Lecture 3, part 1 Flashcards
What is the purpose of comparison?
Summarize relationship btwn exposure and dz by comparing at least 2 measures of dz frequency in different pops
It’s the essence of epidemiology
The rate of dz in an exposed group says nothing about whether exposure is a RF for (or cause of) the dz
-Can only be evaluated by comparing dz occurrence in an exposed grp to another grp (unexposed or referenced grp)
What are measures of effect?
Difference in dz occurrence btwn 2 grps of ppl who differ with respect to causal characteristics
An effect is the endpoint of a causal mechanism
Measures what would happen to one pop under 2 possible conditions (counterfactual)
What are measures of association
Preferred term when characteristic under study is not causal (or of unknown causality)
Theoretically equivalent to the effect measure
Compares occurrence in 2 different pops
What are relative effects?
Ratio of risks or probability
What do relative effects do?
Calculate the ratio of 2 measures of dz frequency
-Measure in exposed grp
-Measure in unexposed grp
Examples of relative effects
Risk ratios
Odds ratios
What are absolute effects?
Differences in risks
What do absolute effects do?
Calculate the difference btwn 2 measures of dz frequency
-Measure in exposed grp
-Measure in unexposed comparison grp
Examples of absolute effects
Risk differences
Attributable proportions
Purpose of risk ratio
Gives info on the relative effect of the exposure on the dz (or outcome)
-Estimate of how many times higher (or lower) the risk of dz is among the exposed (index group) as compared to the unexposed (comparison group)
-Commonly used in etiologic research (attempt to ID causes of dz)
Formula for risk ratio
Risk in exposed group/risk in unexposed group
What is the implication when RR = 1.0?
Risk in exposed is same as risk in non-exposed
No association between exposure and dz
What is the implication when RR > 1?
Risk in exposed > risk in non-exposed
Positive association; causal?
What is the implication when RR < 1?
Risk in exposed < risk in non-exposed
Negative association; protective
To what group can risk ratios and odds ratios be compared if there are more than 2 different groups?
The reference group
What is the default value for the reference group in RRs or ORs?
1.00
Describe reference group selection
It’s arbitrary, often based on study question or clinical relevance
Usually the group with the lowest risk of the dz/outcome
Definition of odds of an event
The ratio of the probability that the event will occur to the probability that the event will not occur
Definition of odds of an exposure
The ratio of the probability that the exposure was present to the probability that the exposure was absent
What do odds ratios do?
Quantifies the relationship between an exposure and a health outcome
-Used when do not know the total pop in the margin of a two-by-two table, or when do not know the rate or risk of dz
When does an odds ratio approximate the risk ratio?
When the health outcome is uncommon
Odds ratio formula
ad/bc
What is the implication of OR = 1.0?
Exposure is not related to dz
No association; independent
What is the implication of OR > 1.0?
Exposure is positively related to dz
Positive association
What is the implication of OR < 1.0?
Exposure is negatively related to dz
Negative association
