3: How do you describe disease occurrence? Flashcards
Why are we interested in disease within populations rather than individuals?
- Population = more individuals = more information
- Every individual might not be available for examination
- Occurrence in populations can inform us about individual risk
- Some things only happen at a population level
How do we define populations broadly?
A group of units, elements, individuals connected by some common characteristics.
When does the definition of a population differ?
It is different depending on the discipline.
How do we define populations in epidemiology?
Connected by their relevance to the outcome of interest (population at risk)
For a population definition to be useful you need to determine …
What, where, when?
(Characteristics, space, time)
What are different population characteristics?
- Structure (contiguous or separate or meta-populations)
- Managed/unmanaged
- Dense/sparse
- Homogenous/heterogeous
What populations are easier/harder to count?
Contiguous = difficult to count
Separated = easier to count
Can we study the entire population?
No (in most cases)
What is a target population?
Population about which we are drawing inferences.
What is a source population?
Population from which the sample is drawn.
What is a study population?
Group of individuals selected for the study (sample).
Why might target and source populations be different?
- Access
- Bias
What do we want in a sample?
To be as representative of the target as possible.
How do we sample populations?
Non-probability sampling - some individuals do not have any probability to be sampled.
- Convenience
- Purposive
Probability sampling - all individuals have a chance to be sampled.
- Simple random sampling
- Systematic random sampling
- Stratified random sampling
- Clustered random sampling
What is convenience sampling?
An example of non-probability sampling.
Ex. Sampling from a pasture that is closest to access.
How do we quantify disease occurrence?
Ratio, proportion, and rates.
What is a ratio?
A value obtained by dividing one quantity (the numerator) by another (the denominator).
What is a proportion?
A ratio in which the numerator is part of the denominator (x/x+y).
What is a rate?
A ratio that indicates the change in one quantity with respect to one or more others over time.
What is prevalence?
The proportion of the population that has a specific disease at a given time.
What are prevalent cases?
Number of cases of a disease at a given time.
How do you calculate prevalence?
Number of cases / size of population
A population has a disease prevalence of 0.13. What does that value mean?
There is a 13% chance that an individual in that population has that disease.
What factors can affect a prevalence estimate?
- Test specificity & type of test
- Case definition
- Population definition
How can you describe the dynamic of a disease over time?
- Incident cases
- Incident risk
How is incidence risk calculated?
The number of new cases / the number of individuals initially at risk
Within a defined time period
What is incidence risk?
The average risk of developing a disease in a population within a specific time period.
(aka. cumulative incidence)
What individuals are not included in an incidence risk calculation?
Those that are already sick before the period starts.
How is incidence rate/incidence density calculated?
Number of new cases in a time period / the sum of length of time at risk of developing the disease (in all individuals)
What value can incidence rate be?
Any number between 0 and infinity.
What is the unit for incidence rate?
/animal-week, /animal-year, etc.
How do you calculate the approximate incidence rate?
IR approx = Number of new cases / (Duration*(number at risk at the start of the time period + number at risk at the end of the time period)/2)
When would you use the approximate incidence rate?
If you don’t have an exact timeline for all animals.
When would you choose prevalence vs incidence?
Depends on needs and research question.
Prevalence:
- Focus on existing cases
- Measure of disease burden
- Evaluate diagnostic tests
Incidence:
- Study dynamic of disease
- Exploring causal theories
- Evaluating preventive measures
What is an attack rate?
Cumulative incidence at beginning of outbreak or following short exposure.
What is the cumulative mortality?
Disease-related death incidence risk (rate).
What is case fatality?
Number of death/number of diseased animals
What is death rate?
Deaths from all causes.
What is infection?
Invasion by and multiplication of microorganisms in a bodily part or tissue, which may produce subsequent tissue injury and progress to overt disease through a variety of cellular or toxic mechanisms.
What is disease?
Deviation from the normal structure or function of any body part, organ, or system that is manifested by a characteristic set of symptoms and signs and whose etiology, pathology, and prognosis may be known or unknown.
What is the effect of sampling?
You have a sub-group of individuals.
How does sampling impact the measure of occurrence?
It is slightly different every time / for every sample. (= random error)
How do we express when reporting results? (this sentence doesn’t make a ton of sense to me but its right from the notes)
Confidence interval
What does a confidence interval mean?
When sampling many times and constructing CI, most (95%) of them will include the true occurrence value.
How does confidence interval change with sample size?
Small sample size = large confidence interval
