Public Health Formulas

Question 1

Sensitivity

Answer 1

1 - FN rate

Ruling out disease (screening)

Question 2

False Negative Rate

Answer 2

FN / FN + TP

Question 3

Specificity

Answer 3

1 - FP rate

Ruling in disease

Question 4

Incidence

Answer 4

# new cases / # of people at risk         
(per unit of time)   

EXCLUDE PATIENTS THAT ARE ALREADY DIAGNOSED

Question 5

Negative likelihood ratio

Answer 5

(1 - sensitivity) / specificity

(FNrate) / (TNrate)

probability of negative result in patient with disorder / probability of negative result in patient without disorder

Question 6

Positive likelihood ratio

Answer 6

Sensitivity / (1 - specificity)

(TP rate) / (FP rate)

probability of positive result in patient with disorder / probability of positive result in patient without disorder

Question 7

Prevalence

Answer 7

of existing cases / total # of people in a population

(at a point in time)

Question 8

Prevalence / 1 - prevalence

steady state population

Answer 8

incidence rate X average duration of disease

Question 9

Relative Risk

Answer 9

[a/(a+b)] / [c/c+d)]

incidenceof disease in exposed group / incidenceof disease in unexposedgroup)

Question 10

Relative Risk Reduction

Answer 10

1 - RR

Question 11

Odds Ratio

Answer 11

(a/c) / (b/d)

a x d) / (b x c

Question 12

Exposure Attributable Risk

Answer 12

(incidence ratein exposedgroup) -(incidence ratein unexposedgroup)

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

Question 13

Population Attributable Risk

Answer 13

(incidence ratein the entire studypopulation) -(incidence ratein the unexposedgroup)

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

Question 14

Attributable Risk %

Answer 14

[(RR-1) / RR] X 100

(Attributable Risk / Incidence of disease in the exposed group) X 100

Question 15

Attributable Risk Reduction (ARR)

Answer 15

(absoluteriskin the unexposedgroup) -(absolute riskin the exposedgroup)

c/(c + d) – a/(a + b)

Question 16

Number Needed to Treat (NNT)

Answer 16

1 / ARR

Question 17

Number Needed to Screen (NNS)

Answer 17

1 / ARR

Question 18

Number Needed to Harm (NNH)

Answer 18

1 / AR

Question 19

Hazard Ratio

Answer 19

Incidenceof outcome in exposed group / incidenceof outcome in unexposedgroup

[a / (a + b)] / [c / (c + d)]

Question 20

Absolute Risk Reduction is also known as _______.

Answer 20

Risk difference

Question 21

Case fatality rate is also known as _______.

Answer 21

Lethality

Question 22

Case Fatality Rate

Answer 22

(# deaths from a specific condition / # cases with the same specific condition) X 100

Question 23

Attack rate is also known as _______.

Answer 23

Cumulative incidence

Question 24

Attack Rate

Answer 24

People who become ill / Total people exposed

DO NOT SUBTRACT DEATHS FROM DENOMINATOR

Question 25

Mortality rate is also known as _______.

Answer 25

Crude death rate

Question 26

Mortality Rate (crude death rate)

Answer 26

of deaths / total population size

Question 27

Cause Specific Mortality Rate

Answer 27

of deaths of particular disease / total population size

Question 28

Standardized Mortality Ratio

Answer 28

Observed # of deaths / Expected # of deaths

Question 29

Maternal Mortality Rate

Answer 29

(# of maternal deaths / # of live births) X 100,000

Question 30

Perinatal Mortality Rate

Answer 30

[(# of stillbirths + # of infant deaths during first week after birth) / total number of births] X 1,000

Question 31

Crude Birth Rate

Answer 31

of live births / total population size

Question 32

Fetal Mortality Rate

Answer 32

[(# of infant deaths during the first 24hours after birth) / (total number of live births)] X 1,000

Question 33

Reliability Coefficient

Answer 33

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

Question 34

Standard Deviation (σ)

Answer 34

√variance

Question 35

Variance

Answer 35

SDˆ2

Question 36

Standard Error of the Mean

Answer 36

Standard deviation / √(sample size)

Question 37

Standard normal value(Z-score,Z-value, standard normalized score)

Answer 37

(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

Question 38

Confidence Interval for Sample Mean

Answer 38

Mean +/- Z(standard error of the mean)

Mean+/- Z (standard deviation / √samplesize)

Confidence interval (CI) = 1 – α.

Question 39

Coefficient of Determination

Answer 39

rˆ2

Amount of variance in one variable that can be explained by variance in another variable