Equations

Question 1

Sensitivity

Answer 1

True positive rate (proportion of all people with disease who test positive)
TP/(TP+FN) = 1 - (false negative rate)

Question 2

Specificity

Answer 2

True negative rate (proportion of people without disease that test negative)
TN/(TN+FP) = 1 - (false positive rate)

Question 3

PPV

Answer 3

positive predictive value = proportion of positive test results that are true positive
= TP/(TP+FP)

Question 4

NPV

Answer 4

Negative predictive value = proportion of negative test results that are true negative
= TN/(TN+FN)

Question 5

PPV varies _____ with prevalence or pretest probability

Answer 5

directly

Question 6

NPV varies ______ with prevalence or pretest probability

Answer 6

indirectly

Question 7

Incidence rate:

Answer 7

(# new cases) / (# people at risk) during a time period

Question 8

Prevalence rate:

Answer 8

(#existing cases)/(# people at risk) at a point in time

Question 9

Prevalence = what other value’?

Answer 9

pretest probability

Question 10

odds ratio

Answer 10

odds that the group with disease was exposed to a risk factor divided by the odds that the group without disease was exposed
OR = (a/c)/(b/d) = ad/bc

Question 11

relative risk

Answer 11

risk of developing disease in the exposed group divided by risk in the unexposed group
RR = a(a+b) / c(c+d)

Question 12

Attributable risk

Answer 12

proportion of disease occurrences that are attributable to the exposure

AR = (a/a+b) - (c/c+d)

Question 13

Relative risk reduction

Answer 13

proportion of risk reduction attributable to intervention as compared to control

RRR = 1 - (Relative risk)

Question 14

Absolute risk reduction

Answer 14

Difference in risk attributable to intervention vs. control

ARR = (C/C+D) - (a/a+b)

Question 15

NNT

Answer 15

number of patients who need to be treated for 1 patient to benefit

NNT = 1/ARR

Question 16

NNH

Answer 16

number of patients who need to be treated for 1 patient to be harmed

NNH = 1/AR

Question 17

SEM

Answer 17

standard error of the mean

SD/sqrt(n)

Question 18

CI

Answer 18

CI = mean +/- Z(SEM)

Question 19

For 95% CI, Z=

Answer 19

1.96

Question 20

For 99% CI, Z =

Answer 20

2.58