Equations

Question 1

Sensitivity

Answer 1

proportion of all people with disease who test positive, or the probability that a test detects disease when disease is present
PID = positive in disease
SNOUT = Sensitivity rules out

Question 2

Sensitivity equation

Answer 2

= TP/(TP + FN)

= 1 - false negatives

Question 3

Specificity

Answer 3

proportion of all people without disease who test negative, or the probability that a test indicates non-disease when disease is absent
NIH = negative in health
SPIN = specificity rules in

Question 4

Specificity equation

Answer 4

= TN/(TN + FP)

= 1- false positives

Question 5

Positive predictive value

Answer 5

proportion of positive test results that are true positive

varies directly with prevalence or pretest probability

Question 6

Positive predictive value equation

Answer 6

= TP/(TP + FP)

Question 7

Negative predictive value

Answer 7

proportion of negative test results that are true negative

varies inversely with prevalence or pretest probability

Question 8

Negative predictive value equation

Answer 8

= TN/(TN + FN)

Question 9

Incidence

Answer 9

of new cases in a specified time/population at risk

Question 10

Prevalence

Answer 10

existing cases/population
=incidence rate x average disease duration
greater than incidence for chronic disease

Question 11

Odds ratio

Answer 11

used in case control studies.
Odds that the group with the disease was exposed to a risk factor divided by the odds that the group without the disease (controls) was exposed.
(a/c)/(b/d) = ad/bc (ODDS had 2 D’s, CC study has 2 C’s)

Question 12

Relative Risk

Answer 12

used in cohort studies.
Risk of developing disease in the exposed group divided by the risk in the unexposed group
=[a/(a+b)]/[c/(c+d)]

Question 13

Attributable risk

Answer 13

the difference in risk between exposed and unexposed groups, or the proportion of disease occurrences that are attributable to the exposure
=(a/(a+b)) - (c/(c+d))

Question 14

Absolute risk reduction

Answer 14

absolute reduction in risk associated with a treatment as compared to a control.

Question 15

Number needed to treat

Answer 15

number of patients who need to be treated for 1 patient to benefit.
=1/absolute risk reduction

A risk reduction is a positive thing–> use for # needed to treat which is a positive thing

Question 16

Number needed to harm

Answer 16

number of patients who need to be exposed to a risk factor for 1 patient to be harmed.
=1/attributable risk

Attributable risk is a negative thing–> use for # needed to harm which is a negative thing