heart disease 5,595,211 persons
= .003107 heart disease / resident / year

Prevalence vs. Incidence

Prevalence is the number of existing cases of disease in the population during a defined period.

Incidence is the number of new cases of disease that develop in the population during a defined period.

Incidence

Incidence rate is a measure of the probability of the event among persons at risk.

Incidence Rates

Population denominator:

I = number of new events during a period of time

number of persons at risk during this time period


Example (Incidence Rate)
During a six-month time period, a total of 53 nosocomial infections were recorded by an infection control nurse at a community hospital. During this time, there were 832 patients with a total of 1,290 patient days. What is the rate of nosocomial infections per 100 patient days?

Prevalence

Prevalence: The number of existing cases in the population during a given time period.

PR = # existing cases during time period

population at same point in time

Prevalence rates are often expressed as a percentage.

Measures
Morbidity: Refers to the presence of disease in a population

Mortality: Refers to the occurrence of death in a population

Mortality Rates: Examples

Maternal mortality: Ratio of death from childbearing for a given time period per number of live births during same time period

Mortality Rates: Examples

Infant mortality: Rate of death for children less than 1 year per number of live births


Neonatal mortality: Rate of death for children less than 28 days of age per number of live births