Key principles of epidemiology Flashcards
define frequency
number of occurrences of an outcome within a given time period
two main approaches to epidemiological study
descriptive and analytical
what information does descriptive epidemiology provide?
distribution of health outcomes by age, population type, geography or over time.
what information does analytical epidemiology provide?
investigates which factors may be responsible for increasing or decreasing the probability of an outcome
difference between sufficient and component cause
sufficient cause refers to a factor or sort of factors that produce the outcome. the factors that form a sufficient cause are called component causes
for an association to be causal, the …. must occur before the ….
exposure then outcome
what can effect causality? (alternative explanation for an association)
chance, bias and confounding
what is chance and how do you reduce it?
possibility that there is a random error and reduced by increasing sample size, using random selection or randomisation
what is bias
refers to systematic differences between comparison groups which may misrepresent the association being investigated
what is confounding
caused when another factor, independently associated with both the outcome and exposure, influences the association being investigated
two approaches to analytical epidemiology
observation or intervention
difference between observational and intervention study
observational compares the frequency of an outcome in groups or individuals with and without the exposure of interest. invervention is an experiment - evaluates the effect of reducing a risk factor or increasing a protective factor on the frequency of an outcome.
define prevalence
the number of existing cases in a defined population at a defined point in time divided by the total number of people in that population at the same point in time - it is a proportion
how do you measure prevalence?
population surveys or cross sectional studies
define incidence
frequency of new cases in a defined population during a specified time period
how do you measure incidence?
ecological or cohort studies
three ways to consider incidence
risk, odds and incidence rate
define risk
total number of new cases in a specified time period divided by the total number of individuals at risk in the population at the start of the time period
how do you work out secondary attack rate
number of new cases among contacts in a specified time period divided by total number of contacts of a primary case in that time period
define odds
ratio of two proportions so number of new cases in a specified time period divided by number who did not become a case during that time period
what do you use if it is an open population rather than a closed population to work out incidence rate?
population time at risk
examples of relative measurers that use ratios
prevalence ratio, risk ratio, odds ratio, incidence rate ratio
what does relative risk measure
how much more likely it is that an exposed individual will develop the outcome compared with the unexposed individual
what studies do you use to calculate risk ratio?
ecological, cohort or intervention