Observational Studies

Question 1

Why are observational studies done?

Answer 1

Hypothesis generating, RCTs are expensive, study of rare events, ethics

Question 2

What is the difference between cases and controls?

Answer 2

Cases have the disease of interest

Controls don’t have the disease and are usually age and sex matched with cases

Question 3

What is the selection bias in case-control studies?

Answer 3

Cases may not represent exposure distribution in all cases in the source population, control may not represent exposure distribution in those without disease

Question 4

How is selection bias minimised?

Answer 4

Minimised at design stage = ensure cases and controls are representative of and come from the same source population

Question 5

What is observer bias in case-control studies?

Answer 5

Knowledge of case/control status may influence data collection
Impact = identify more (spurious) risk factors in cases

Question 6

How is observer bias minimised?

Answer 6

Use of standardised objective instruments and blind researchers to case/control studies

Question 7

What is recall bias and how can it be minimised?

Answer 7

Cases and controls recall prior exposures differently

Minimise period of recall and measure exposure data objectively (medical notes, third party verification)

Question 8

What does survivor bias cause?

Answer 8

Will detect factors that increase survival among the diseased as risk factors for the disease

Question 9

What are the strengths of case-control studies?

Answer 9

Rapid and cheap, ideal for rare diseases/outcomes, useful for diseases with long latent periods, can simultaneously examine a large number of potential exposures

Question 10

What are the weaknesses of case-control studies?

Answer 10

Bias, temporal relationship can be difficult to establish, can’t compare incidence rates

Question 11

What are the uses of cohort studies?

Answer 11

Impact of infrequent/unusual exposure, multiple outcomes related to infrequent exposure, disease incidence, temporal sequence, how risk changes over time

Question 12

What questions must be considered when calculating risk?

Answer 12

What is the risk of outcomes for those exposed?

How do the risks compare?

Question 13

How is the risk of disease in exposed individuals calculated?

Answer 13

Disease present/(disease present + disease absent)

Question 14

How is the risk of disease in unexposed individuals calculated?

Answer 14

Disease present/(disease present + disease absent)

Question 15

What is the relative risk ratio?

Answer 15

Compares risk across exposure groups

Risk of disease in exposed/risk of disease in unexposed

Question 16

What does the relative risk ration mean?

Answer 16

RR >1 = exposure harmful
RR <1 = exposure protective
RR = 1 then exposure does nothing

Question 17

What determines the type of cohort study done?

Answer 17

The type of follow up needed

Question 18

What are some features of prospective cohort studies?

Answer 18

Time consuming, expensive, defined cohort (detailed exposure records with standardised measurement)

Question 19

What are some features of retrospective cohort studies?

Answer 19

Faster answers, don’t have to employ people to conduct regular follow-up (cheap), quality of records must be checked

Question 20

What are the advantages of cohort studies?

Answer 20

Temporality, no recall bias, can study multiple outcomes associated with rare exposures, measures incidence

Question 21

What are the disadvantages of cohort studies?

Answer 21

Requires large investment of time and money if prospective, requires large sample size, loss of follow-up bias, inefficient for rare diseases, uncontrolled confounding

Question 22

What biases are present in cohort studies?

Answer 22

Selection = initial cohort not representative of population, loss of follow up
Information = misclassification of exposure/outcome
Confounding = unmeasured day-to-day exposures

Question 23

What is the odds ratio in case-control studies?

Answer 23

Risk of disease given exposure, when disease studies is rare

Question 24

What is a cross-sectional study?

Answer 24

Carried out a single point in time, most frequently as a survey

Question 25

What are the benefits of cross-sectional studies?

Answer 25

Particularly good for estimating point prevalence, quick and cheap

Question 26

What is the disadvantage of cross-sectional studies?

Answer 26

Can’t establish causation

Question 27

What should be considered in a critical appraisal?

Answer 27

Where research came from, how the research was done, what research showed, how reliably it can be applied to clinical care

Question 28

What do CASP checklists contain?

Answer 28

Set of eight critical appraisal tools

Question 29

What is the null hypothesis?

Answer 29

Hypothesis that there is no significant difference between specified populations

Question 30

What does a p value <0.1 mean for the null hypothesis?

Answer 30

There is weak evidence against the null hypothesis

Question 31

What does a p value <0.05 mean for the null hypothesis?

Answer 31

There is strong evidence against the null hypothesis

Question 32

How is the null hypothesis affected if the p value is <0.01?

Answer 32

There is very strong evidence against the null hypothesis