Formulas for Exam 1 Flashcards
Cumulative Incidence
= 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 %
Attack rate
=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
Case-Fatality Rate
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
Incidence Rate (IR)
IR = Number of new cases of disease/ Total at-risk person-time of observation
*during time period
TRUE RATE
“Incidence density” another name
Mortality Rate
Mortality Rate = Total number of deaths/
Total person-time of follow-up
mid-year population rate of dying during a time period
Mortality (cumulative incidence of death)
Mortality = Total number of deaths/
Total population during time of relevance
NOT a rate but a CI of dying
RISK
IR and CI relationtionship
CI ≈ IR * Time Period
*for CI < 10%
Prevalence
P = Number of existing cases of disease / Total number of people in population at that time
A proportion
Survival
Survival = Number still alive by time(X) / Total number with disease at start of follow-up
not a rate
Birth defect rate
Birth defect “rate” = Number of children born with defects/ Total number of births
NOT a rate
Point prevalence and incidence prevalence relationship
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)
Direct Adjustment Computation
Age-adjusted rate = (age-specific rate × weight)
Absolute measures (CI or Prevalence)
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
Absolute Measure (IR)
Incidence Rate:
= a/(PTexp) among exposed
= c/(PTunexp) among unexposed
= (a+c)/( PTtot) in total sample
Risk Difference “Attributable risk”
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
For Cumulative Incidence and Prevalence
Excess Number of Cases = RD × Number Exposed
For Incidence Rate
Excess Number of Cases = RD ×Person-Years Exposed
Excess number of cases= APe x number cases exposed
Population Risk Difference (PRD)
PRD = Rtot− Runexp PRD = RD × Pexptot
impact of exposure on the total pop
Attributable Proportion Among exposed
APexp = Rexp−Runexp/ = RD/ Rexp
Rexp
Also called etiologic fraction, attributable risk percent, attributable risk percent among exposed
no units
Attributable proportion among total population
APtot=Rtot – Runexp/ = PRD/ Rtot
Rtot
The Study of Epidemiology
The study of the distribution and determinants of disease frequency in human populations and the application of this study to control health problems.
what is scientific about epi
the use of representative sampling of people, diseases and exposures to draw inferences about causation in general
components of epidemiology
- eDistribution: Descriptive epi, person, place time, who, when, where
- Determinants: etiologic, causation, risk factors
- Frequency
Distinguishing factor of epidemiology from medicine and public health
populations
Primary prevention
reducing risk or occurrence of disease in people who do not have disease
Secondary prevention:
prevent asymptomatic disease from becoming symptomatic (e.g., early diagnosis and prompt treatment), or preventing transmission of infectious diseases from cases to susceptible individuals
Tertiary prevention:
reduce damage of symptomatic disease, e.g., rehabilitation, halting progression and complications
Descriptive epidemiology
who what where and when
NOT how
purpose of descriptive epi
- clues about disease causation and prevention
- assess and qualify the health status of a pop.
- Allocate resources efficiently and target populations for disease prevention and health promotion efforts
Uses of descriptive epi
- hypothesis generator
- public health planning
- Research: determining best prevention methods
Goals of measures of frequency
- measure distribution of disease
- measure burden
- to compare between groups to asses disease determinants
Components of measures of frequency
- count: number affected
- source of population
- time
types of populations
- Fixed: membership is permanent
2. Dynamic: membership is tansient
Most important determinant of mortality
Age
Another excess cases
Excess number of cases = Exposed cases -Runex x Total Exposed
Attributable proportion of the exposed (alt formula)
APexp= Excess cases / Exposed cases
Attributable proportion of the total pop (alt formula)
APtot= excess cases / total cases
Excess relative risk
ERR= 1- RR *100 expressed as a percent
