Ch 2 Measuring outcome frequency Flashcards
What is “frequency”
the number of occurrences in a defined population over a defined period
What are “cases”?
Individuals with the outcome of interest
What is the “case definition”
The criteria used to define a case.
What are ways epidemiologists count cases?
clinical assessments,
diagnostic tests,
registry or facility record entries, observation,
or even self-reporting in population surveys
What is a “study population”?
What is a “study sample”?
individuals with specific relevant characteristics being studied.
A sample of the population being studied.
Define “population at risk”
only those individuals who could develop the outcome.
Define “prevalence”, how is that different than “incidence”. Each as most useful to study what?
Prevalence measures the number of existing cases in a population at a defined time point. Useful for studying the burden of chronic diseases, or for initial rapid assessment of an outcome to identify implications for health services
Whereas incidence, measures the number of new cases in a population over a defined period. A better measure of frequency for studying causation.
What are the 3 ways that incidence can be presented?
Explained via example using musical chairs, where outcome = sitting on a chair.
- Risk counts those sitting after a specified time (e.g. 10 minutes), compared with the total participants at the start of the game.
- Odds counts those sitting after a specified time (e.g. 10 minutes), compared with those standing after the same time.
- Incidence rate counts those sitting at any point during the game and considers the total time that each person participates, allowing for children who join the game late or leave early.
Define “risk”. Another term for risk is?
Also known as “cumulative incidence” because it refers to the total number of new cases in a defined ‘population at risk’ during a specified period..
Define “risk”. Another term for risk is?
Risk = (# new cases in specified time)/ (total pop. at risk at start of the period)
Also known as “cumulative incidence” because it refers to the total number of new cases in a defined ‘population at risk’ during a specified period. Expressed in %. Specified period is key to define.
Or probability that an individual will develop the outcome in a specified period.
The pop. at risk excludes pre-existing cases at the start or the period.
What is “secondary attack rate”?
A specific form of risk used in disease outbreaks.
Formula:
secondary attack rate = (# of new cases in a specified time)/ (Total # at-risk contacts of a primary case over a specified period)
What is “secondary attack rate”?
A specific form of risk used in disease outbreaks. Differentiates community transmission and transmission that occurs in close contact spaces like households.
Formula:
secondary attack rate = (# of new cases in a specified time)/ (Total # at-risk contacts of a primary case over a specified period)
What is “odds”
a different way of representing incidence, because it compares the probability of becoming a case to the probability of not becoming a case.
Formula:
Odds = (new cases/ total at risk) / (non-cases/ total at risk) = (cases/total) x (total/ non-cases) = (# of new cases in specified time)/ (# still at risk at the end of specified time)
Thus adding the numerator and denominator should = total pop. at risk.
What formula is used to convert risk to odds?
risk = odds/ (1+odds)
How is incidence rate different than risk and odds?
Both risk and odds assume that the population at risk is followed over a specified time, and that all those who are included at the beginning of the time are also counted at the end. This is called a ‘closed’ population.
incidence uses a ‘dynamic’ or ‘open’ population, in which people enter and exit the population at risk at different points and are therefore at risk for different lengths of time.
