Critical Evaluation Of The Evidence

Question 1

What is a ‘cause’ of a disease?

Answer 1

A cause is an exposure that produces the disease

Question 2

Epidemiological studies usually assess risk factors instead of cause. Why?

Answer 2

• Few diseases arise from a single cause
• some exposures cause multiple diseases
• misclassification of disease and exposure status can occur
• latency (lead time) between exposure and disease

Question 3

What is a risk factor?

Answer 3

Exposure that increases risk of disease. Individuals exposed have a higher risk of disease

Question 4

A risk factor is not necessarily a cause - why?

Answer 4

• chance
• indirect causation
• reverse causation
• confounding

Question 5

What is indirect causation?

Answer 5

There can be an association between poverty and malnutrition but there is a mitigating factor in between: poverty causes starvation which in turn causes malnutrition

Question 6

What is reverse causation?

Answer 6

A certain drug may be associated with asthma mortality. However, the drug isn’t causing the mortality rates, the association appears because the drug is ONLY given to severe cases, who are therefore more likely to die

Question 7

What is a systematic review?

Answer 7

An overview of all primary studies that contain explicit statements of objectives, materials, and methods and has been conducted according to explicit and reproducible methodology

Question 8

What is a meta-analysis?

Answer 8

A study involving the quantitative pooling of data from two or more studies

They are subsets of systematic reviews

Question 9

What are the 5 steps to a systematic review?

Answer 9

1. Frame question
2. Identify relevant literature
3. Assess quality of literature
4. Summarise the evidence
5. Interpret the findings

Question 10

What three areas does quality usually cover in epidemiological studies?

Answer 10

1. Selection of the groups
2. Comparability of the groups
3. Ascertainment of exposure for case-control studies/outcome for cohort studies

Question 11

Give an example of a scale used for quality assessment in randomised controlled trials?

Answer 11

Jadad scale: gives a score between 3 and 5 and can be used to determine a cut-off for inclusion into a meta-analysis. It asks:

• did they use randomisation? (Was the method described and appropriate?)
• was the study double-blind?
• did they analyse everyone recruited? (Description of withdrawals/dropouts)

Question 12

Is a meta analysis a qualitative or quantitative method?

Answer 12

It is quantitative as it uses statistical methods to give combine results from identified studies and provides a pooled measure of risk

Question 13

What are the stages of a meta-analysis?

Answer 13

1. Calculate a measure of effect/risk and CI for each study
2. Display the results graphically (Forest plot)
3. Assess heterogeneity (variation between results of included studies)
4. Pool the data

Question 14

How do you look for heterogeneity?

Answer 14

Examine the forest plot - do the CIs of the study overlap with the pooled CIs?

Use I-squared: 0% = no heterogeneity, 50% means 50% of the variation in the meta-analysis is due to heterogeneity, 100% then all of the variation in the meta-analysis is due to heterogeneity

If heterogeneity I-squared = 85-100% then you shouldn’t do a meta analysis

If 0-40% use a fixed effect method
If 40-85% use a random effect method

Question 15

What is publication bias?

Answer 15

Positive studies are more likely to be published. This can be related to time or study size