Cross-sectional studies

Question 1

What is a cross sectional study?

Answer 1

A snapshot of the frequency of outcomes and exposures in a particular population at a specific time point.

Question 2

How do you begin a cross sectional study?

Answer 2

• Target population - Define a population of
  interest
• Selected sample- Invite a representative
  sample of individuals to take part
• Study sample - Obtain information on both
  the outcome of interest and exposure at a
  particular point in time on as many of those
  as possible

Question 3

What are the strengths of cross-sectional studies?

Answer 3

+ Good quality data as collected for the
purpose of research (not from routine data
collections)

+ Relatively quick and cheap to conduct (no
long periods of follow up)

+ Can study multiple exposures and outcomes

Question 4

What are the weaknesses of cross-sectional studies?

Answer 4

• Temporal sequence is unknown - cannot be
  sure exposure came before outcome
  (Reverse causality)
• Prone to bias (measurement)
• Confounding (could other factors related to
  exposure and outcome explain the
  association)

Question 5

What is a confounder?

Answer 5

A third variable that provides an alternative explanation for an observed association between exposure and outcome

Question 6

What is a risk ratio? ( also known as relative risk)

Answer 6

A measure of the risk of a certain event happening in one group compared to the risk of the same event happening in another group.

Estimated using regression models - outcome must be binary.

Question 7

What is the equation for risk ratio?

Answer 7

Risk Ratio (RR) = Risk in exposed group /
Risk in unexposed group

Question 8

What is the equation for risk in the exposed group?

Answer 8

= number with outcome / number at risk of
developing outcome

Question 9

What is the equation for risk in the unexposed group?

Answer 9

= number with outcome / number at risk of developing outcome

Question 10

What does a Risk Ratio value of 1 (null value) mean?

Answer 10

= No difference between exposure groups

Question 11

What does a Risk Ratio value of <1 mean?

Answer 11

= Exposure reduces the risk

Question 12

What does a Risk Ratio value of 0.5 mean?

Answer 12

= Exposure halves the risk

Question 13

What does a Risk Ratio value of 0.75 mean?

Answer 13

= Exposure reduces the risk by 25%

Question 14

What does a Risk Ratio value of >1 mean?

Answer 14

Exposure increases the risk

Question 15

What does a Risk Ratio value of 2 mean?

Answer 15

Exposure doubles the risk

Question 16

What does a Risk Ratio value of 10 mean?

Answer 16

Exposure increases the risk by 10 fold

Question 17

What is the definition of representativeness?

Answer 17

How representative the selected sample is of the target population