Measures of Occurrence Flashcards
what is incidence
number of new cases occurring as a percentage or rate - must specify time units and population
what is prevalence
proportion of existing cases
what is point prevalence
proportion of existing cases in a population at a single point of time - only measure obtainable rom cross sectional surveys
what is ascertainment
you have counted all the individuals of interest in your study, poor record keeping may mean you don’t have all the information
what is mortality and case fatality
number dying from a disease in a population
CF - number dying from a disease out of those with the disease in a population
what are standardising rates
used to enable comparison between areas with different population structures
what is probability
between 1 and 0 chance of getting some outcome from some event - may be a percentage risk
what is relative risk
the ratio of the risk of an outcome in one group (eg exposed) compared to the risk of the outcome in another group (unexposed group) eg treatment group vs control group
RR = risk in exposed group / risk in unexposed group
what does it mean when RR is less than 1
the risk in exposed group is less than the risk in unexposed group
what does it mean when RR is greater than 1
the risk in the exposed group is higher than the risk in the unexposed group
how do you find the percentage reduction for RR less than 1
subtract RR from 1 ie (1 - RR) x 100
what are odds
the probability of an event occurring divided by the probability of the event not occurring
divide the number of times that the event happens by the number of times it did not happen
what are odds ratios
odds of the event in one group (eg exposed to a drug) divided by the odds in another group not exposed
what is the difference in odds ratios (OR’s) when they are greater or less than 1
less than 1 - the odds in exposed group in less than the odds in the unexposed group
more than 1 - the odds in the exposed group is higher than the odds in the unexposed group
what is the 95% CI used for in OR’s
95% confidence interval used to estimate the precision of the OR
ie large CI - low precision of the OR
small CI - higher precision of the OR
what is implied if the CI crosses 1 ie between 0.9 and 2.5
implies no statistical significant difference between exposed and unexposed groups
when would you use RR compared to OR
RR = randomised control trials, cohort studies and prospective sudtisues - where the subjects are initially chosen y their exposure
OR = case control and retrospective studies
what is risk difference
difference in risk of outcome between
eg the exposed and unexposed groups or treatment group and control
why is the RD (risk difference important)
meaningful for understanding if findings of a study are worthy of incorporating into practice
the magnitude of the risk difference may not warrant changing practice
what is the NNT
number needed to treat
1/risk difference
Not is the number of people who need to receive the treatment in order to achieve the required outcome in one of them
