Lecture 13: Descriptive epidemiology

What is descriptive epidemiology?

The field of descriptive epidemiology classifies the occurrence of disease according to person (who is affected), place (where the condition occurs), and time (when and over what time period the condition has occurred). It is concerned with characterizing the amount and distribution of health and disease within a population.

What are the key variables of descriptive epidemiology?

Person (who is affected), Place (where the condition occurs), and Time (when and over what time period the condition has occurred).

What are some activities supported by descriptive epidemiology?

Prevention of disease

Design of interventions

Conduct of additional research

What are the uses of descriptive epidemiologic studies?

Permit evaluation of trends in health and disease

Provide a basis for planning, provision, and evaluation of health services

Identify problems to be studied by analytic methods and suggest areas for further investigation

What are the three types of descriptive epidemiologic studies?

1. Case reports

2. Case series

3. Cross-sectional studies

What is a case report? Provide an example.

Accounts of a single occurrence of a noteworthy health-related incident or a small collection of such events.

Example: Rabies in two bison in Colorado:

- Two adult bison (>5 years) found dead with no prodrome, confirmed rabid by viral sequencing showing the skunk variant of the rabies virus.

- Noteworthy due to the rare disease, dramatic outcome, and atypical geographic presentation.

What is a case series? Provide an example.

A larger collection of cases of disease, often grouped consecutively and listing common features (e.g., characteristics of affected patients).

Example: A Case of Biting Dogs:

- Summary of 227 biting dogs (117 male, 110 female).

- History of aggression in 75.6% of the dogs.

- Adult people were the most common victims, most untreated.

- Commonalities (e.g., breed, sex, age, provocation) serve as potential predictors.

What are the advantages of case reports and case series?

Can aggregate cases from disparate sources to generate hypotheses and describe new syndromes.

Simple to write, simple to read, and contains lots of information.

What are the limitations of case reports and case series?

Cannot test for statistical association due to the lack of a comparison group.

Based on individual exposure, which may be coincidental.

Cannot establish causality, only identify potential associations.

What is a cross-sectional study?

A type of investigation that examines the relationship between diseases (or other health-related characteristics) and other variables of interest as they exist in a defined population at one particular time. It is a type of prevalence survey or study that analyzes data from a population at one point in time.

What are the advantages of cross-sectional studies?

Can aggregate cases from disparate sources to generate hypotheses and describe new syndromes (e.g., Hepatitis, AIDS).

Simple to write, simple to read, and contains lots of information.

What are the limitations of cross-sectional studies?

Cannot test for statistical association because there is no relevant comparison group.

Based on individual exposure, which may simply be coincidental.

Cannot establish causality; only identifies potential associations.

Provide an example of a cross-sectional study.

Beer and Obesity: This type of study analyzes data collected from a population at one point in time to examine relationships between health-related characteristics and variables.

What is the definition of an epidemic?

The occurrence of an infectious disease clearly in excess of normal expectancy, generated from a common or propagated source.

What is the definition of an endemic?

The habitual presence (or usual occurrence) of a disease within a given geographic area.

What is the definition of a pandemic?

A worldwide epidemic affecting an exceptionally high proportion of the global population.

What is clustering?

A closely grouped series of events or cases of a disease or other health-related phenomena with well-defined distribution patterns in relation to time or place or both. Often used to describe the aggregation of uncommon conditions such as leukemia.

What are the two types of clustering?

1. Spatial clustering: Aggregation of events in a geographic region.

2. Temporal clustering: Denotes the occurrence of events related to time.

What are potential causes of clustering?

Common exposure to an etiologic agent.

Chance occurrences.

What are secular trends? Provide examples.

Gradual changes in the frequency of disease over long time periods.

Examples:

- Yearly suicide rates of U.S. females: Firearm-associated suicides decreased, but those by hanging increased.

- Age-adjusted prevalence of hypertension: No secular trend evident.

How do descriptive studies contribute to identifying problems and establishing hypotheses?

Provide a basis for generating hypotheses.

Connect intimately with the process of epidemiologic inference.

Epidemiologic inference is initiated with descriptive observations

How are person variables used to formulate hypotheses in acute disease outbreaks and chronic disease trends?

Age: Diseases like mumps and chickenpox occur more often in children, while chronic disease rates increase with age. Leading causes of death for young adults include unintentional injuries.

Sex: Males generally have higher age-specific mortality rates. Differences exist in diseases like cancers of the genital system.

Race/Ethnicity: Identifies disparities in health outcomes across groups.

Socioeconomic Status (SES): A composite measure including income, education, and occupation. Lower SES correlates with higher morbidity and mortality.

Other Variables: Marital status, nativity, migration, and religion may also influence disease patterns.

How are place variables used to formulate hypotheses in acute disease outbreaks and chronic disease trends?

International Differences: Highlight disparities in global disease distribution.

National Patterns: Reflect within-country variations.

Urban-Rural Differences: Indicate environmental or socioeconomic influences.

Localized Patterns of Disease: Identify hotspots for particular health conditions.

How are time variables used to formulate hypotheses in acute disease outbreaks and chronic disease trends?

Secular Trends: Show gradual changes over long periods.

Cyclic (Seasonal) Trends: Relate to seasonal variations, e.g., flu outbreaks.

Point Epidemics: Focus on outbreaks tied to specific times and locations.

Clustering: Can signal temporal or spatial aggregations of events.

What are the potential sources of bias in descriptive epidemiology?

Chance: Role of randomness, largely determined by sample size. Usually the greatest source of error in these studies.

Selection Bias: Subjects (cases) may not be representative of the entire population at risk. Example: In the beer study, beer drinking might be rare, reducing validity of conclusions.

Observation Bias: Systematic differences in how data are obtained. Example: Some data may rely on recall, while other data are directly measured.

How does descriptive epidemiology differ from analytical epidemiology?

Relatively inexpensive and less time-consuming.

Describes patterns of disease occurrence, e.g., who gets sick, where rates are highest and lowest, and temporal patterns of disease.

Results are useful for epidemiologists (as the first step in risk factor determination) and public health administrators (for resource allocation).

NOT useful for determining causality.

Focuses on classifying disease occurrence by person, place, and time.

Generates hypotheses for exploration by analytic studies.

Includes case reports, case studies, and cross-sectional studies.

Basic form of characterizing a health event or trend; reporting prevalence or