4) Observational Studies

Question 1

Why use an observational study

Answer 1

• Randomisation unethical - harmful exposure
• Randomisation impractical/impossible
• rare diseases adverse events
• LT outcomes
• Descriptive questions

Question 2

Types of observational studies

Answer 2

• cohort studies (Prospective or retrospective)
• case control studies
• cross sectional
  Ecological

Question 3

Prospective Cohort study

Answer 3

• how the present affects the future

- start with a pop, follow up and measure outcome

Question 4

Prospective cohort study strength

Answer 4

• highest level of evidence for causality after RCT
• outcome measured after exposure to risk has occured
• can look at multiple risk factor + disease
• examine incidence and natural history or clinical course of a disease

Question 5

Prospective cohort study weaknesses

Answer 5

• Expensive
• time consuming
• no randomisation - confounding

Question 6

Retrospective cohort study strength

Answer 6

• less costly or time consuming as used already collected data
• useful for diseases/outcomes that require a long time to develop
• can involve large administrative surveillance/datasets which can be useful for detecting rare outcomes/adverse events

Question 7

Prospective study weaknesses

Answer 7

• less control over what is collected
• bias
• may be inacurracies of data collection

Question 8

Selection bias

Answer 8

• distortion in results because of how the participants were selected
• control and comparison groups need to be as similar as possible
• lack of randomisation
• loss to follow yp

Question 9

How to reduce selection bias in cohort studies

Answer 9

• sample selection: restriction and matching
• Loss to follow up: exclude participants that are likely to drop out/ get contact detail/ maintain regular contact
• Analysis stage: adjust for confounders, measurement error
• Blinding

Question 10

Case control studies strength

Answer 10

• Good for rare diseases - require fewer participants

- good for diseases that take a long time to develop

Question 11

Case control studies weaknesses

Answer 11

• can’t estimate prevalence and incidence
• can only study one disease
• more susceptible to bias

Question 12

Biases

Answer 12

• selection bias
• measurement bias
• Recall bias

Question 13

Reducing measurement bias

Answer 13

• use previously collected data that was recorded before the outcome occured
• structured questionnaires/ interview scripts
• use of memory aids to aid recall of past exposures
• blinding

Question 14

Cross sectional studies

Answer 14

Observational studies conducted at one poitn in time withouit a follow up period
- risk factor and outcome are measured in selected sample at one point in time

Question 15

Two main types of cross sectional stidies

Answer 15

• Descriptive: how common something is

- Analytic: examines relationship between predictors and outcome

Question 16

Cross sectional studies strength

Answer 16

• Useful for establishing the prevalence of a disease - sample must be representative
• no follow up : not time consuming or expensive

Question 17

Cross sectional studies weaknesses

Answer 17

• difficiult to establish a cause-effect relationship
• hypothesis generation only
• prevalence- incidence bias

Question 18

Ecological studies

Answer 18

• looks at relationships between two things that are measured at a population level rather than an individual level

Question 19

Ecological fallacy

Answer 19

• relationships observed in groups also hold for individuals

- confounding factors

Question 20

Ecological studies strength

Answer 20

• often uses available data that doesnt have to be collected and doesnt need an ethics application
• cheap

Question 21

Ecological studies weaknesses

Answer 21

• possibility that many other things could be responsible for the observed difference
• hypothesis generation only