Cross-sectional and Ecological Studies

Question 1

What are the two types of epidemiology?

Answer 1

Descriptive epidemiology:
- person, place, time
- oberservational (who, what, when - when the study took place or a season or relation to specific time of an event, where - geographical location of study)

Analytic epidemiology:
- Associations: exposures and outcomes
- causation
- observational or intervention studies
- why

Question 2

What are cross-sectional studies and what do they measure?

Answer 2

Measures exposures and/or outcomes at one point in time (a particular date, event or period of time)
They measure prevalence (the proportion of a defined population who have a disease at a point in time)
- Affected by incidence and duration
- Duration could be shorter due to people recovering quickly or dying quickly once they get the illness

Question 3

What can cross sectional studies be used for?

Answer 3

To compare, the generate hypotheses, to plan (health services), to describe prevalence

Question 4

Describe the GATE frame

Answer 4

Graphic Appraisal Tool for Epidemiological studies

Triangle:
- whole triangle represents the population, with the shaded orange part being the sample (those who meet the study criteria and enter the study)

Circle:
- top half = exposed group
- bottom half = comparison group

Square:
- top left = positive outcome for exposed group
- top right = negative outcome for exposed group
- bottom left = positive outcome for comparison group
- bottom right = negative outcome for comparison group

negative being did not get the illness
positive being had/got the illness

Time: on the bottom, can be pointing along or upwards depending on how the duration of the study was measured (if they are measured at the same time it will be pointing up)

Can calculate prevalence using the equation and the square with the outcomes

Question 5

What is a prevalence ration and how do you calculate it?

Answer 5

It is used to form a hypothesis and is generated by dividing the prevalence of the exposed group getting the outcome over the prevalence of the comparison group getting the outcome.

Question 6

What are the limitations of cross sectional studies?

Answer 6

• When exposure and outcome are assessed at the same time it is hard to tell what cam first (temporal sequencing)
• Measures prevalence not incidence
• Not good for studying rare outcomes or exposures
• not good for assessing variable and transient exposures or outcomes

Question 7

Why do we do cross-sectional studies?

Answer 7

• can assess multiple exposures and outcomes
• depends on your research question
  
  • prevalence, and distribution of prevalence in the population
  • stable exposures and outcomes
  • hypothesis generating
• can be less expensive than some other study designs
• relatively quick

Question 8

What are ecological studies and what are they used for?

Answer 8

They compare exposures and outcomes across GROUPS not individuals
Used for:
- to compare between populations
- to access population level factors
- to consider hypotheses (NOT to prove hypothesis)

Question 9

What are the limitations of ecological studies?

Answer 9

• Ecological fallacy (can’t describe the characteristics of the group to the individuals in the group)
• cannot control confounding
• cannot show causation

Question 10

Why do ecological studies?

Answer 10

Depends on the research question
- population level exposures
- consideration of hypotheses
Data is often routinely collected
- may be relatively easy to do
- may be relatively inexpensive