Lecture 13: Evidence Evaluation: Observational Studies II Flashcards
What kind of study is case-control?
Observational, analytic study
What is the starting point of a case-control study
Outcome (ex.disease)
Steps of a case-control study
1a. Identify cases (individuals with outcome or disease)
1b. Identify controls (individuals without the outcome or disease of interest)
- Look at exposure histories
Case-control study flow
Look at canvas
Why can’t we measure risk of an outcome when analyzing case-control studies?
Because we don’t know when they occur (study design does not involve a follow up)
Does relative risk apply to a case control study?
No
What is the relevant concept when analyzing case-control studies
Odds
What are odds?
The odds of an even can be defined as the ratio of the number of ways the event can occur to the number of ways the even cannot occur
Interpretation is there a relationship? OR > 1
Association: Exposed group have higher odds of outcome
compared to unexposed group
Interpretation is there a relationship? OR = 1
No association
Interpretation is there a relationship? OR < 1
Inverse association: Exposed group have lower odds of
outcome compared to unexposed group
Interpretation: What is the strength of the relationship: OR > 1
% increase = (OR - 1) x 100
Interpretation: What is the strength of the relationship: OR < 1
% decrease = (1 - OR) x 100
What is the problem with mis-labelling?
“Case-control” which refers to the design of the study is often
confused with “cases” and “controls” which refer to participants in a
study
- Cohort study is often mislabelled as case control study
What are the advantages of a case-control study?
Advantages
* Good for less common or rare
outcomes
* Versus a cohort study where one
would have to study many
participants over a long follow-up
for outcomes to occur
* Quick(er) and less expensive
(compared to cohort study)
* Can study multiple exposures
What are disadvantages to case-control studies?
- Cannot establish incidence
- Cannot study multiple outcomes
What is a cross-sectional study? Analytic or descriptive?
An observational design that surveys exposures and/or outcomes at a single point in time (‘snapshot’)
- Can be analytic (i.e. measure association between exposure and disease)
- Can be descriptive (i.e. focus on exposure or disease)
Describe a cross-sectional study as an analytic study
Temporal sequence of exposure and outcome impossible to determine (don’t know which occurred first) à main limitation (compared to cohort and case-
control studies)
Describe a cross-sectional study as a descriptive study
Examines patterns (of disease, health behaviours)
* Measures prevalence not incidence
What measure is used for an analytic cross-sectional study?
Odds Ratio
What do you study in a descriptive cross-sectional study?
Studying:
* Frequency of disease
* Frequency of health behaviour(s)
What do descriptive studies describe/measure?
Occurrence of an outcome (i.e. disease)
Do descriptive studies evaluate intervention?
No
What is the start and end of a descriptive study?
Start: no hypothesis
End: possible hypothesis
What kind of studies are case report and case series?
Observational, descriptive study
What are case reports and case series?
Detailed presentation
* Case report – 1 or 2 cases
* Case series - > 2 cases
What do case reports and case series do?
- Report a new or unique condition
- Describe previously undescribed disease
- Show unexpected link between diseases
- Show unexpected new therapeutic effect
- Report adverse events
What don’t case reports and case series do?
Measure disease incidence
* Case series assess prevalent
(existing) disease
* Identify risk factors
* Identify cause of disease
When outcome is rare what study do we use?
Case-control
When there are multiple exposures what study do we use?
Case-control
When investigating an outbreak what study do we use?
Case-control
