PPHC 08 and 09: Evidence Evaluation (Epidemiology) – What are features of observational studies? Flashcards
What are the three types of epidemiology?
- descriptive (observational)
- analytic (observational)
- experimental
What is the goal of descriptive epidemiology?
to examine patterns (of disease, health behaviours)
What is the goal of analytic epidemiology?
to evaluate relationships between risk/protective factors and disease
What is the goal of experimental epidemiology?
to evaluate effect of treatment/intervention on disease
What are the 2 types of experimental studies?
- randomized controlled trial
- non-randomized
What are the 2 types of observational studies?
- descriptive
- analytic
What are the 3 types of descriptive observational studies?
- case report
- case series
- cross-sectional
What are the 3 types of analytic observational studies?
- cross-sectional
- cohort
- case-control
What are observational studies?
individuals are followed ‘observed’ in real-world settings
- no intervention/exposure assignment by researcher
What is the goal of descriptive vs. analytic observational studies?
- descriptive: examine patterns (of exposure or outcome)
- analytic: evaluate relationship (between exposure and outcome)
Do descriptive vs. analytic observational studies evaluate an intervention?
- descriptive: no
- analytic: yes – assigned by researcher
Is there a hypothesis at the start of descriptive vs. analytic observational studies?
- descriptive: no hypothesis
- analytic: defined hypothesis
Is there a hypothesis at the end of descriptive vs. analytic observational studies?
- descriptive: possible hypothesis
- analytic: confirm or reject hypothesis
What are descriptive observational studies?
describe occurrence of an outcome (ie. disease)
- do not evaluate an intervention
- ‘hypothesis generating’ – no hypothesis at start, then possible hypothesis at end
What are case reports?
detailed presentation of 1 case (n of 1)
What do case reports do? (4)
- report a new or unique condition
- describe previously undescribed disease
- show unexpected new therapeutic effect
- report adverse events
(same as case series)
What do case reports NOT do? (3)
- measure disease incidence (describe prevalent disease)
- identify risk factors
- identify cause of disease
(same as case series)
What are case series?
detailed presentation of 2 or more cases
What do case series do? (4)
- report a new or unique condition
- describe previously undescribed disease
- show unexpected new therapeutic effect
- report adverse events
(same as case reports)
What do case series NOT do? (3)
- measure disease incidence (describe prevalent disease)
- identify risk factors
- identify cause of disease
(same as case series)
What are cross-sectional studies?
observational study design that surveys exposures and/or outcomes at a single point in time (snapshot)
What type of study are cross-sectional studies?
analytic – ie. measure association between exposure and disease
- evaluate association between exposure and outcome
- temporal sequence of exposure and outcome impossible to determine (don’t know which occurred first) – main limitation compared to cohort and case-control studies
descriptive – ie. focus on exposure or disease
- examines patterns (of disease, health behaviours)
- measures prevalence, not incidence
What are analytic observational studies?
describe relationship between exposure and outcome
- disease can be the exposure or outcome
- can evaluate an intervention – though not assigned by researcher
Analytic Observational Studies
What is the exposure?
- drug (ie. statin)
- intervention (ie. diet)
- policy (ie. drug plan coverage
Analytic Observational Studies
What is the outcome?
- clinical measure (ie. cholesterol)
- disease outcome (ie. occurrence of cancer)
- patient satisfaction
What are key concepts for analytic epidemiologic studies?
- population: who we want to learn about
- sample: who we will study
Analytic Cross-Sectional Study
What is an analytic cross-sectional study?
observational study design that surveys exposures and/or outcomes at a single point in time (snapshot)
Analytic Cross-Sectional Study
What are the 4 analyses?
- A: exposed, have outcome
- B: exposed, no outcome
- C: not exposed, have outcome
- D: not exposed, no outcome
Analytic Cross-Sectional Study
Why can’t you measure the risk of an outcome?
because we don’t know when outcome occurs
- exposure and outcome are assessed at the same time in analytic cross-sectional study
- therefore relevant concept is odds
What is odds?
ratio of the number of ways the event can occur to the number of ways the event cannot occur
odds = event / (1 - event)
What is odds ratio?
measure of association
odds ratio = (odds of outcome among exposed) / (odds of outcome among unexposed) = (a/b) / (c/d)
What is OR > 1?
association
- exposed group have higher odds of outcome compared to unexposed group
- % increase = (OR - 1) x 100
What is OR = 1?
no association
What is OR < 1?
inverse association
- exposed group have lower odds of outcome compared to unexposed group
- % decrease = (1 - OR) x 100
Cohort Study – Analytical Observational Study
What is the starting point of a cohort study?
exposure
Cohort Study – Analytical Observational Study
What are the 3 steps of a cohort study?
- draw study sample from population
- identify exposure status (exposed vs. not exposed)
- follow individuals over time to see who develops outcome
Cohort Study – Analytical Observational Study
What is the key to cohort studies?
- outcome occurs after exposure
- participants are followed up to that outcome(s) so researchers know when that outcome(s) occurred
What is relative risk/risk ratio (RR)?
measure of association
relative risk = (risk of outcome among exposed) / (risk of outcome among unexposed) = a/(a+b) / c/(c+d)
What is RR > 1?
association
- Exposed group have higher risk of outcome than unexposed group
- % increase = (RR - 1) x 100
What is RR = 1?
no association
What is RR < 1?
inverse association
- exposed group have lower risk of outcome than unexposed group
- % decrease = (1 - RR) x 100
Case-Control Study – Analytical Observational Study
What is the starting point for case-control studies?
outcome (ie. disease)
Case-Control Study – Analytical Observational Study
What are the steps to case-control studies?
- identify cases (individuals with outcome or disease)
- identify controls (individuals without the outcome or disease or interest) – note: cases and controls usually match on some factors or characteristics (ie. age, gender)
- look at exposure histories
Case-Control Study – Analytical Observational Study
When might case-control studies be used?
when outcome (case) is rare
Case-Control Study – Analytical Observational Study
What measure of association does case-control studies use?
odds
Case-Control Study – Analytical Observational Study
Why is it important to analyze case-control studies?
cannot measure risk of an outcome because we don’t know when outcome occurs
- case-control study design does not involve follow-up
- in case-control study, one starts at the outcome
- so relevant concept is odds
What is the problem with mis-labelling ‘case-control’?
‘case-control’ refers to design of the study, but is often confused with ‘cases’ and ‘controls’ which refer to participants in a study
