Evidence Based Medicine

Question 1

What is a confounding factor?

Answer 1

Confounder is associated with exposure but not a consequence of exposure. Also associated with outcome independent of exposure

Question 2

Types of selection bias

Answer 2

Recruitment of unrepresentative sample population

• Sampling bias
• Response bias

Question 3

Attrition bias

Answer 3

Number of dropouts differ significantly in different arms

Reduced with intention to treat analysis

Question 4

What is a cohort study?

Answer 4

• observational study
• group of ppl followed over time to establish incidence of a condition
• identify risk factors for certain condition
• prospective or retrospective

Question 5

What are the advantages of a cohort study?

Answer 5

• demonstrate an appropriate temporal relationship between exposure and outcome
• estimate incidence rates in exposed and non-exposed groups
• can assess multiple outcomes
• can study uncommon exposures
• insight into latent and incubation periods for communicable/non-communicable diseases

Question 6

What are the disadvantages of cohort study?

Answer 6

• require large sample size
• long follow-up periods
• frequent re-evaluation of exposure
• portion of cohort lost to FU
• outcome misdiagnosed/misallocated if diagnostic criteria changed over time
• outcome assessment vulnerable to diagnostic suspicion bias

Question 7

What are the disadvantages of cohort study?

Answer 7

• require large sample size
• long follow-up periods
• frequent re-evaluation of exposure
• portion of cohort lost to FU
• outcome misdiagnosed/misallocated if diagnostic criteria changed over time
• outcome assessment vulnerable to diagnostic suspicion bias

Question 8

What is a CONSORT statement?

Answer 8

Consolidated Standard of Reporting Trials

set of recommendations for papers reporting the results of randomised clinical trials.

Question 9

What guidance does the statement provide?

Answer 9

• 25 item check list
• template participant flow diagram
• guidance for each section of paper (abstract, intro, methods, results, discussion, other information

Question 10

What is a PRISMA statement?

Answer 10

Preferred Reporting Items for Systematic Review and Meta-Analysis

Set of evidence-based guidelines intended to improve quality of reports for systemic reviews

Question 11

What guidance does PRISMA provide?

Answer 11

• 27 item check list
• Template flow diagram diagram to describe the identification and inclusion/exclusion of trials/studies in the review.

Question 12

What is impact factor?

Answer 12

Measure of a particular journal’s relative importance in its field

Question 13

How do you calculate impact factor?

Answer 13

Number of citations in a given year / number of articles published

Question 14

Any other bibliometrics similar to impact factor?

Answer 14

• Immediacy index
• Cited half-life

Question 15

What is h-index?

Answer 15

a simple metric that provides a guide to an individual researcher’s impact on the field

It is the rank of the paper at which position in ranking equals citaiton count

Question 16

Outline 1 a, b, c of oxford system for studies of intervention

Answer 16

1a: systematic reviews or RCT
1b: high quality RCTs with narrow CI
1c: all or nothing RCTs

Question 17

Outline 2 a, b, c of oxford system for studies of intervention

Answer 17

2a: systematic review of cohort studies
2b: individual cohort study or low quality RCT
2c: Ecology studies

Question 18

Outline 3 a, b of oxford system for studies of intervention

Answer 18

3a: systematic reviews of case control studies
3b: individual case-control studies

Question 19

Outline level 4 and 5of oxford system for studies of intervention

Answer 19

4: case series, poor quality cohort or case control study
5: expert opinion

Question 20

Alternative systems to classify level of evidence

Answer 20

NICE

SIGN (Scottish intercollegiate guideline network)

Question 21

What is a meta-analysis?

Answer 21

Use of statistical methods to combine results of individual studies

Question 22

What is systematic review?

Answer 22

Literature review conducted according to strict protocol to identify studies relevant to question under consideration

Question 23

What elements comprise a good systematic review/ meta-analysis?

Answer 23

• well-constructed clinical question in PICO terms
• objective inclusion and exclusion criteria
• Key outcomes defined
• Structured search strategy
• Efforts to obtain missing data to avoid publication bias
• Data should be extracted by two or more observers independently using a prepared extraction pro forma

Question 24

What are commonly used pooled outcome measures in meta-analyses?

Answer 24

• Weighted mean difference: continuous variables
• Pooled odds ratio: categorical variables in interventional studies
• Risk ratio/ relative risk: categorical variables in epidemiological studies

Question 24

What are commonly used pooled outcome measures in meta-analyses?

Answer 24

* Weighted mean difference: continuous variables * Pooled odds ratio: categorical variables in interventional studies * Risk ratio/ relative risk: categorical variables in epidemiological studies

Question 25

What are the advantages of meta-analysis compared to traditional literature reviews?

Answer 25

* evaluation for bias * greater statistical power to detect effect over single study * can control between study variation * more reliable synthesis of literature, reduce info overload * less likely influenced by local factors

Question 26

Disadvantages to Meta-analysis

Answer 26

* vulnerable to publication bias * pooling of several small studies may not predict results of large study * unreliable results if source studies poorly designed

Question 27

What is publication bias?

Answer 27

Studies with positive results more likely to be published than those with negative results

Question 28

How is publication bias detected?

Answer 28

Using a funnel plot

Question 29

Describe a funnel plot

Answer 29

X-axis: sample size Y-axis: effect size (standard error of effect estimate)

Question 30

How to identify publication bias from funnel plot?

Answer 30

asymmetric funnel

Question 31

Possible causes of asymmetrical funnel plot

Answer 31

* Publication bias * Heterogeneity * Poor methodology leading to inflated effects in smaller studies

Question 32

What is a forest plot

Answer 32

graphical representation of data from a meta-analysis

Question 33

Describe the format of a forest plot

Answer 33

Box: effect estimates form single studies Diamond: pooled result Horizontal line: confidence interval Vertical line: line of no effect

Question 34

What is stratification?

Answer 34

a method of ensuring an equal distribution of key confounding factors between arms of a randomised trial