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Assessing Disease FrequencyPowerPoint Presentation

Assessing Disease Frequency

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South Asian Cardiovascular Research Methodology Workshop. Basic Epidemiology. Assessing Disease Frequency. Thomas Songer, PhD. Why should we be concerned with monitoring disease(s)?. Disease Control and Prevention. Good monitoring does not necessarily ensure the making of right decisions,

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### Surveillance:

### So you have identified the frequency of disease in a given area….

Research Methodology Workshop

Basic Epidemiology

Assessing Disease Frequency

Thomas Songer, PhD

Good monitoring does not necessarily

ensure the making of right decisions,

but it reduces the risk of wrong ones.

Languimer, 1963

There are several decisions regarding disease monitoring

- What level of resources should be allocated to disease monitoring?
- What outcomes do we want to achieve?
- What benefits are obtained from these items and to whom do the benefits accrue?
- Can the counting methods be readily accepted into the community?

Approaches Towards Monitoring Disease and Injury

Death Certificates

Population Surveys

Surveillance

Registries

Screening

Systematic, regular ascertainment of incidence using methods distinguished by their practicality, uniformity, and frequently their rapidity, rather than by complete accuracy.

Last, 1990

Types of Surveillance

- Active
- Passive

Active Surveillance

- the collection of data on a disease by regular outreach. Designated medical personnel are called at regular intervals to collect information on the new cases of disease.

monitoring domestic violence in

emergency departments

Active Surveillance

Health Dept.

Passive Surveillance

- data generated without contact by the agency carrying out the surveillance. Reportable diseases fall under this type of surveillance.

Passive Surveillance

Health Dept.

Comparison of Disease Counting Approaches

Population

Communicable

Registries

Screening

Disease

Surveillance

Source

Academia

Academia

Health Depts

Speed

Slow

Slow

Fast

Cost/Case

High

High

Low

Ascertain

>90 %

65-75%

Low and

-ment

Variable

What do you do with it?

death rates area….

disability rates

hospitalization rates

incidence rates

prevalence rates

Epidemiology is a Science of RatesRates area….

- 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 disease in a community or population

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

Number of events

in time period

numerator

:

denominator

Number at risk

for the event

Why area…. are rates useful?

- Can help to identify groups with an elevated risk of disease
- can target interventions to these groups
- these groups can be studied to identify risk factors

Page, Cole 1995

Rates area….

- Relate health events to a population base
- This provides a basis for making valid comparisons of health events by considering the number at risk in each population

Page, Cole 1995

Injury Deaths, Australia, 1992 area….

Harrison, 1995

Injury Death Rates, Australia, 1992 area….

Harrison, 1995

- Mortality area….: is one of the major measures of disease in the population
- information available from death certificates (required by law)
- Death rate:

Number of deaths

in time period

=

Number at risk

of dying

Three common types of rates area….

- Crude rates
- consider the entire population

- Specific rates
- consider differences among subgroups of the population

- Adjusted rates
- adjust for differences in population composition

Page, Cole 1995

number of deaths area….

in time period

- Crude rates
- Specific rates

Crude death rate =

total population

number of deaths in age

group in time period

Age-specific

death rate

=

population in age group

Adjusted Rates area….

- Use statistical procedures to adjust for differences in characteristics between populations
- Age is the most frequent factor adjusted for because age is related to both death and disease
- Adjusted rates do not describe actual occurrence, but are hypothetical given certain assumptions

Crude and Age-Adjusted Death Rates area….United States, 1940-1992

crude death rate

per 100,000 population

age-adjusted death rate

Morbidity area….

- any departure from health
- i.e. the extent of illness, injury or disability in a defined population
- morbidity rates are used as indicators of health
- in epidemiology, the main measures of morbidity are incidence and prevalence

Measures of disease frequency area….

- Incidence rates
- crude incidence
- cumulative incidence

- Prevalence rates
- crude prevalence
- period prevalence
- point prevalence

Paneth

- Incidence area….: is one of the major measures of disease in the population
- information available from surveys, registries, or investigations
- Incidence rate:

Number of new cases

of disease in population

in time period

=

Number at risk of

developing disease in

same time period

x 1000

The numerator has to come from the population at risk for developing disease

The denominator may change over time as people develop disease

The denominator does not include persons with the disease

numerator

Incidence Ratedenominator

in large studies, the denominator is often the mid-year population

in small studies, the denominator does not include persons with the disease

numerator

Incidence Ratedenominator

However, in practice

What is the incidence rate from population

October 1, 1990 to Sep 30, 1991?

Cumulative incidence population

- Number of new cases of disease occurring over a specified period of time in a population at risk (at the beginning of the interval)

Number of new cases of disease

identified over a given time interval

Cumulative

incidence

rate

=

Estimated population

at beginning of interval

Prevalence population

- Prevalence population: is another major measure of disease in the population
- information available from surveys, registries, or investigations

Number of existing cases

of disease in population

in time period

Prevalence

Rate

=

Population at risk

in same time period

x 1000

Numerator: New cases occurring during a given time period population

Denominator: Number at risk of developing disease

Numerator: All cases present (new and existing) during a given time period

Denominator: Number in population

Difference between incidence rates and prevalence ratesIncidence

Prevalence

Several factors may affect populationprevalence

- Incidence
- Duration of disease
- Disease treatments

Point Prevalence population

- Number of individuals in a specified population at risk who have the disease of interest at a given point in time

Number of cases of disease

at a given point in time

Point

prevalence

rate

=

Estimated population

at the same point in time

What is the point prevalence on April 1? population

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