Block 5 Flashcards
Clinical approach focuses on _____ and Epidemiological approach focuses on _____
Individuals; Population
Success of clinical approach depends on… (2)
- Etiology being known
- Correct diagnosis or correct list of diagnoses
Success of epidemiological approach….
Can be achieved without knowing the etiological agent or mechanism of action between exposure and disease
Clinical approach
Establish a diagnosis or small list of possible diagnoses: physical exam and history, generate list of ddxs, further examination and diagnostic testing. Treat against the diagnosis or list of diagnoses.
Epidemiological approach (4)
1) Describe the occurance of disease in populations.
2) Look for differences in disease occurance between groups in populations (groups with vs without certain exposures).
3) Identify those exposures that cause disease.
4) Apply measures to reduce those exposures.
Population
A defined collection of individuals who share one or more observable characteristics, as specified in the definition.
Descriptive Epidemiology
Describe the health status of specified populations
Study design of descriptive epidemiology
Surveys – goals is to estimate, with some defined precision, the frequency and distribution of selected outcomes in a defined population, based on measurements in a sample of that population
Analytical Epidemiology (3)
1) Epidemiologic research to identify potential causal associations between exposures and health-related outcomes.
2) The aim is to make causal inferences about relationships between exposure and disease.
3) Scientific process to seek understanding of a phenomenon.
Census vs Sample
Census = every individual in the population is evaluated
Sample = only a subset of individuals drawn from the population are evaluated (more feasible than a census)
Sampling
The process of selecting study subjects
What source of error does sampling introduce into epi studies
Sampling variability/random error
The opposite of random error is _____
Precision (an estimate with little random error may be described as precise)
Precision can be improved by ______
Increasing the sample size
Statistical inference
The process of drawing conclusions about a population based on data from a sample of that population. It allows us to deal with random error.
What is the only source of error in epidemiological studies?
Random error
Bias occurs due to _____
systematic error
Extent of bias (can/cannot) be determined
Cannot; there is no formal method to deal with systematic error (as there is with random error).
Probability sampling methods (4)
1) Simple random sampling
2) Systematic random sampling
3) Stratified random sampling
4) Cluster sampling
Simple random sampling
Each subject in the population has an equal chance of being selected. Requires a sampling frame of all subjects in the population (disadvantage).
Systematic random sampling
Doesn’t require sampling frame, only an estimate of the total population size and a means of sampling subjects at a predefined interval.
Stratified random sampling
Prior to sampling, divide populations into mutually exclusive strata (=groups) based on factors likely to affect disease occurrence (eg sex, breed..). Within each stratum, select a sample (simple or systematic random sample).
Advantages of stratified random sampling (3)
1) Ensures that all strata are represented in sample
2) May increase precision of overall estimates
3) Produces estimates of stratum-specific disease occurrence
Cluster sampling
A cluster is a natural collection of study subjects (eg farm, herd, litter, pen…)
One stage cluster sampling
Randomly select clusters, and sample all subjects within selected clusters
Two stage cluster sampling
Randomly select clusters, then randomly select subjects from within those clusters
Advantages of cluster sampling (2)
1) Only need sampling frame of clusters (eg all farms in district).
2) May be more cost-effective than simple random sampling.
Point estimate
Use the sample data to calculate a single number to estimate the proportion of diseased animals in the population
Interval estimate
Use the sample data to calculate a range of reasonable values that are intended to contain the true proportion of diseased animals in the population. This method accounts for random error and provides an indication of the precision of our estimate.
Problem with point estimate
Does not account for random error
Prevalence
The proportion of the population that is diseased
Point prevalence
The proportion of the population with the disease/condition at a single point in time
Period prevalence
The proportion of the population with the disease/condition over a specific period of time
T/F: Period prevalence does not distinguish existing cases from new cases
True
Factors that increase prevalence (6)
1) Increase in new cases
2) Longer duration of disease
3) Prolongation of life without cure
4) In-migration of cases
5) Out-migration of health animals
6) Improved diagnostics
Factors that decrease prevalence (6)
1) Decrease in new cases
2) Shorter duration of disease
3) High case-fatality rate from disease
4) In-migration of health animals
5) migration of cases
6) Improved cure rate
Incidence
Measures how frequently susceptible individuals become diseased as they are observed over time
Incidence risk can only be measured in what type of population?
Closed population (no additions to the population and no (or only few) losses over the period of the study; the entire population at risk must be followed from the start of the study until the end)
Incidence rate is expressed as:
The number of cases per animal-time unit at risk
Incidence rate can be measured in ____ populations
Open (animals are entering and leaving the population throughout the study period).
Prevalence is a function of…
Incidence (rate at which new cases arise) and Duration of Disease (which is determined by the rate at which existing cases of disease become non-cases, by dying or being cured).
Attack rate
Incidence risk during an outbreak
When would you use an attack rate
When period of risk is limited and any cases arising from exposure are likely to occur in the risk period
Mortality risk/rate
Incidence risk/rate where death is the outcome of interest
Cause-specific mortality risk/rate
Mortality risk/rate due to a specific disease/event
Case-fatality rate
Proportion of cases of a specific disease that are fatal, within a defined time period following disease onset or diagnosis.
Exposure
Any potential determinant of disease or health status (pathogen, toxin, host factors, env factors….).
An exposure may increase or decrease the occurrence of disease OR have no effect at all.
Once an exposure is shown to be associated with the disease, it is a determinant (a risk/protective factor–not necessarily causal).
Once an exposure is shown to be associated with the disease, it is a _____
Determinant (a risk/protective factor–not necessarily causal)
Outcome
A result or response, usually a disease or some other change in health status.
There is an association between an exposure and a disease if (3):
1) The proportion of individuals with the disease is significantly higher (+ve) or lower (-ve) in those exposed than in those who are not exposed
2) The number of new cases of disease that arise over a specified time period is significantly higher (+ve) or lower (-ve) in those exposed than in those who are not exposed
3) Exposure is significantly more common (+ve) or less common (-ve) in those with the disease than in those without the disease
What are measures of association? And what two things do they do?
Used to assess the magnitude of the relationship between an exposure and the disease of interest in the population.
They 1) determine if there is an association, and 2) measure the strength of the association
How are measures of association expressed
As a relative measure calculated as the ratio of two estimates of disease occurence
If the measure of association = 1
There is no association (measure of occurrence is the same in both groups)
If the measure of association > 1
There is a positive association between exposure and disease. Exposure is a ‘risk factor’ for disease (not necessarily causal).
If the measure of association < 1
There is a negative association between exposure and disease. Exposure is a ‘protective factor’ for disease (not necessarily causal).
