Epidemiology Flashcards
What is risk?
Probability an event will occur
Think about it in terms of prevalence and incidence
What is prevalence
Proportion of sampled individual affected AT A GIVEN POINT IN TIME.
Includes both old and new cases, and is typically expressed as a percent.
What is incidence?
Number of new cases occurring in a specified population over a SPECIFIED PERIOD OF TIME.
Typically expressed as cases per individual per unit of time.
What is the formula for prevalence using the 2x2 table?
Prevalence = (a+c) / (a+b+c+d)
All animals that actually have the disease / total population
What is sensitivity?
TRUE POSITIVE RATE
Likelihood of a positive test in individuals known to have the disease based on the gold standard
What is specificity?
TRUE NEGATIVE RATE
Likelihood of negative test in individuals known NOT to have the disease based on the gold standard
Do sensitivity and specificity change with disease prevalence?
NO - they are inherent properties of the diagnostic tests and do not change with disease prevalence in the population in question
Predictive values will change with disease prevalence
What is the formula for sensitivity using the 2x2 table?
Sensitivity = a / (a+c)
True positives / everyone that actually has the disease
What is the formula for specificity using the 2x2 table?
Specificity = d / (b+d)
True negatives / everyone that doesn’t have the disease
What is positive predictive value?
Likelihood of a positive test in an animal truly affected by the disease
Increases as prevalence of disease increases
What is negative predictive value?
Likelihood of a negative test in an animal truly negative for the disease.
Decreases as prevalence of disease increases
A negative test result can be trusted less in a disease that is super common, but can be trusted more for a disease that is rare because it’s more likely to be true
T/F: Positive and negative predictive value do not change with prevalence.
FALSE - they do. They are not determined by test characteristics, they are determined by prevalence of disease
What is the formula for positive predictive value using the 2x2 table?
PPV = a / (a+b)
True positives / all test positives
What is the formula for negative predictive value using the 2x2 table?
NPV = d / (c+d)
True negatives / all test negatives
What is relative risk/risk ratio?
Ratio of risk in individuals exposed vs. not exposed to a risk factor
Relative risk = (risk in exposed individuals) / (risk in unexposed individuals)
What is the formula for relative risk using the 2x2 table?
Relative risk = [a/(a+b)] / [c/(c+d)]
RR = 1 means what?
There is no difference in the risk of a given outcome between individuals exposed and unexposed to the risk factor/intervention
RR<1 means what
There is less risk of the outcome in individuals exposed to the risk factor/intervention than those unexposed.
THE INTERVENTION IS PROTECTIVE
RR >1 means what
There is more risk of the outcome in individuals exposed to the risk factor/intervention
What is an odds ratio?
Probability of an event happening vs. not happening
When is odds ratio useful?
Useful to measure associations when incidence cannot be calculated (I.e. case-control studies)
Also useful when the frequency of outcome is low
What is the formula for odds ratio using 2x2 table?
Odds ratio = (a x d) / (b x c)
probability that case was expose / probability that a control was exposed
OR = 1 means what
NO association between the factor and outcome
OR < 1 means what
NEGATIVE association between factor and outcome
OR > 1 means what
POSITIVE association between factor and outcome
