Calculations Flashcards
Ratio
any number divided by another number
Proportion
a special type of ratio in which everything or
everyone in the numerator is also counted in the
denominator
Rate
one measure against some other quantity or
measure, and should contain a measure of time
Attack rate
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
Crude death rate
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
Disease-specific death rate
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
Case fatality ratio
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
Lifetime risk
a measure of the risk that a certain event will
happen during a person’s lifetime
1 in (1/cumulative incidence)
Point prevalence
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
Period prevalence
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
Cumulative incidence / incidence proportion
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
Incidence rate
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
Relationship between prevalence and incidence
if prevalence is low: P = incidence rate x average duration
if prevalence is high: P/(1-P) = IR x D
Odds ratio
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]
Risk ratio
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)]
Sensitivity
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)
Specificity
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)
Positive predictive value (PPV)
the probability (%) that someone who tested positive actually has the condition of interest
True Positives (a) / All test positive (a+b)
Negative predictive value (NPV)
the probability (%) that someone who tested negative truly doesn’t have the condition of interest
True Negatives (d) / All test negative (c+d)
Accuracy
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)
