Part I: Fundamentals of Epidemiology Flashcards
Epidemiology
The study of the distribution and determinants of disease frequency in human populations and application to control health problems
Population
Group of individuals with a shared characteristic
Distribution
Analysis of disease patterns according to person, place, and time
Objectives of Epidemiology
- Identify Causes of Disease and risk factors
- Determine extent of disease in community
- Examine natural history of disease
- Evaluate effectivness of interventions and treatments
- Input on public policy
Assumptions of Epidemiology
- Human disease does not occur at random, factors increase or decrease likelihood of disease
- Factors can be identified by systematic investigation of populations
Determinants
Factors that change a person’s health or make a difference in health (exposures)
Endemic
disease regularly found among a population in a certain area
Epidemic
Occurrence of disease in excess of normal expectancy
Pandemic
Epidemic occuring worldwide, or crossing international boundaries
Epidemic Intensity
(# of deaths / (Global population + Epidemic duration)
Sequence of Epidemiological Investigation
–> Suspect Exposure influences disease occurence –> Form hypothesis about exposure-disease association –> Conduct epidemiological studies to measure relationship –> Judge whether association is valid –> Evaluate preventions and treatment –>
Descriptive Epidemiology
Analysis of dease patterns according to characteristics of person, place, and time
Types of Epidemiology
- Descriptive
2. Analytic / Scientific
Roles of Descriptive Epidemiology
Describe disease patterns
1. Monitor the publics health
2. Evaluate success of intervention programs
3. Generate hypothesis
Studies: (Case Report, Case Series, Cross-Sectional, Ecologic Study)
Limitation: Cannot Uncover Cause of Disease
Roles of Analytical Epidemiology
Search for Disease Causes and Preventions
1. Evaluate hypothesis about causes of disease
2. Evaluate success of intervention programs
Studies: (Clinical Trial, Experimental Study, Case-Control, Cohort)
Person
Permanent Characteristics - Age, Sex, Race Biological Characteristics, - Immune status, etc. Acquired Characteristics - Marital Status Activities - Occupation, Leisure, Medications, Drugs Conditions Under which one lives - SES
Person: Marriage
Protective Hypothesis - Marriage provides an environment conductive to health
Selective Hypothesis - people who marry are healthier than people who never marry
Place
Benefit: Can help us determine where diseases are occuring and if frequency varies by location
What is it?
- Residency, site of employment, school district, birthplace, etc.
- Suburban, Urban, Rural
Can determine if certain communities are at an increased risk of disease
Time
Occurrences of disease change over time, some are predictable and others are not
Patterns of disease occurence critical for:
- Monitoring disease occurrence in a community
- Assessing whether ph interrventions made a difference
4 Components for Measuring Disease Frequency
- Population (Which group of people)
- Cases of Disease (Numerator)
- Size of Population (Denominator)
- Time (Be explicit)
Population (Measuring Disease Frequency)
Used as the base group from which we count disease frequency
Population at Risk
Members of the population who are capable of developing the disease being studied
Does not include:
- Those who already have the disease
- Those who cannot get the disease
Source Populations
a. Fixed Population: Membership defined on the basis of some event, membership is permanent (e.g. People born in 1982, or survivors of Hiroshima)
b. Dynamic Population: Membership is defined by being in a state or condition, membership is transient (e.g. residents of boston, parents of a teenager)
Numerator of all measures of frequency
Cases of disease
Disease
Any Health outcome (positive or negative)
Methods of Disease Assesment
Clinical records, diagnostic tests, disease registries, surveillance programs, self reports
Denominator for All Measures
Size of the population
Full or sample of population
Necessary for comparison of disease across populations
Time (Measures of Disease Frequency)
Single Point in Time
- At study enrollment, at birth, on a particular date
Interval of Time
- A period of follow-up, from birth to 10 years of age
Types of Measures
Ratios, Proportions, Rates
Count
The number of cases of a disease or other health phenomenon being studies
Ratio
Definition: Division of one number by another
E.g. Ratio = (Observed number of AIDS cases in country A during june) / (Expected number of AIDS cases in country A during June)
Proportion
Definition: Division of two related numbers, (the numerator is a subset of the denominator)
Often expressed as a percentage
Rate
Definition: Division of one number by another in which TIME is an intrinsic part of denominator
E.g: Breast cancer incidence rate - number of new cases of breast cancer among 100,000 women over a one year period - 125 cases/100,000 women per year OR 125 cases / 100,000 woman-years
Epidemiological Measures of Disease Frequency
Three Types:
- Prevalence
- Cumulative Incidence
- Incidence Rate
All measures share three characteristics:
Number of cases (numerator), size of population (denominator), Measure of time
Prevalence
Definition: Measure presence of EXISTING CASES of a disease in a population during a specified time period
- A snapshot describing the burden of disease
- Involves being in a specific state
Denominator includes the TOTAL POPULATION
Proportion of population who have disease
Point Prevalence
Proportion of population that has the disease at a single point in time
Prevalence = (# of Existing cases at a time point) /
(# in total population at a time point)
Relevant time period does not appear in formula but must be stated using words
Incidence
Definition: Measures occurrence of new cases of disease in a population during a specified time period
- Involves a TRANSITION from one state to another
Denominator only includes population at risk
- Exclude those who already have the disease
- Exclude those who are immune
Prevalence vs. Incidence
Prevalence Incidence
What:
Existing Disease. New Disease
Why:
1. Asses burden of disease. 1. Etiologic research
2. Use when incidence hard. 2. Evaluation of
preventions/treatments
Cumulative Incidence
- Time is described in words
- Measures occurrence of new cases of disease in a population during a specified time period
Critical Assumption: All people in the population have been followed for the entire time period (NO LTF)
- CI = (# New cases during time period) / (# in population at risk at start of time period)
Incidence Rate
- Time is an intrinsic part of denominator
- Measures SPEED at which NEW CASES of disease occur in population
- Does not assume no LTF
- Time is intrinsic part of denominator
- Formula: IR = (# New cases during time period) / (Total person-time of observation in population at risk)
- Units = Person time
Attack Ratio (Incidence Measure)
Proportion of individuals exposed to infectious agent who become infected over a certain time period
Case Fatality Ratio (Incidence Measure)
Proportion of individuals with a disease who die of that disease
Limitations of Cumulative Incidence
No LTF
- Not a perfect measure in a dynamic population or a fixed population that loses members over time
Person Time
Amount of time each person at risk is under observation
- Accrued only among persons at risk
- Accrued only while being followed
Ends when:
- Person is no longer part of population at risk
- Observation periods end
Converting B/w Prevalence & Incidence Rate
Formula: P = IR * D
D = average duration of disease
Assumption
- Population is in a steady state (In = out)
- Prevalence is low (<10%)
How/Why Do We Compare Measures of Disease Frequency?
How: Exposure Status
Why: If disease occurs more often or less often in one group than in the other, then there is an association b/w exposure and disease
Comparisons
Index group (Exposed) vs. Comparison/Reference Group (Unexposed)
Measures of Association
Definition: Comparisons of measures of disease frequency
Approaches
- Absolute
- Relative
Absolute Measure
Def: Calculate the difference b/w two measures of disease frequency
Types:
- Prevalence Difference
- Risk Difference
- Rate Difference
Formula: RD = R_E - R_U
Relative Measure
Def: Calculate the ratio of two measures of disease frequency
Types:
- Prevalence Ratio
- Risk Ratio
- Rate Ratio
Formula: RR = R_E / R_U
Prevalence Differennce
(Based on Prevalence)
Def: Difference in prevalence b/w index and comparison groups
Formula: PD = P_E - P_U (No units)
Risk Difference
(Based on Cumulative Incidence)
Def: Difference in cumulative incidence b/w index and comparison groups
Formula: RD = CI_E - CI_U (No units)
Rate Difference
(Based on Incidence Rate)
Def: Difference in incidence rate b/w index and comparison groups
Formula: RD = IR_E - IR_U (Person Time)
Interpreting Absolute Measures of Association
RD = Difference comparison of prevalence/risk/rate of disease in the exposed group to that of unezposed
Note:
- Focus on the excess disease in exposed grroup
- Specify comparison group
- Specify time period
Prevalence Ratio
(Based on Prevalence)
Def: Ratio of prevalence b/w index and comparison groups
Formula: PR = P_E / P_U (No units)
Risk Ratio
(Based on Cumulative Incidence)
Def: Ratio of cumulative incidence b/w index and comparison groups
Formula: RR = CI_E / CI_U (No units)
Rate Ratio
(Based on Incidence Rate)
Def: Ratio of incidence rate b/w index and comparison groups
Formula: IRR = IR_E / IR_U (No Units)
Relative Risk
Generic term repersenting all relative measures of association
Interpreting Relative Measures of Association
Relative Risk = Ratio comparison of prevalence/risk/rate of disease in the exposed to that of the unexposed group
E.g. if RR = 7.45
- Women with ___ were 7.45 x as likely to have __ over a __ year period compared to women without ___
- Or had 7.45 x the risk of ___
Excess Relative Risk
(RR-1) x 100%
Can be negative
Risk Difference vs. Relative Risk
RD : Repersents measure of public health impact of exposure on disease occurence
RR: Repersents measure of strength or magnitude of the association
Association
- An identifiable relationship b/w expsure and disease
- Indicates that the exposure might cause disease
- Comparisons of risk or rates yields an association
Outbreak Investigation Sequence
- Confrm
- Describe
- Determine the Cause
- Control
Outbreak Investigation Detailed
- Confirm that there is an outbreak
- Formulating case definitions
- Conducting case confirmation
- Establishing the descriptive epidemiology of the outbreak cases
- Generating and testing hypothesis about the causes of the outbreak
- Collecting and testing environmental samples
- Implementing control measures
- Interacting with the press and public to disseminate information
Epidemic Curves
- Used to describe outbreaks
- Plot date or time of illness on x-axis, number of cases on y-axis
- Tell us: distribution of cases over time, outbreaks magnitude, pattern of spread, time of exposure, shape tells us about mode of transmission
Point Source Outbreak
- Persons are exposed over a brief time to the same source
Number of cases rises rapidly to peak and then falls gradualy
Most cases within one incubation period
Continuous Common Source Outbreak
- Persons exposed to the same source but exposure is prolonged over a period of time
- # of cases might peak and fall, but all cases do not fall within a single incubation
Propogated Outbreak
- There is no common source, outbreak is person to person
- Curve will cycle through progressivley taller peaks that are often one incubation period apart
