week five

Question 1

What are the different types of observational studies

Answer 1

Descriptive: no comparison group

• Ecological
• Case study

Analytical: comparison group

• Cross sectional
• Cohort
• Case Control

Question 2

What is an observational study

Answer 2

A study which allows nature to chose its own course

Question 3

Ecological Studies

What are they

Answer 3

Examines populations, not individuals. ·

Compares disease frequencies between:

– Different populations during the same period of time, or
– Same population at different time periods

Question 4

advantages of ecological

Answer 4

Fast, easy, cheap

Hypothesis generating

Question 5

disadvantages of ecological

Answer 5

Highly susceptible to bias & confounding
Associations not causation
May confuse characteristics of group for characteristics of individuals

Question 6

case studies

Answer 6

· Detailed report by one or more health professionals on the profile of a single patient
· Good for adverse events

Question 7

case series

Answer 7

· Case series is a report on a series of patients with an outcome of interest – when case studies become common, there is a trend within them they get grouped together

Question 8

Advantages of case studies

Answer 8

identification of rare disease/exposures, which leads to further rigorous studies

Question 9

Disadvantages of case studies

Answer 9

Selection bias

Confounders

Question 10

cross sectional studies

Answer 10

· Units of analysis are individuals (individual level, ecological is population level)
· Measures:
– Prevalence of disease, and
– Presence or absence of exposure

Question 11

advantages of cross sectional

Answer 11

Cheap, easy
Estimate prevalence of outcome(s) of interest
Useful for understanding disease etiology and for the generation of hypotheses

Question 12

disadvantages of cross sectional

Answer 12

Interpret associations with caution
Difficult to make causal inference
Only a snapshot in time
The situation may provide differing results if another time-frame had been chosen

Question 13

Case control

Answer 13

· Simple method of investigating rare diseases
· Compares the occurrence of possible cause in cases and controls
· Data is collected at one point in time
· Exposures are collected at a previous point in time

They are retrospective

Question 14

advantages of case control

Answer 14

Good for rare outcomes and ‘long diseases’
– Thalidomide available in the 1950s (morning sickness in pregnant women)
– The late 1950s and early 1960s saw an increase in birth defects (case control study was used to identify that this drug caused these defects)
Relatively easy, fast and cheap (as everything has already happened)

Question 15

disadvantages of case control

Answer 15

Control selection crucial
– What to match?
Retrospective
– High probability of recall bias, selection bias and measurement error

Question 16

Cohort studies
What are they
How do they work

Answer 16

• The incidence of an outcome is compared between those exposed and those not exposed to a risk factor during the study time
• Provides good evidence of cause and effect relationship (as we measure the exposure before the outcome, forward in time)

· Participants are selected on the basis of exposure
· Participants are then followed up to identify whether or not they have the outcome

Question 17

how do we define a cohort

Answer 17

· Needs to be accessible
· Needs to be outcome free prior to exposure
· Needs to be some within the group who will potentially be exposed and thus may present with the outcome
· Need to be some within the group who don’t present with the outcome

Question 18

follow up of a cohort

Answer 18

Length needed varies depending on
– Objectives of the study
– Outcome(s) being assessed 
· The longer they go for, the potential for more information but have the problem of loss to follow-up (people dropping out) 

Question 19

Advantages of cohort

Answer 19

– Identify the natural history of a disorder

– Identify the temporal sequence between cause and outcome

– Good for rare exposures and common outcomes

– Very rigorous epidemiological design
- Also high on the evidence pyramid

Question 20

Disadvantages of cohort

Answer 20

– Selection bias 
– Exposed and non-exposed subjects differ on important predictors of outcome 
– Insufficient to study rare diseases 
– Loss to follow-up (exposure status) 
– Expensive and time consuming

Question 21

Retrospective cohort studies

How do they work?

Answer 21

• participants are identified on the basis of a previously recorded exposure

Measures exposure first but looks back in time for the direction of inquiry

Question 22

difference between case control and cohort studies

Answer 22

Case control: groups based on outcome

Cohort: groups are always divided up based on exposure which has already happened
Groups are based on exposure