Public Health Formulas Flashcards
Sensitivity
1 - FN rate
Ruling out disease (screening)
False Negative Rate
FN / FN + TP
Specificity
1 - FP rate
Ruling in disease
Incidence
# new cases / # of people at risk (per unit of time)
EXCLUDE PATIENTS THAT ARE ALREADY DIAGNOSED
Negative likelihood ratio
(1 - sensitivity) / specificity
(FNrate) / (TNrate)
probability of negative result in patient with disorder / probability of negative result in patient without disorder
Positive likelihood ratio
Sensitivity / (1 - specificity)
(TP rate) / (FP rate)
probability of positive result in patient with disorder / probability of positive result in patient without disorder
Prevalence
of existing cases / total # of people in a population
(at a point in time)
Prevalence / 1 - prevalence
steady state population
incidence rate X average duration of disease
Relative Risk
[a/(a+b)] / [c/c+d)]
incidenceof disease in exposed group / incidenceof disease in unexposedgroup)
Relative Risk Reduction
1 - RR
Odds Ratio
(a/c) / (b/d)
a x d) / (b x c
Exposure Attributable Risk
(incidence ratein exposedgroup) -(incidence ratein unexposedgroup)
a / (a + b) - c / (c + d)
Population Attributable Risk
(incidence ratein the entire studypopulation) -(incidence ratein the unexposedgroup)
(a + c) / (a + b + c + d) - c / (c + d)
Attributable Risk %
[(RR-1) / RR] X 100
(Attributable Risk / Incidence of disease in the exposed group) X 100
Attributable Risk Reduction (ARR)
(absoluteriskin the unexposedgroup) -(absolute riskin the exposedgroup)
c/(c + d) – a/(a + b)
Number Needed to Treat (NNT)
1 / ARR
Number Needed to Screen (NNS)
1 / ARR
Number Needed to Harm (NNH)
1 / AR
Hazard Ratio
Incidenceof outcome in exposed group / incidenceof outcome in unexposedgroup
[a / (a + b)] / [c / (c + d)]
Absolute Risk Reduction is also known as _______.
Risk difference
Case fatality rate is also known as _______.
Lethality
Case Fatality Rate
(# deaths from a specific condition / # cases with the same specific condition) X 100
Attack rate is also known as _______.
Cumulative incidence
Attack Rate
People who become ill / Total people exposed
DO NOT SUBTRACT DEATHS FROM DENOMINATOR
Mortality rate is also known as _______.
Crude death rate
Mortality Rate (crude death rate)
of deaths / total population size
Cause Specific Mortality Rate
of deaths of particular disease / total population size
Standardized Mortality Ratio
Observed # of deaths / Expected # of deaths
Maternal Mortality Rate
(# of maternal deaths / # of live births) X 100,000
Perinatal Mortality Rate
[(# of stillbirths + # of infant deaths during first week after birth) / total number of births] X 1,000
Crude Birth Rate
of live births / total population size
Fetal Mortality Rate
[(# of infant deaths during the first 24hours after birth) / (total number of live births)] X 1,000
Reliability Coefficient
Variance of the true value (population variance) / Variance of the measured value (sample variance)
Reliability coefficient =1 →thevarianceof the samplemeanis equal to thevarianceof the truemeasure →the study/test is highly reliable
Standard Deviation (σ)
√variance
Variance
SDˆ2
Standard Error of the Mean
Standard deviation / √(sample size)
Standard normal value(Z-score,Z-value, standard normalized score)
(value -populationmean) / standard deviation
Z-scorefor a 90%confidence interval= 1.64
Z-scorefor a 95%confidence interval= 1.96
Z-scorefor a 97.5%confidence interval= 2.24
Z-scorefor a 99%confidence interval= 2.58
Confidence Interval for Sample Mean
Mean +/- Z(standard error of the mean)
Mean+/- Z (standard deviation / √samplesize)
Confidence interval (CI) = 1 – α.
Coefficient of Determination
rˆ2
Amount of variance in one variable that can be explained by variance in another variable
