Ecological and Cross Sectional Studies

Question 1

Types of studies

Answer 1

• Interventional
  o Clinical trials
  o Experimental study designs
• Observational

o Descriptive
 Ecological
 Migrant
 Cross sectional

o Analytical
 Cross-sectional
 Case-control
 Cohort

Question 2

What are ecological studies

Answer 2

• Compare occurrence of disease between different groups of individuals
  o Groups do not need to be countries
• OR compare the same population at different points in time

Question 3

What is ecological fallacy

Answer 3

• Mistaken belief that inferences can be made about individuals based on differences between the groups from which they belong

Question 4

Advantages ecological studies

Answer 4

o Use secondary data
o Simple and inexpensive
o Simple analysis
o Good for hypothesis generation

Question 5

Disadvantages ecological studies

Answer 5

o Alternative explanations for results cannot be tested
o No individual data so cannot adjust for other factors
o Ecological fallacy
o Cannot examine causality

Question 6

What is migrant study

Answer 6

• Study which exploits the fact that movement of people has offered to investigate gene-environment aetiology

Question 7

Advantages migrant study

Answer 7

o Simple and inexpensive
o Usually uses secondary data
o Good for hypothesis generation

Question 8

Disadvantages migrant study

Answer 8

o Difference in disease identification between country A and B -> not actually due to environment
o Healthy migrant effect -> people who move maybe due to economic reasons and are healthier, do not represent population leaving behind
o Migration does not mean acculturation

Question 9

What is cross sectional study

Answer 9

• Observational and descriptive person-based studies that uses data from a single point in time
• Used to measure prevalence of outcomes and exposures

Question 10

What is source population

Answer 10

o Population from which study subjects are selected

Question 11

Where can source population be from

Answer 11

o Can be many things: electoral role, phone book, NHS register etc
o NHS register best as 96% of population registered and is patient based data

Question 12

Forms of sampling methods

Answer 12

o Non-probability sampling (avoid as generates non-representative sample)
 Convenience sampling – man in the street
 Purposive sampling – predefined specific groups
 Snowball sampling – subject to subject

o Probability sampling
 Random sampling – eliminates sampling bias, each member of source population has equal probability of selection
 Assign random number to each patient, order from lowest to highest, use first n

Question 13

Considerations about outcome

Answer 13

• Must be clearly defined how it is recorded
• Consider how data is obtained
  o Questionnaire, lab/physical measurement etc

Question 14

Advantages cross section study

Answer 14

o Relatively quick and simple
o Generalisable findings (if good sampling methods)
o Can measure prevalence of many outcomes and exposures
o Can compare between different areas

Question 15

Disadvantages cross section study

Answer 15

o Cannot measure incidence or temporal relationships
o Susceptible to selection and information bias
o Not good for: rare diseases, diseases with high fatality rate or quick fatality