Lecture 17: Cross sectional and Ecological Studies

Question 1

What is a cross-sectional study?

Answer 1

Measures exposures and/or outcomes at one point in time

Question 2

What do we mean by a ‘point in time?’

Answer 2

Particular date
Specific event
Specific period in time

Question 3

What do cross-sectional studies
measure?

Answer 3

Prevalence: the proportion of a defined population who have a
disease at a point in time

Question 4

Prevalence - calculation

Answer 4

Prevalence = number of people with disease at a given point in time/
Total number of people in the population at that point in time

Question 5

What are cross-sectional studies used to measure?

Answer 5

describe prevalence
compare prevalence
generate hypotheses
plan (e.g. health service delivery)

Question 6

Limitations of cross-sectional studies (1)

Answer 6

Temporal sequencing

Question 7

Limitations of cross-sectional studies (2)

Answer 7

Measures prevalence not incidence

Question 8

Limitations of cross-sectional studies (3)

Answer 8

Not good for studying rare outcomes or exposures

Question 9

Limitations of cross-sectional studies (4)

Answer 9

Not good for assessing variable and transient exposures or
outcomes

Question 10

So why do cross-sectional studies?

Answer 10

• Can assess multiple exposures and outcomes
• Can be less expensive than some other study designs
• Relatively quick

Question 11

Ecological studies

Answer 11

Compare exposures and outcomes across GROUPS not individuals

Question 12

What are ecological studies used for?

Answer 12

• To compare between populations
• To assess population level factors
• To consider hypotheses

Question 13

Limitations of ecological studies (1)

Answer 13

Ecological fallacy
“derives from ascribing to individuals the characteristics of the
group.”

Question 14

Limitations of ecological studies (2)

Answer 14

Cannot control for confounding or causation