Measures of Occurrence

Question 1

what is incidence

Answer 1

number of new cases occurring as a percentage or rate - must specify time units and population

Question 2

what is prevalence

Answer 2

proportion of existing cases

Question 3

what is point prevalence

Answer 3

proportion of existing cases in a population at a single point of time - only measure obtainable rom cross sectional surveys

Question 4

what is ascertainment

Answer 4

you have counted all the individuals of interest in your study, poor record keeping may mean you don’t have all the information

Question 5

what is mortality and case fatality

Answer 5

number dying from a disease in a population

CF - number dying from a disease out of those with the disease in a population

Question 6

what are standardising rates

Answer 6

used to enable comparison between areas with different population structures

Question 7

what is probability

Answer 7

between 1 and 0 chance of getting some outcome from some event - may be a percentage risk

Question 8

what is relative risk

Answer 8

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

Question 9

what does it mean when RR is less than 1

Answer 9

the risk in exposed group is less than the risk in unexposed group

Question 10

what does it mean when RR is greater than 1

Answer 10

the risk in the exposed group is higher than the risk in the unexposed group

Question 11

how do you find the percentage reduction for RR less than 1

Answer 11

subtract RR from 1 ie (1 - RR) x 100

Question 12

what are odds

Answer 12

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

Question 13

what are odds ratios

Answer 13

odds of the event in one group (eg exposed to a drug) divided by the odds in another group not exposed

Question 14

what is the difference in odds ratios (OR’s) when they are greater or less than 1

Answer 14

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

Question 15

what is the 95% CI used for in OR’s

Answer 15

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

Question 16

what is implied if the CI crosses 1 ie between 0.9 and 2.5

Answer 16

implies no statistical significant difference between exposed and unexposed groups

Question 17

when would you use RR compared to OR

Answer 17

RR = randomised control trials, cohort studies and prospective sudtisues - where the subjects are initially chosen y their exposure

OR = case control and retrospective studies

Question 18

what is risk difference

Answer 18

difference in risk of outcome between

eg the exposed and unexposed groups or treatment group and control

Question 19

why is the RD (risk difference important)

Answer 19

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

Question 20

what is the NNT

Answer 20

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