epidemiology Flashcards
Epidemiology
Study of the distribution and determinants of disease frequency in human population
Epidemic/Outbreak
Occurrence of a disease in members of a defined population, clearly in excess of the number of cases usually or normally found in the population
Pandemic
Occurring throughout the population of a country, people, or the world
Purposes of Epidemiologic Methods
Quantifying the magnitude of health problems
Identifying the factors that cause disease
Providing quantitative guidance for the allocation of public health resources
Monitoring the effectiveness of prevention strategies using population-wide surveillance programs
John Snow
From data, postulated that cholera was transmitted by contaminated water
Charred frequency and distribution of water
Ascertained cause/determinants of cholera
Distribution
When? Where? Who?
Questions may consist of comparisons between different populations at a given time, or between different subgroups, or between different periods of observation
-Integral in describing disease patterns, and in the formulation of hypotheses pertaining to possible causal or preventive factors
Disease Frequency
Quantification of existence or occurrence of disease
This info is necessary for investigation of disease occurrence in human populations
Determinants of Disease
Derived from distribution and disease frequency
Necessary to test epidemiologic hypotheses
Hypothesis
Statement derived from a theory predicting the relationship among variables representing concepts, constructs, or events
What the researcher expects to find
Key Assumption of Epidemiology
-Majority of human disease doesn’t occur at random
-Causal and preventive factors of human disease that can be identified through investigation of different populations or subgroups of individuals within a population
Primary Units of Concern
Groups of persons - not separate individuals
Groups of persons must be studied in order to answer questions relating to etiology and prevention of disease and to allocate effort and resources in healthcare facilities and communities
Count
Count of the number of persons in the group studied who have particular disease or characteristic
Ex. 40 of 90 KP290 students have asthma
Ex.
Rez A… 10 students of 30 w2ith STDs
Rez B… 25 of 100 students with STDs
Bigger problem with STDs is group A
Ratio
Expression of a relationship between 2 numbers
100 males : 150 females
1 male : 1.5 females
Proportion
Special type of ratio
Numerator is part of the denominator and resulting proportion is expressed as percentage
Using last example
100 males / 250 (males + females) x 100%
Rate
Certain kind of proportion
Involve or imply time relationship
Prevalence and incidence… indicators of morbidity
Prevalence rate
Refers to the number of pre-existing cases of a condition within a specific population, at a specific time, per 100 of the population at risk
Denominator includes everyone in the population
-Also referred to as point prevalence rate
Key to this definition - one specific time
of cases of certain disease existing in a population at a specified period of time / x 100 population
Ex. 310 people in pop. of 2477 have cataracts… what is prevalence rate
P.R. = 310/2477 x 100
= 0.12151 x 100
= 12.51
Incidence Rate
Number of new cases of certain condition of disease commencing during a specified time per 100 of the population at risk during the same period of time
-Population at risk is the population minus the number of people who have already contracted the disease or condition
of new cases of a particular disease or condition commencing during a specified period of time x 100
Population - pre-existing cases
Descriptive Study Designs
Concerned with disease distribution, including consideration of the following:
-Who?
-Where?
-When?
-Who, where, when… info obtained from the above may lead to the development of epidemiologic hypotheses
Descriptive Study Designs include 2 type of designs
Cross-sectional
Ecological
Cross-sectional design
Measure causes and effects at a certain point and look at relationships
Cross-sectional design advantages
Allows researchers to study many different variables at the same point in time
Less time consuming
Cross-sectional design Limitations
Outcome and exposure are measured at the same point in time
Cannot make statements about cause and effect
Cannot examine longitudinal relationships
Ecological Design
Use existing data sources for exposure and disease outcomes to compare and contrast rates of disease by specific characteristics of an entire population
Ecological Design advantages
-Inexpensive
-Relatively easy to complete
Ecological Design Limitations
-Level of analysis is population, not individual
-Cannot control for the effects of the factors
-Can produce “spurious” results (false results)
Epidemiologic Study Designs
Analytic Study Designs
Analytic Study Designs
-Focus on disease determinants by testing hypotheses, formulated from descriptive studies
-Ultimate goal is to determine whether exposure to a certain factor causes or prevents disease in question
Analytic Study Designs
2 design types
Cohort studies
Case-control studies
Cohort studies
-Prospective studies
-group of people and follow over a period of time; Amount of exposure each person in the group will vary; can never be identical
-Try to establish cause and effect
Cohort Studies Advantages
-Good for rare exposure
-Good for understanding multiple effects of a single exposure
Temporal sequence - you know exactly the time between the exposure and outcome
Cohort Studies Limitations
Difficult to do (because of large n)
Costly (because of large n)
Loss to follow-up (especially when study lasts for many years
Some disease outcomes are ++ rare; may be difficult to get a sufficient number of cases for analyses, even when the cohort is very large
How to Select a cohort
Accessibility (ease of selection)
History of previous exposure
Random sample of people who haven’t been exposed (you know who has it, now you want to know who doesn’t)
Case-Control Studies
-retrospective studies
-Select the participants from a group with a disorder and compare with cases without the disorder have a group of people who already have (they are known as the cases)
How to select cases
Physicians
Employers
Healthcare providers
Medical records
Death certificates
Subject themselves
Proxies for subjects
Case-Control Studies Advantages
Results can be seen in a short period time
Enable hypothesis testing for multiple exposures for a single disease outcome
Establishing Causation
Temporal sequence
consistency
Strength of association
Specificity of effect
Biological gradient
Existing data and theory
Problems of Error: Bias
any error in design, conduct or analysis that could render the data inaccurate
—>selection
—>information
—>confounding
Problems of Error: Random variation
-Chance differences between groups
-May be a result of un-representatives in the comparison of the 2 groups
Problems of Error: Random misclassification
-Subject could be in wrong group
-Exposed person could be in the group with non-exposed, and vice-versa
How do we Control for Error?
matching
Homogeneous grouping
Stratified sampling
History of Epidemiology
In the 5th century B.C. Hippocrates (father of modern medicine) suggested that the development of human disease may be related to eternal and personal environment of individual
Cause of disease was considered for next several hundred years but no attempt to measure impact
In 1662, John Graunt analyzed weekly reports of births and deaths:
- First to quantify patterns of disease
- Noted increase in # of men that were born or died
- Noted high infant mortality rate
- Also noted seasonal variations with respect to mortality
Little further application until WIlliam Farr (2 centuries later)
In 1839, Farr setup system of routine compilation of numbers and causes of deaths
Compared mortality rates with several different characteristics
2 decades later, info collected by Farr enabled John Snow to formulate and test hypothesis concerning origin of cholera epidemic
Cholera
an acute bacterial infection of the small intestine characterized by severe diarrhea and vomiting, muscular cramps, dehydration and depletion of electrolytes
What is the primary difference between a descriptive and an analytic epidemiologic study design?
Descriptive studies focus on describing the distribution of disease (who, where, when), while analytic studies aim to identify the determinants of disease by testing hypotheses about exposure-disease relationships.
What is the main difference between prevalence and incidence? Provide an example to illustrate your point
Prevalence refers to the total number of existing cases of a disease in a population at a specific time, while incidence refers to the number of new cases that develop over a specific period. For example, the prevalence of diabetes in a community might be 10%, while the incidence might be 5 new cases per 1000 people per year.
Case-Control Studies: Limitations
Exposure information is obtained after the disease has been diagnosed
May be influenced by recall bias
Can be hard to recruit appropriate control group (can therefore affect the odds ratios)
May be influenced by selection bias
Errors in Data Collection
Clinical Observation
Disease diagnosis
death records
Medical Charts
Questionare Reasponses
Laboratory Data
