Flashcards in REVISION GUIDE: CRITICAL NUMBERS Deck (34)

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1

## what is prevalence probability?

### probability of having a disease at a given point in time

2

## what is incidence probability?

### probability of getting a disease during a specified point in time

3

## what is the incidence rate?

### average rate of change over time

4

## what is the hazard rate?

### the instantaneous rate of change

5

## what are the 2 equations that can be used to calculate probability?

###
p = no. favourable outcomes / all possible outcomes

p = no. of cases / total population

6

## what is conditional probability?

### the probability that something will happen, given an event has already happened

7

## how would you calculate odds ratio?

### odds = cases / non-cases

8

## what are the ranges for probability and odds ratios?

###
probability = 0-1

odds = 0 to infinity

- more than 1 = positive association

- 1 = no association

9

## what are the problems with prevalence probability?

###
- length-time bias - conditions with a longer duration are more likely to be captured in prevalence

- a difference in prevalence can simply indicate shorter/longer average duration of disease or a different treatment success

10

## what are the problems with incidence probability?

###
- issues with competing risk - ie death due to another cause

- ignores time to event

11

## how can you avoid problems with incidence probability?

### use incidence rate instead

12

##
how would you calculate absolute risk difference?

drug A = 0.6 drug B = 0.2

###
absolute risk difference = A - B

= 0.6 - 0.2

= 0.4 = 40%

13

##
how would you calculate risk ratio?

drug A = 0.6 drug B = 0.2

###
risk ratio = A / B

= 0.6 / 0.2

= 3

14

##
how would you calculate relative risk difference?

drug A = 0.6 drug B = 0.2

###
relative risk difference = (A - B / B) x 100

= (0.6 - 0.2 / 0.2) x 100

= (0.4 / 0.2) x 100

= 2 x 100 = 200%

15

## what is relative risk?

###
exaggerates risk

they do not indicate a baseline

16

## what is risk difference?

###
gives an absolute measure of the association of exposure on disease occurrence

gives a clearer sense of public health impact

17

## what is risk ratio?

###
gives a relative measure

give a clearer sense of strength of effect

18

## what is the definition of number of people needed to treat/harm?

###
it is the number of people you need to treat in order to prevent one outcome

it indicates the potential benefit of a clinical intervention

19

## how do you calculate the number of people needed to treat/harm?

### NNT = 1 / Risk Difference

20

## what is a cohort study?

###
- forward prospective study

- population-based

- onset of study -> follow-up

21

## what are the strengths of a cohort study?

###
- useful for demonstrating causal effects

- multiple diseases can be studies

- multiple exposures can be studied

22

## what are the weaknesses of cohort studies?

###
- expensive and time consuming

- not suitable to rare diseases

- need to deal with confounding factors

23

## what are cross-sectional studies?

###
- investigates what is happening at a certain point in time

- outcome and exposures are measured simultaneously

24

## what are the strengths of cross-sectional studies?

###
- relatively fast and inexpensive

- rapid feedback on current events

- multiple outcomes and exposures can be studied

25

## what are the weaknesses of cross-sectional studies?

###
- not suitable for rare diseases

- very limited potential to establish disease aetiology

- can be affected by selection bias and confounding factors

26

## what are case-control studies?

###
- retrospective - past exposures are measured

- sample is taken based on a disease

27

## what are the strengths of case-control studies?

###
- quick and inexpensive

- suitable for rare diseases

- multiple exposures can be studied

- suitable for transient exposures eg outbreaks

28

## what are the weaknesses of case-control studies?

###
- only a single disease can be studied

- not suitable for rare exposures

- exposure data needed before start of study

- can be affected by selection bias and confounding factors

29

## what are randomised control trials (RCTs)?

###
- have an entry criteria and split randomly into groups

- this prevents confounding problems

- trials can be double blinded

- prevents bias

30