Formulas for Exam 1

Question 1

Cumulative Incidence

Answer 1

= Number of new cases of disease/ Number at-risk in population
*during specified time period

This estimates the probability (risk) that a person will develop disease during a specified time

PROPORTION expressed as a %

Question 2

Attack rate

Answer 2

=Number of new cases of disease /Total population at risk

*for “limited” period of observation
NOT actually a rate, a fraction
For acute epidemics and outbreaks

Question 3

Case-Fatality Rate

Answer 3

Case-Fatality Rate = Number of deaths from disease/
Number of cases of that disease
*in a specified time period

Another CI measure
NOT a rate but a fraction

Reflects severity of disease

Question 4

Incidence Rate (IR)

Answer 4

IR = Number of new cases of disease/ Total at-risk person-time of observation

*during time period

TRUE RATE

“Incidence density” another name

Question 5

Mortality Rate

Answer 5

Mortality Rate = Total number of deaths/
Total person-time of follow-up

mid-year population rate of dying during a time period

Question 6

Mortality (cumulative incidence of death)

Answer 6

Mortality = Total number of deaths/
Total population during time of relevance

NOT a rate but a CI of dying
RISK

Question 7

IR and CI relationtionship

Answer 7

CI ≈ IR * Time Period

*for CI < 10%

Question 8

Prevalence

Answer 8

P = Number of existing cases of disease / Total number of people in population at that time

A proportion

Question 9

Survival

Answer 9

Survival = Number still alive by time(X) / Total number with disease at start of follow-up

not a rate

Question 10

Birth defect rate

Answer 10

Birth defect “rate” = Number of children born with defects/ Total number of births

NOT a rate

Question 11

Point prevalence and incidence prevalence relationship

Answer 11

time duration* IR = P / (1-P)

P ~ IR * D when P is <10%

The time duration is the average time that the disease lasts in the population (afterdiagnosis, before recovery, emigration or death)

Question 12

Direct Adjustment Computation

Answer 12

Age-adjusted rate = (age-specific rate × weight)

Question 13

Absolute measures (CI or Prevalence)

Answer 13

Prevalence or cumulative incidence 
(depending upon sampling)
= a/(a+b) among exposed
= c/(c+d) among unexposed
= (a+c)/(  a+b+c+d) among total sample

Question 14

Absolute Measure (IR)

Answer 14

Incidence Rate:
= a/(PTexp) among exposed
= c/(PTunexp) among unexposed
= (a+c)/( PTtot) in total sample

Question 15

Risk Difference “Attributable risk”

Answer 15

RD = Rexp− Runexp

For CI: RD = CID = CIexp- CIunexp= [a / (a+b)] – [c / (c+d)]
For IR: RD = IRD = IRexp- IRunexp= [a / PTexp] – [c / PTunexp]
For Prev: RD = PD = Pexp - Punexp= [a / (a+b)] – [c / (c+d)]

excesss risk in the exposed group

Question 16

For Cumulative Incidence and Prevalence

Answer 16

Excess Number of Cases = RD × Number Exposed

Question 17

For Incidence Rate

Answer 17

Excess Number of Cases = RD ×Person-Years Exposed

Excess number of cases= APe x number cases exposed

Question 18

Population Risk Difference (PRD)

Answer 18

PRD = Rtot− Runexp
PRD =   RD × Pexptot

impact of exposure on the total pop

Question 19

Attributable Proportion Among exposed

Answer 19

APexp = Rexp−Runexp/ = RD/ Rexp
Rexp
Also called etiologic fraction, attributable risk percent, attributable risk percent among exposed

no units

Question 20

Attributable proportion among total population

Answer 20

APtot=Rtot – Runexp/ = PRD/ Rtot

Rtot

Question 21

The Study of Epidemiology

Answer 21

The study of the distribution and determinants of disease frequency in human populations and the application of this study to control health problems.

Question 22

what is scientific about epi

Answer 22

the use of representative sampling of people, diseases and exposures to draw inferences about causation in general

Question 23

components of epidemiology

Answer 23

1. eDistribution: Descriptive epi, person, place time, who, when, where
2. Determinants: etiologic, causation, risk factors
3. Frequency

Question 24

Distinguishing factor of epidemiology from medicine and public health

Answer 24

populations

Question 25

Primary prevention

Answer 25

reducing risk or occurrence of disease in people who do not have disease

Question 26

Secondary prevention:

Answer 26

prevent asymptomatic disease from becoming symptomatic (e.g., early diagnosis and prompt treatment), or preventing transmission of infectious diseases from cases to susceptible individuals

Question 27

Tertiary prevention:

Answer 27

reduce damage of symptomatic disease, e.g., rehabilitation, halting progression and complications

Question 28

Descriptive epidemiology

Answer 28

who what where and when

NOT how

Question 29

purpose of descriptive epi

Answer 29

1. clues about disease causation and prevention
2. assess and qualify the health status of a pop.
3. Allocate resources efficiently and target populations for disease prevention and health promotion efforts

Question 30

Uses of descriptive epi

Answer 30

1. hypothesis generator
2. public health planning
3. Research: determining best prevention methods

Question 31

Goals of measures of frequency

Answer 31

1. measure distribution of disease
2. measure burden
3. to compare between groups to asses disease determinants

Question 32

Components of measures of frequency

Answer 32

1. count: number affected
2. source of population
3. time

Question 33

types of populations

Answer 33

1. Fixed: membership is permanent

2. Dynamic: membership is tansient

Question 34

Most important determinant of mortality

Answer 34

Age

Question 35

Another excess cases

Answer 35

Excess number of cases = Exposed cases -Runex x Total Exposed

Question 36

Attributable proportion of the exposed (alt formula)

Answer 36

APexp= Excess cases / Exposed cases

Question 37

Attributable proportion of the total pop (alt formula)

Answer 37

APtot= excess cases / total cases

Question 38

Excess relative risk

Answer 38

ERR= 1- RR *100 expressed as a percent