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Y2 RESS > Measures of Association > Flashcards

Measures of Association Flashcards

1
Q

What types of studies are pertinent to measures of association?

A
  1. RCT
  2. case-control
  3. cohort studies
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2
Q

Why are outcomes measured in a variety of ways?

A

there are different data types

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3
Q

What are the 3 types of outcome measures?

A
  1. continuous objective e.g. blood glucose level
  2. continuous subjective measures e.g. pain levels
  3. categorical e.g. positive or negative disease test
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4
Q

In which 3 ways can the effects of new treatments with dichotomous outcomes be expressed?

A
  1. relative risk
  2. risk difference
  3. odds ratio
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5
Q

What do the measures (relative risk, risk difference, odds ratio) show?

A

they show the direction and magnitude of association between 2 variables

e.g. treatment and outcome

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6
Q

What is the definition of probability?

A

a measure of the chance of getting an outcome of interest from an event

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7
Q

Between which 2 numbers do probabilities always lie between?

A

probabilities always lie between 0 and 1

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8
Q

What does a probability of 0 and 1 mean?

A

a probability of 1 means that the outcome is certain

a probability of 0 means that the outcome cannot happen

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9
Q

In what other way can probabilities be measured?

A

as percentage chances

this is the probability x 100 so it ranges from 0-100%

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10
Q

What is risk a way of expressing?

A

risk is a way of expressing treatment effects

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11
Q

What is the definition of risk?

A

the probability of an outcome or event occurring

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12
Q

How is absolute risk calculated?

A

by dividing the number of events by the number of people at risk

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13
Q

What would the absolute risk be if 1 in every 100 patients suffers a side effect?

A

absolute risk = 1/100 = 0.01

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14
Q

What is meant by relative risk?

A

the ratio of the risk of an outcome in one group (e.g. exposed) compared to the risk of the outcome in another group (e.g. unexposed)

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15
Q

What is the calculation for relative risk?

A

risk in exposed/treatment group

/

risk in unexposed/control group

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16
Q

What does it mean when relative risk = 1?

A

the risk in the exposed group is the same as the risk in the unexposed group

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17
Q

What does it mean when the relative risk is < 1?

A

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

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18
Q

What does it mean when relative risk > 1?

A

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

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19
Q

For relative risks <1, how is the percentage reduction calculated?

A

subtracting the relative risk from 1

(1-RR) x 100

20
Q

Why is the size of the observed relative risk important?

A

Large values indicate a strong association

21
Q

What should relative risk always be interpreted in association with?

A

confidence interval

22
Q

What is meant by ‘odds’?

A

the probability of an event occurring divided by the probability of the event not occurring

23
Q

How are odds calculated?

A

by dividing the number of times an event happens by the number of times it does not happen

24
Q

If 1 in 100 patients suffer a side effect, what are the odds?

A

1/99 = 0.0101

25
Q

What must be calculated before odds can be used to interpret results?

A

an odds ratio

26
Q

What is an odds ratio?

A

the odds of the event in one group (e.g. exposed) divided by the odds in another group (e.g. non exposed)

27
Q

What is the calculation for odds ratio?

A

odds in exposed group

/

odds in exposed group

28
Q

What does it mean when the odds ratio = 1?

A

the odds in the exposed group is the same as the odds in the unexposed group

29
Q

What does it mean when the odds ratio < 1?

A

the odds in exposed group are less than the odds in the unexposed group

30
Q

What does it mean when the odds ratio > 1?

A

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

31
Q

What values can an odds ratio take?

A

any value from 0 to infinity

32
Q

What is the 95% confidence interval used for when reported with RR or OR?

A

It is used to estimate the precision of the OR or RR

33
Q

What does a large and small confidence interval show when presented with OR or RR?

A

Low CI shows low precision of the OR, RR

Small CI shows high precision of the OR, RR

34
Q

What does it mean if the confidence interval crosses 1?

A

this implies no statistical significant difference between exposed and unexposed groups

35
Q

When is RR usually used in a study?

A

In studies where the subjects are initially chosen by their exposure and followed up over time to see their outcome

e.g. cohort studies, randomised controlled trials

36
Q

When should relative risks not be used?

A

case-control or other retrospective studies

37
Q

When should odd ratios be used in a study?

A

in case-control or other retrospective studies

they are commonly reported in logistic regression analysis

38
Q

If a disease/outcome is rare, what is the difference between RR and OR?

A

The RR will be similar to OR

39
Q

What is meant by risk difference?

A

the difference in risk of outcome between the exposed/treatment group and the unexposed/control group

40
Q

Why is risk difference used in clinical practice?

A

it is meaningful for understanding if findings of a study are worthy of incorporating into practice

41
Q

Does risk difference always warrant changing clinical practice?

A

the magnitude of the risk difference may not warrant changing practice

42
Q

What is another way of evaluating clinical importance of treatment?

A

number needed to treat (NNT)

43
Q

What is the calculation for NNT?

A

1/risk difference

44
Q

When is NNT usually used?

A

in studies comparing an intervention and standard treatment

45
Q

What is Akobeng’s definition of NNT?

A

the number of people who need to receive the treatment in order to achieve the required outcome in one of them

Y2 RESS (9 decks)

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