Behavior Equations Flashcards
Sensitivity
true-positive rate
Proportion of all people with disease who test positive, or the probability that when the
disease is present, the test is positive
= TP / (TP + FN)
= 1 – false-negative rate
SN-N-OUT = highly SeNsitive test, when Negative, rules OUT disease
Specificity
true-negative rate
Proportion of all people without disease who test negative, or the probability that when the disease is absent, the test is negative
= TN / (TN + FP)
= 1 – false-positive rate
SP-P-IN = highly SPecific test, when Positive, rules IN disease.
Positive predictive value
Proportion of positive test results that are true positive
Probability that a person who has a positive test result actually has the disease
PPV = TP / (TP + FP)
Negative predictive value
Proportion of negative test results that are true negative
Probability that a person with a negative test result actually does not have the disease
NPV = TN / (TN + FN)
Odds ratio
used in *case-control studies
OR = (a/c)/(b/d)= (ad)/(bc)
Relative risk
used in *cohort studies
RR = [a/(a + b)]/[c/(c + d)]
Attributable risk
The difference in risk between exposed and
unexposed groups, or the proportion of disease occurrences that are attributable to the exposure
AR = a/(a + b) - c/(c + d)
Relative risk reduction
The proportion of risk reduction attributable to the intervention as compared to a control
RRR = 1 − RR
Absolute risk reduction
The difference in risk (not the proportion) attributable to the intervention as compared to a control
ARR = c/(c + d)− a/(a + b)
Number needed to treat
Number of patients who need to be treated for 1
patient to benefit.
NNT = 1/ARR
Number needed to harm
Number of patients who need to be exposed to a
risk factor for 1 patient to be harmed.
NNH = 1/AR
