Chapter 3: Cohort Studies And Case Control Studies

Question 1

What is a cohort study?

Answer 1

Observational study design where the starting point is exposure to a particular risk factor

Question 2

Steps to a cohort study

Answer 2

• Determine exposed or not exposed
• This forms the cohort group
• Determine outcome

Question 3

What is the direction of inquiry in cohort studies

Answer 3

Direction of inquiry is forward in time

Question 4

Types of cohort studies

Answer 4

1. Prospective (current)
2. Retrospective (historical)
3. Ambidirectional

Question 5

Advantages of cohort studies

Answer 5

• Exposure is measured at the start prior to outcome – measurement of exposure of not biased by the presence of absence of an outcome
• Temporal sequence of events can be established
• Can provide information about time course of outcomes, natural history
• Can examine multiple outcomes
• Rare exposures can be study (not necessarily rare outcomes as you will need a large cohort)
• Can measure incidence rates directly

Question 6

Disadvantages of cohort studies

Answer 6

• Slow and usually expensive
• Inefficient for rare diseases
• Differential loss to follow-up can introduce bias
• Exposure status can change
• Can become historical

Question 7

How to conduct cohort study

Answer 7

1. Research question
2. Identify study population
3. Measure exposure
4. Follow up participants and determine outcomes
5. Data analysis
6. Interpret results
7. Assess possible sources of error

Question 8

Interpreting results in a cohort study

Answer 8

If RR = 1 - no association
If RR > 1 - risk in exposed greater than risk in not exposed
If RR < 1 - risk in exposed less than risk in not exposed

Question 9

What is risk ratio and when to use it?

Answer 9

• When follow up time is identical (or nearly) for all cohort members
• For both the risk ratio and the risk difference it would be best practice to calculate a 95% confidence interval (CI). Statistical construct that provides information about a range in which the true values of the risk estimate lies within a 95% probability

Question 10

Formula for risk ratio

Answer 10

Risk in exposed/ risk in not exposed

** to calculate risk - outcome(yes)/ total

Question 11

What is rate ratio and when to use it?

Answer 11

• When there is variation between cohort members in duration of follow-up time
• Person time can be measured as the total number of days, months, years of follow up. Unit will depend on time between exposure and outcome
• Best practice to calculate corresponding 95% CI ratio

Question 12

Formula for rate ratio

Answer 12

Rate ratio = rate in exposed/ rate in not exposed

*rate in exposed = outcome(yes)/ person-time in exposed
*rate in non-exposed = outcome(yes)/ person-time in not exposed

Question 13

What is personae time?

Answer 13

• Particularly important in a cohort study that is dynamic
• For an individual: accounts fro the total time period during which an individual could develop/ is at risk of the outcome of interest
• For a population/ cohort study: total person-time risk is the sum of all individuals person-time, the total time during which outcomes could occur and would be considered events in the population of interest
   

Question 14

What is a case control study?

Answer 14

Case control study is a type of observational study in which two existing groups differing in outcome are identified and compared on the basis of some supposed causal attribute

Question 15

Advantages of case control study

Answer 15

• Relatively quick and inexpensive, can have relatively small sample sizes
• Useful for studying rare outcomes
• Useful for studying diseases with long latent periods
• Can study multiple exposures

Question 16

Disadvantages of case-control studies

Answer 16

• Selection bias
• Information bias
• Possibility for reverse causality
• Not suitable for studying rare exposures (unless choose nests c-c study)
• Cannot estimate disease incidence or prevalence

Question 17

How to conduct a case control study

Answer 17

1. Research question and population
2. Selecting cases
3. Selecting controls
4. Matching appropriately
5. Matching exposure
6. Analysis of case control studies
7. Assess possible sources of error

Question 18

Analysis of case control studies

Answer 18

• Compares frequency of exposure among the cases with the frequency of exposure among the controls
• Cannot estimate incidence from a case-control study
• Compare the odds of exposure among cases and controls suing the odds ratio(OR)

Question 19

Formula for odds ratio

Answer 19

Odd of exposure (cases)/ odds of exposure (controls)

*odds of exposure (cases) = exposed cases/ not exposed cases
*odds of exposure (controls) = exposed controls/ not exposed controls

Question 20

Assessing possible sources of error in case control studies

Answer 20

• Chance, bias or confounding
• Selection bias in selecting cases and controls
• Information bias in obtaining exposure information from cases and controls (recall bias a particular concern in self-reporting)
• Misclassification bias (how exposure or outcome is identified) – is it non-random (non-differential)?
  o When proportion misclassified in the same in cases and controls = non-differential e.g. medical records
  o Non-differential misclassification differs between cases and controls; this means that the association will either be under-estimated or over-estimated
• Reverse causality is when the exposure is a consequence of the outcome
  o E.g. poor infant growth lead to increased breast feeding, breast feeding did not lead to poor infant growth
  o To limit this, exposure time period of interest should be limited

Question 21

What are the two types of epidemiological study design?

Answer 21

• Interventional studies
  o Clinical trials
  o Experimental study designs
• Observational studies
  o Descriptive studies: ecological studies, migrant studies, cross-sectional studies
  o Analytical studies: cross-sectional studies, case-control studies, cohort studies