Evidence for population health I & II

Question 1

What are descriptive studies and what is their purpose?

Answer 1

Those that describe cases or a population. The purpose is to know how much disease a region or group has.

Question 2

What are the various ways in which descriptive/observational studies may be conducted?

Answer 2

• Anecdote/case series
• Cross-sectional survey
• Measuring incidence
• Ecological studies
• Case-control study
• Cohort study

Question 3

What are the pros of an anecdote/case series?

Answer 3

• Quick
• Easy to perform in a clinic
• Provides new previously unobserved conditions
• Provides new potential risk factors

Question 4

What are the cons of an anecdote/case series?

Answer 4

• Not scientific - not able to test a hypothesis
• Seriously affected by observer bias
• Difficult to make inference about disease cause

Question 5

What is a cross-sectional survey?

Answer 5

Count number of people with disease in short time period (hence cross-sectional) in a pre-defined population.

Question 6

What are the pros of cross-sectional surveys?

Answer 6

• Quick

- Good at estimating prevalence or burden

Question 7

What are the cons of cross-sectional surveys?

Answer 7

• Only represents that point in time.
• Cannot estimate incidence
• Sampling frame may lead to bias (e.g. missing workers)

Question 8

What does measuring incidence involve?

Answer 8

Recording new cases in a pre-specified population. Cases are registered via GP systems, death certificates, newspaper adverts etc…

Question 9

What is the counterfactual method?

Answer 9

Would the disease have happened at the same time in the same person if the factor of interest was not present?

Question 10

What are ‘ecological’ studies?

Answer 10

Studies where the unit of observation is a group for at least one variable.

Question 11

What are the pros of ecological studies?

Answer 11

• Less expensive
• Less prone to bias due to participation
• Easy to perform using routinely collected data
• Provides new hypotheses about the causes of a disease or condition
• Provides new potential risk factors

Question 12

What are the cons of ecological studies?

Answer 12

• Ecological fallacy - do population level measures hold for the individual?
• Assume average value of the risk factor applies to all individuals
• Assume average incidence applies to all the individuals in a population
• Data collection may vary e.g. coding systems

Question 13

What is a case-control study?

Answer 13

Compare two populations (exposed and non exposed) and look for differences in disease incidence.

Question 14

What are the pros of a case-control study?

Answer 14

• Good for rare disease and/or rare exposure.

- Fairly quick (no need for follow up)

Question 15

What are the cons of a case-control study?

Answer 15

• Prone to selection bias
• Especially prone to participation bias
• Finding a suitable control group can be difficult
• Difference in recall, leading to bias

Question 16

What is a cohort study?

Answer 16

Follow two cohorts (exposed and non-exposed) over a period of time and observe outcome. Exposed cohort must be at risk of but free from outcome at the start.

Question 17

What are the pros of a cohort study?

Answer 17

• Good for rare exposures
• Can look at multiple outcomes
• Reduces information bias, survivor bias
• Direct measurement of incidence

Question 18

What are the cons of cohort studies?

Answer 18

• Inefficient for rare diseases
• Expensive
• Retrospective is much quicker
• Loss to follow-up

Question 19

What is the counterfactual theory of causation?

Answer 19

The basic idea of counterfactual theories of causation is that the meaning of causal claims can be explained in terms of counterfactual conditionals of the form “If A had not occurred, C would not have occurred”

Question 20

What is a randomised controlled trial?

Answer 20

Randomise study population into treatment group and control group.

Question 21

What is the difference between a preventive RCT and a therapeutic RCT?

Answer 21

Use currently healthy people for preventive trial, currently sick people for therapeutic trial.

Question 22

What are the important features of an RCT?

Answer 22

Randomisation - to eliminate bias
Blinding - patient cannot be influenced by self, or clinician
Placebo-controlled - compare the drug you are testing against a placebo

Question 23

What are the pros of RCTs?

Answer 23

• Strongest evidence for causality
• If randomised, selection bias and confounding removed
• If blinded, less observer bias

Question 24

What are the cons of RCTs?

Answer 24

• Not real life
• High cost
• Inappropriate/unethical for many research questions

Question 25

What is the difference between a cohort study and a case-control study?

Answer 25

Case-control - divides people by exposure but looks into the past (ie identifies those who are healthy and those who aren’t and if there’s a link)
Cohort - divides well people by exposure and follows them into the future (to see which develop illness)

Question 26

What is disease mapping?

Answer 26

Disease maps are visual representations of intricate geographic data that provide a quick overview of said information. Mainly used for explanatory purposes, disease maps can be presented to survey high-risk areas and to help policy and resource allocation in said areas.

Question 27

What is a longitudinal study?

Answer 27

A longitudinal study is an observational research method in which data is gathered for the same subjects repeatedly over a period of time. E.g cohort studies and panel studies.