Lecture 20 Case control study

Question 1

Analytic studies types

Answer 1

observational

• cross sectional
• ecological
• cohort
• case control

intervention
- randomised controlled trial

Question 2

Limitations of cohort studies

Answer 2

Can be inefficient with rare/slow to develop outcomes

Can be inefficient with transient/acute exposures

Question 3

case control study

Answer 3

Designed for rare/slow to develop outcomes

Can efficiently examine acute or transient exposures

Question 4

Cohort vs case-control studies

Answer 4

cohort
- Ascertain exposure status,
then find out outcome(s)

case-control
- Ascertain outcome status, then find out exposure(s)

Question 5

Randomised controlled trials

Answer 5

Intervene in individual studies

drug trial

Question 6

Cohort study Can be inefficient with rare/slow to develop outcomes

Answer 6

Follow up long time
Incidence low
Cancers

Question 7

cohort study Can be inefficient with transient/acute exposures

Answer 7

Brief increase in risk of outcome

Injuries

Question 8

in case control study

greater odds of exposure (cases)

Answer 8

exposure more common in cases

potential risk factors

Question 9

In case control study

greater odds of exposure (controls)

Answer 9

Exposure more common in controls

Potential protective factor

Question 10

Logic of case-control studies

If controls good representation of source population

Answer 10

Ratio of odds of exposure may indicate association between exposure and outcome

Question 11

In case controlled study the case is

Answer 11

Defined by only one outcome (cohort has multiples)

Define outcome clearly

• Dont want people who like they have outcome but dont
• Introduce bias
• Eg death from alzheimer’s vs dementia

Comprehensive case finding

Question 12

Prevalent cases

Answer 12

Association of developing disease (outcome) and association of how long have it

Survival or lack of cure with it

Question 13

in case control

Relevant exposure period

Answer 13

Just before the outcome

Question 14

in case control

Irrelevant exposure period

Answer 14

Right at the beginning of the follow up

Won’t affect the risk

Question 15

Case controlled study allows

Answer 15

Look at relevant exposure period

Because we get people who’ve had the outcome and look before it

Question 16

Odds of exposure in cases

Answer 16

People with exposure / people without exposure

Question 17

Odds of people in control

Answer 17

People in controls with exposure / people in controls without exposure

Question 18

Measure of association

Answer 18

Odds ratio (not incidences)

How many times as likely cases are to have the exposure compared to controls

Question 19

What is the null value for the odds ratio?

Answer 19

Null value for ratios = 1

Question 20

calculate odds ratio

Answer 20

Odds of exposure in cases / odds of exposures in controls

Question 21

Relative risks and odds ratios

Answer 21

Interpret exactly same way

Question 22

OR when disease rare

Answer 22

Will approximate RR

Question 23

case control study

Can use RR interpretation

Answer 23

Exposed group
Value
Outcome
Comparison group

Question 24

case control

Null value 1 below

Answer 24

Protective factor

Question 25

case control Above null value 1

Answer 25

Risk factor

Question 26

What happens in a case controlled study

Answer 26

1. Identify Source Population 2. Identify people with outcome (cases) 3. Sample people without outcome from same population (controls) 4. Measure exposure prior to outcome in cases and controls 5. Compare odds of exposure to calculate measure of association (odds ratio)

Question 27

Controls don’t have outcome | So when do you measure exposure for controls?

Answer 27

Can't measure exposure just prior to outcome because by definition dont have it Index dates

Question 28

Index dates

Answer 28

Point define when the controls had the outcome

Question 29

control case Why use odds?

Answer 29

Can’t calculate prevalence or incidence of outcome Have selected number of people in study with and without outcome

Question 30

Measure of association for case-control studies:

Answer 30

odds ratios

Question 31

Interpreting the odds ratio | Different interpretation to the RR

Answer 31

People with outcome are x times as likely to have the exposure than people without the outcome

Question 32

interpret RR

Answer 32

People with exposure are x times as likely to develop the outcome as people without the exposure

Question 33

When disease is rare,

Answer 33

OR approximates the RR (rare disease assumption) Can interpret OR just like RR Use RR interpretation in this course

Question 34

case control interpretation

Answer 34

Expose group value (x times as likely) outcome comparison group

Question 35

what are Index dates?

Answer 35

Pretend control had event on same date as case (or close to it)

Question 36

Case selection: | important points

Answer 36

Defined by outcome, so only one Clear outcome definition and identification Comprehensive case finding E.g. Death from Alzheimer's vs death from dementia. Routine data collections Medical records

Question 37

Case selection: | prevalent cases

Answer 37

Usually try to identify incident cases Sometimes just recruit people with the outcome (prevalent cases)

Question 38

Control selection: | important points

Answer 38

Need to represent the exposure distribution in source population - Good representation of the odds of exposure in source population Must be capable of becoming a case Often select multiple controls per case for statistical power

Question 39

Exposure measurement: | important points

Answer 39

Need to measure exposure period before outcome

Question 40

Exposure measurement: | important points

Answer 40

Differential recall | Exposure measurement must be comparable

Question 41

Differential recall

Answer 41

Cases trying to work out what made them sick | Outcome may affect recall ability

Question 42

Exposure measurement must be comparable

Answer 42

Dead cases vs alive controls | Interviewers may act differently for cases and controls

Question 43

Cohort vs case-control studies

Answer 43

Cohort | - Ascertain exposure status, then find out outcome(s)

Question 44

case-control studies

Answer 44

case-control studies | - Ascertain outcome status, then find out exposure(s)

Question 45

Case-control studies | strengths

Answer 45

- Rare outcome, transient exposures - Multiple exposures - Temporal sequencing - Often comparatively quick and inexpensive

Question 46

Case-control studies | Limitations

Answer 46

- Usually can only study one outcome - Difficult to select appropriate control group - Can be susceptible to selection and recall bias

Question 47

General population

Answer 47

Ideal | - Most likely to represent the odds of exposure in source population

Question 48

Hospital controls

Answer 48

- Looking at serious outcome tends to lead to hospital treatment - Convenient (know where they're gonna be)