Calculations

Question 1

Ratio

Answer 1

any number divided by another number

Question 2

Proportion

Answer 2

a special type of ratio in which everything or
everyone in the numerator is also counted in the
denominator

Question 3

Rate

Answer 3

one measure against some other quantity or

measure, and should contain a measure of time

Question 4

Attack rate

Answer 4

proportion (%) of people who became sick among
those who ate (or did not eat) each type of food

of people who became sick / total # who ate (did not eat) each type of food

Question 5

Crude death rate

Answer 5

a measure of the number of deaths (in general) in
a population, scaled to the size of that population

of new deaths in the specified year / (average) # of population during that year

Question 6

Disease-specific death rate

Answer 6

a measure of the number of deaths (with a
specific disease) in a population, scaled to the
size of that population

of deaths with the disease in the specified year / (average) # of the population during that year

Question 7

Case fatality ratio

Answer 7

proportion (%) of people with a given disease who
die from that disease during a specified time
period

of deaths with the disease in the specified year / total # with the disease during that year

Question 8

Lifetime risk

Answer 8

a measure of the risk that a certain event will
happen during a person’s lifetime

1 in (1/cumulative incidence)

Question 9

Point prevalence

Answer 9

proportion (%) of a population with the disease at
a specified point in time

of cases at a specified point in time / # of population at the specified point in time

Question 10

Period prevalence

Answer 10

proportion (%) of a population with the disease at
any point during a specified time period

of cases at any point during a specified time period / (average) # of population during that period

Question 11

Cumulative incidence / incidence proportion

Answer 11

proportion (%) of the defined population that
develop the disease during a specified time period

of NEW cases of disease over a specified time period / # of population at risk at the start of time interval

Question 12

Incidence rate

Answer 12

a measure of how quickly people are developing a disease during the specified time period/the rate at which new cases of a disease occur in a population

of NEW cases of disease over a specified time period / summation of person time or population at risk during that specified time period

Question 13

Relationship between prevalence and incidence

Answer 13

if prevalence is low: P = incidence rate x average duration

if prevalence is high: P/(1-P) = IR x D

Question 14

Odds ratio

Answer 14

Odds of disease in an exposed group divided by
odds in an unexposed or reference group, used in
case-control study

odds among exposed [a/b] / odds among unexposed [c/d]

Question 15

Risk ratio

Answer 15

Risk of disease in one group divided by risk in a
second or reference group, used in cohort study

risk in one group [a/(a+b)] / risk in the reference group [c/(c+d)]

Question 16

Sensitivity

Answer 16

the probability (%) that someone with the
condition of interest will return a positive test
result

True Positives (a) / All cases with the condition of interest (a+c)

Question 17

Specificity

Answer 17

the probability (%) that someone without the
condition of interest will return a negative test
result

True Negatives (d) / All non-cases (controls) without the condition of interest (b+d)

Question 18

Positive predictive value (PPV)

Answer 18

the probability (%) that someone who tested
positive actually has the condition of interest

True Positives (a) / All test positive (a+b)

Question 19

Negative predictive value (NPV)

Answer 19

the probability (%) that someone who tested
negative truly doesn’t have the condition of
interest

True Negatives (d) / All test negative (c+d)

Question 20

Accuracy

Answer 20

proportion (%) of tests in which individuals are
correctly classified as to their true disease status

All correct tests (a+d) / All tested (a+b+c+d)