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**Assessing Disease Frequency**

South Asian Cardiovascular Research Methodology Workshop Basic Epidemiology Assessing Disease Frequency Thomas Songer, PhD

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**Why should we be concerned with monitoring disease(s)?**

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**Disease Control and Prevention**

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**Good monitoring does not necessarily **

ensure the making of right decisions, but it reduces the risk of wrong ones. Languimer, 1963

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**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?

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**Approaches Towards Monitoring Disease and Injury**

Death Certificates Population Surveys Surveillance Registries Screening

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Surveillance: Systematic, regular ascertainment of incidence using methods distinguished by their practicality, uniformity, and frequently their rapidity, rather than by complete accuracy. Let’s spend some more time talking about each of the major systems for identifying injuries. The primary means to identify injuries is to establish a surveillance system. What is a surveillance system? John Last has defined surveillance as the regular assessment of disease or injuries, with a common method, and often an approach that is simplistic rather than complex. One of the key words here is regular assessment. Injury monitoring, such as that with death certificates, is set up to evaluate injuries over time. With common methods, this allows a researcher to examine changes over time in these events. Last, 1990

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Types of Surveillance Active Passive

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**monitoring domestic violence in emergency departments**

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. Active surveillance involves the regular monitoring of surveillance sites by designated persons. These persons often call up a site to gather information on injury events that happened in the previous month or week. One example of this process would be the surveillance of injuries from domestic violence in emergency departments. Research personnel would contact emergency departments on a regular interval to identify injury events from domestic violence. As no data sources routinely identify domestic violence injuries, this type of active surveillance is necessary to identify the incidence of domestic violence events. monitoring domestic violence in emergency departments

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Active Surveillance Health Dept.

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Passive Surveillance data generated without contact by the agency carrying out the surveillance. Reportable diseases fall under this type of surveillance. More commonly surveillance systems are passive. By definition, researchers or health department personnel do not go out into the community to find cases. Rather, they develop instruments that persons in the community have to send into them (e.g. death certificates) as a means of identifying events. Reportable diseases such as AIDS and malaria, etc. are monitored in this fashion. Laboratories, physicians, and hospitals have to report these events to the health department when they identify a case at their institution. In some states, spinal cord injuries are a reportable condition and are followed with passive surveillance.

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Passive Surveillance Health Dept.

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**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

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

What do you do with it?

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**Epidemiology is a Science of Rates**

death rates disability rates hospitalization rates incidence rates prevalence rates Epidemiology is a science based upon rates. Incidence and prevalence rates provide information on the importance of a disease or injury in a defined population. Understanding how many injuries occur (the absolute number) provides only one part of the puzzle; the numerator. The second piece is the population in which they occur; the denominator. As we will see, the denominator is a very important factor in defining the significance of a problem. Effective injury control is based upon an understanding of injury rates; i.e. how many injuries occur within a given population.

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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 disease in a community or population

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**numerator denominator : Number of events in time period Number at risk**

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 numerator : denominator Number at risk for the event

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Why 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

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**Rates 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

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**Injury Deaths, Australia, 1992**

In this slide, we see the number of injury deaths in Australia by age and gender. The graph suggests that injury deaths are particularly significant in young adults and men. Harrison, 1995

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**Injury Death Rates, Australia, 1992**

In this slide, we see the injury death rates in Australia by age and gender. The message of the graph has changed from that previously. Here we see that injuries are significant in both the young and the very old (75+ years). The previous slide considered only the “numerator”; the number of injury deaths. It’s message is different from this slide, which included a denominator (the population in the respective age and gender categories). Thus, numerator and denominator data are both important to defining the magnitude of the problem. Harrison, 1995

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**= Number of deaths in time period Number at risk of dying Death rate:**

Mortality: 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

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**Three common types of rates**

Crude rates Specific rates Adjusted rates Page, Cole 1995

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**Three common types of rates**

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

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**Crude rates Specific rates = number of deaths in time period**

Crude death rate = total population number of deaths in age group in time period Age-specific death rate = population in age group

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Adjusted Rates 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

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**Crude and Age-Adjusted Death Rates United States, 1940-1992**

crude death rate per 100,000 population age-adjusted death rate

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**Morbidity 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

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**Measures of disease frequency**

Incidence rates crude incidence cumulative incidence Prevalence rates crude prevalence period prevalence point prevalence Paneth

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**of disease in population**

Incidence: 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

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**numerator denominator Incidence Rate**

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 Epidemiology is a science based upon rates. Incidence and prevalence rates provide information on the importance of a disease or injury in a defined population. Understanding how many injuries occur (the absolute number) provides only one part of the puzzle; the numerator. The second piece is the population in which they occur; the denominator. As we will see, the denominator is a very important factor in defining the significance of a problem. Effective injury control is based upon an understanding of injury rates; i.e. how many injuries occur within a given population.

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**numerator denominator Incidence Rate However, in practice**

in large studies, the denominator is often the mid-year population in small studies, the denominator does not include persons with the disease Epidemiology is a science based upon rates. Incidence and prevalence rates provide information on the importance of a disease or injury in a defined population. Understanding how many injuries occur (the absolute number) provides only one part of the puzzle; the numerator. The second piece is the population in which they occur; the denominator. As we will see, the denominator is a very important factor in defining the significance of a problem. Effective injury control is based upon an understanding of injury rates; i.e. how many injuries occur within a given population.

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**What is the incidence rate from**

October 1, 1990 to Sep 30, 1991?

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**What is the incidence rate from October 1, 1990 to Sep 30, 1991?**

4 4 / 14

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**= Cumulative incidence**

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

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Prevalence

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**Number of existing cases of disease in population**

Prevalence: 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

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**Difference between incidence rates and prevalence rates**

Numerator: New cases occurring during a given time period Denominator: Number at risk of developing disease Numerator: All cases present (new and existing) during a given time period Denominator: Number in population

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**Several factors may affect prevalence**

Incidence Duration of disease Disease treatments

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**Incidence rate x average**

Prevalence rate Incidence rate x average duration of disease =

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**Number of cases of disease at the same point in time**

Point Prevalence 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

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**What is the point prevalence on April 1?**

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**What is the point prevalence on April 1?**

7 7 / 18

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