Case Control Studies

Question 1

Describe how a case control study works

Answer 1

Case group (people who have the disease) 
- measure the exposure in the exposed group and unexposed group 

Control group (people without the disease) 
- measured exposure and unexposure

Question 2

What do case control studies do

Answer 2

Identify individuals with a disease (cases)

Identify `similar’ individuals without the disease (controls)

Determine previous exposure

Relate information on exposure to disease

Question 3

What are the types of observational studies

Answer 3

• Cohort
• Case control studies
• Cross sectional studies

Question 4

What are the types of interventional studies

Answer 4

• Randomised controlled studies

Question 5

How do you source cases in a case control study

Answer 5

Source of cases

• Want representative of all people with the disease of interest
• Could be incident cases from disease registry
• Could be hospital based recruitment but this may give a biased sample

Question 6

How do you source controls in a case controls study

Answer 6

Source of controls

• same population as cases
• If hospital based ensure the reason for being in hospital not also related to exposure of interest
• Usually more than one control per case

Question 7

Why do we use matching

Answer 7

Why

• Know about potential confounders eg age/gender
• Not interested in examining the association of these confounders

Question 8

Give an example of how we can use matching

Answer 8

How

If using GP register then can select by age and sex

Question 9

Once matching variable is selected it is….

Answer 9

Once matching variable is selected it is not possible to analyse it as a risk factor later

Question 10

What are the biases in a case control study

Answer 10

Recall bias
- Cases may remember more than controls

Reverse causality
- Has disease caused changes in recent exposures

Selection of cases
- Are they representative of all people with the disease

Selection of controls

• Are they representative of all people without the disease
• Are they similar to the cases

Question 11

What happens in undermatching

Answer 11

• Cases and controls are not similar enough
• E.g. if they don’t match on age cases may be older and therefore more likely to smoke than controls – this would give the impression that smoking is related to the disease when it may not be

Question 12

What happens in over matching

Answer 12

• Cases and controls may be too similar
• E.g. if chose siblings may not differ in exposure of interest for example parental smoking
• This would give the impression that parental (passive) smoking is not related to the disease when it may be

Question 13

What is the difference between a case control study and a cohort study

Answer 13

Cohort study
- in a cohort study you identify your cohort and measure to see if they are exposed or not exposed over time and then see if they get the disease or not

Case control study

• this is when you identify those with the disease and measure to see if they were exposed or unexposed
• and identify those without the disease and measure to see if they were exposed or unexposed

Question 14

What is a nested case control study

Answer 14

• Case control study nested within a cohort study

Question 15

How does a nested case control study work

Answer 15

As an example

• identify a cohort and store the blood sample at the beginning
• then follow up and see who gets the disease and who doesn’t not
• everyone with the disease is called a case so you take the blood sample out of the freezer and look at the biomarkers
• everyone who does nto get the disease are controls
• for every case you might do 3 or 5 controls

Question 16

What is a nested case control study useful for

Answer 16

This is useful for biomarker studies where biomarker is expensive to measure

Question 17

What are the advantages and disadvantages of a nested case control study

Answer 17

Advantages
– Cheap, quick and easy
– Exposure before disease
– Can retrieve stored samples to look at new biomarkers
Disadvantages
- Need cohort study with stored serum samples

Question 18

What type of study out of case control or cohort study would you use to study a risk factor for a rare disease

Answer 18

• Use a case control study

- - if a disease is extremely rare a cohort study may have be impractically large to get enough people with the disease

Question 19

What type of study out of case control or cohort study would you use to study a rare exposure associated with the disease

Answer 19

• Cohort study – if an exposure is rare there may be too few people exposed in a case control study to be able to draw conclusions

Question 20

What type of study out of case control or cohort study would you use if you need to investigate a factor for a disease quickly

Answer 20

case control study

Question 21

What type of study is recall bias a problem in

Answer 21

case control studies and not cohort studies as they are remembering wether or not they can into contact with the exposure

Question 22

Why can you not use relative risk in a case control study

Answer 22

Can’t use relative risk as do not know risk of the disease (as you have started with cases with the disease)

Question 23

What do you you use instead of relative risk in a case control study

Answer 23

odds ratio

- Use odds ratio = odds exposure in cases/ odds exposure in controls

Question 24

How do you work out the odds ratio

Answer 24

odds ratio = odds exposure in cases/ odds exposure in controls

Question 25

if the disease is rare....

Answer 25

If the disease is RARE the odds ratio is a good estimate of the relative risk

Question 26

How do you work out the absolute excess risk

Answer 26

- Risk in exposed – risk in unexposed

Question 27

What is the problem with using absolute risk in a case control study

Answer 27

- you dont know the risk in a case control study

Question 28

Describe an example of how to work out absolute excess risk in a case control study

Answer 28

- Babies on there sides are twice as likely to die as babies on their back From ONS: Population risk of SIDs = 3.4 per 10,000 per year Therefore estimate of risk in babies put on their sides = 2 x 3.4 per 10,000 per year = 6.8 per 10,000 per year Therefore estimate of absolute excess risk in babies put on their sides = 6.8 per 10,000 per year – 3.4 per 10,000 per year = 3.4 per 10,000 per year

Question 29

What is attributable proportion

Answer 29

The same information allows you to calculate the proportion of disease in the exposed group that can be attributed to the exposure

Question 30

describe the formula for attributable proportion

Answer 30

Incidence in population attributable to exposure/ Incidence in population = p(relative risk - 1)/ 1+p(relative risk - 1) P = proportion exposed in population - work this out by using the control variable over the total number of controls

Question 31

How does a cross sectional study work

Answer 31

- Measure existing disease and current exposure | - Sample at 1 point in time without knowledge of disease or exposure

Question 32

What are the advantages and disadvantages of a cross sectional study

Answer 32

• Advantages – Can look at exposures that won’t change eg gender – Gives measures of prevalence and exposure rates • Disadvantages – No use for rare exposures or rare diseases – Not useful for assessing causality

Question 33

Does exposure to blue asbestos cause lung cancer - What study should you do

Answer 33

Cohort

Question 34

Is prevalence of back pain more common in men or women ?

Answer 34

Cross sectional

Question 35

What is the order of the risk of bias in studies

Answer 35

``` Likely - Cross section - case control - cohort - clinical studies Unlikely ```

Question 36

What is the order of the risk of confounding in studies

Answer 36

``` likely - Cross section - case control - cohort - clinical studies unlikely ```

Question 37

What is the order of strength of proof of causality in studies

Answer 37

``` likely - Cross section - case control - cohort - clinical studies unlikely ```

Question 38

What are the reasons for associations between an exposure and disease

Answer 38

* Bias * Reverse causality * Confounding * Incorrect analysis * Chance * Casual

Question 39

A case control study of patient’s with Chron’s disease showed that they were much more likely to have reported ever eating Corn flakes. what is the reason for association between the exposure and disease

Answer 39

Recall bias | Those with Chron’s disease more likely to remember everything they ate. Most people have eaten corn flakes at some time.

Question 40

Non-randomised studies have shown an increased risk of death in preventive (multivessel) percutaneous coronary intervention versus infarct artery only PCI in patients with STEMI what is the reason for association between the exposure and disease

Answer 40

Selection bias Patients who had preventive PCI more likely to be sicker Randomised controlled trials have shown the benefit of preventive PCI

Question 41

A case control study of stomach cancer patients found that they had a higher antacid usage in the 6 months prior to diagnosis than did the controls. what is the reason for association between the exposure and disease

Answer 41

Reverse Causality | People with undiagnosed stomach cancer tend to have the same symptoms for which antacids are taken

Question 42

In a cohort study heavy drinkers were found to be more likely to develop lung cancer. Does drinking cause lung cancer ? what is the reason for association between the exposure and disease

Answer 42

Confounding Heavy smokers are more likely to be heavy drinkers. Smoking causes lung cancer

Question 43

Randomised placebo controlled trial of candesartan in patients with chronic heart failure found it increased risk; RR = 1.6 (95% CI 1.1-2.2) what is the reason for association between the exposure and disease

Answer 43

Chance | This trial was the odd one out

Question 44

What is the criteria for causality

Answer 44

Bradford Hill Criteria for Causation

Question 45

Name the parts that make up the bradford hill criteria for causation

Answer 45

- Strength of association – how extreme is the relative risk – relative risk of 5 or 0.2 or lower or 0.2 or lower is a strong relative risk - Dose response - does increasing the dose increase the risk - Time sequence – exposure preceded the outcome - Consistency of findings – similar studies on different populations - Biological plausibility – e.g. tar is known to be carcinogeni - Coherence of the evidence – cross-sectional, cohort and case-control studies all find the same increased risk - Reversibility – the relative risk for ex cigarette smokers is between that of current smokers and non-smokers

Question 46

Could the association be due to

Answer 46

``` Could the association be due to ? Chance Bias Incorrect analysis Confounding Reverse causality Causality ```

Question 47

How do you work out if the association is casual

Answer 47

Bradford Hill Criteria for Causation