Cohort Studies Flashcards
Descriptive studies
Describe a disease condition (signs, lesions, outcomes, etc)
- case reports, case series, surveys
- no hypothesis, controls, cannot evaluate effect of treatments
- does not offer proof that risk factor causes disease
- goal: to compare findings
Explanatory studes
Seek to identify causes
- has hypothesis and controls
- investigates a treatment, intervention, risk factor in hopes of providing causal evidence
Experimental studies
Element of randomization
- experimenter assigns animal to a treatment (researcher has greatest control)
- does not represent real world conditions
- lab studies, clinical trials
Observational
Epidemiologic
- cohort
- case control
- cross sectional
Which study is best to prove cause or demonstrate efficacy?
Experimental - laboratory
Clinical trials
Assess efficacy of a preventative or therapeutic treatment or intervention
- planned comparison between 2 groups
- animals maintained under field conditions
- disease is naturally occurring
- researcher has less control over animals/housing/challenge than in lab experimental trial
Which study has findings that are more generalizable to field conditions?
Experimental study with clinical trials
Which study is also called “natural studies”
Observational (epidemiologic) studies
- occur freely in nature
- similar to explanatory (goal is to assess the cause)
- controls and planned comparisons between groups
- do not provide sufficient evidence to establish cause
What is the difference between observational and explanatory studies?
In observational studies the allocation of animals to treatment groups is not under control of the researcher
Cohort study
Prospective in time
- feature 2 cohorts: one exposed to a factor and one not exposed to that factor
- subjects followed in time, incidence of 1 or more diseases are compared between 2 cohorts
What is the primary measure of a cohort study?
Simple cumulative incidence
2 x 2 table
- exposure status in left column
- disease (outcome) in top row
- positives go together
What are 3 questions to ask in a cohort study that can be answered using the 2 x 2 table?
- what is the proportion of exposed that develop the disease
- what is proportion of unexposed that develop disease
- how do these compare
Are you looking at incidence or prevalence with a cohort study?
Incidence
When you are looking at how the 2 cohorts compare, you are measuring _______
Relative risk of disease in exposed, compared to unexposed
- risk of disease in exposed is _____ times higher than risk of disease in unexposed
Disease vs factor table
Look at powerpoint and memorize!!
Relative risk formula
a/(a+b) / c/(c+d)
A cohort study is good for diseases that are relatively ______
Common
- cannot compute a relative risk if the disease occurrence is zero
The measure of association for a cohort study is ______
Relative risk
Relative risk > 1 indicates
Increased risk in exposed, compared to unexposed
Relative risk = 1 indicates
Risk in exposed is the same as in unexposed
Relative risk < 1 indicates
Disease in exposed is less than in unexposed
- exposure has a sparing effect, or a reduction in risk associated with exposure
A relative risk is the measure of the ________ between a factor and a disease
Strength of association
- compares probability of disease in a population with the factor/exposure to the probability of disease in a population without the factor/exposure
- can only be estimated in a cohort, cross-sectional, or experimental study
Can relative risk be estimated in a case-control study?
No
Relative risk is proportion ______
with disease in exposed/proportion with disease in unexposed
In a cohort study, is the time sequence of “cause” and disease clear?
Yes
- animals are disease free at the beginning
- exposure documented to occur before disease
Observational studies are well suited to study the effect of ________
Multiple outcomes following a single exposure
- exposure to a single toxin: acute disease, neoplasia, in utero effects
Factor A causes disease X if _______
- factor A occurs before disease X (temporal relationship)
- factor A is present very often in cases, and not in controls (strength of association)
The more _____ you have, the greater the disease chance
Factor A
- dose response relationship
- stratify severity over conditioning
Biological plausibility
Based on what is known, factor A could cause disease X
Consistency of multiple studies
Relationship between factor A and disease X is seen repeatedly, time and time again
Reversible associations
Removal of factor A results in diminished disease
What does risk of disease in unexposed represent?
Control/base line
