Exam 1 Flashcards
Epidemiology
the diagnostic discipline of public health
study of the distribution and the determinants (risk factors) of disease and mortality in the population
Quantifying a Disease
1) Define the disease
2) Measure disease frequency:
a) count the number of individuals affected (numerator)
b) determine the size of the population from which the cases arose (denominator)
c) account for the passage of time
Basic types of incidence rates
1) Cumulative Incidence
2) Incidence rate (aka incidence density: person-time incidence rate)
Incidence
the number of new cases per unit population, during a stated period of time
the rate of development of a disease in a population over a specified period of time
contains the following elements:
a) numerator - number of new cases
b) denominator - population at risk
c) rate base or multiplier
d) time - period during which cases accrued
Population at risk (PAR)
the denominator for incidence rates
are AT RISK of contracting the disease
are DISEASE-FREE at the start of the specified time period
exclude individuals NOT AT RISK in the population from the denominator - important with lifelong diseases and infectious diseases that confer immunity
among large populations (ex. US incidence rates), denominator is based on average population
Rate Base
also known as multiplier
rate base - per 100, 1000, or 100000 (to power of 10)
purpose: the reported rate is expressed as a number equal to or greater than 1
Calculate cumulative incidence of colon cancer among this study population:
60 cases of colon cancer were reported in 5 year study of 311 study subjects, colon cancer-free at start of study
Cumulative incidence rate = number of new cases / population at risk
60/311 = 0.1929
19.3 cases per 100 over 5 years
Calculate cumulative incidence rate for ovarian cancer:
# of new cases = 1085 population at risk = 37105 time period = 8 years
incidence rate = 1085 / 37105 = 0.02924
- 02924 / 8 = 0.003655
- 003655 x 100,000 = 365.5 cases per 100,000 women per year
Cumulative incidence rate
represents a population’s average risk of developing that disease during the time/period of observation
Incidence Rate
also known as incidence density
what to do when members of a population or study group are under observation for different lengths of time
Incidence density equation
Incidence density = # of new cases during the time period / total person-time of observation
If measured in years:
Incidence density = # of new cases during the time period / total person-years of observation
What is the incidence rate of colon cancer among this study population per 100 person-years?
60 cases of colon cancer were reported in 5 year study of 311 study subjects. 10 subjects developed colon cancer and 40 dropped out of study after one year, 35 new cases were reported at end of year two; no cases, but 4 subjects dropped out at end of year three, 10 new cases and 12 subjects dropped out at end of year four, and 5 new cases were reported at end of year 5.
Incidence rate of colon cancer is: 60 cases per 1220 person years
Incidence per 100 person-years: (60/1220) x 100 = 4.9 cases per 100 person years
Advantages of incidence rate over cumulative incidence
advantages in follow up studies:
1) allow for subject losses due to withdrawals, deaths or other reasons
2) accommodates subjects entering a study at different times
Applications of incidence data
1) helps in research on the etiology/causality of disease
2) estimates the risk of developing a disease
Attack Rate (AR)
a cumulative incidence rate used when the nature of the disease or condition is such that a population is observed for a short period of time
alternative form of incidence rate
used for diseases observed in a population for a short time period
with acute infectious disease outbreaks, the population-at-risk increases greatly over a short period of time
ex) salmonella gastroenteritis outbreak
Prevalence
the number of cases (new and existing) of a disease or health condition in a population at or during some designated time
provides an indication of the extent of a health problem BUT is NOT a measure of risk
Point prevalence equation
Point prevalence = number of cases / total number in the group at point in time
Period prevalence
Period prevalence = number of cases of disease during a time period
average population at mid-point
cases are counted even if they die, migrate, or recur
Uses of prevalence
1) describing the burden of a health problem in a population
2) determining allocation of health resources such as facilities and personnel
Interrelationship between Prevalence and Incidence
the prevalence (P) of a disease is proportional to the incidence rate (I) times the duration (D) of a disease
Incidence and Prevalence
if duration of disease is short and incidence is high, prevalence becomes similar to incidence
short duration: cases recover rapidly or are fatal (ex. common cold, flu, ebola)
An epidemiologist conducts a survey of Sargentitis Disease in a population.
The prevalence of Sargentitis
among women is 40 per 1,000 and
among men is 20 per 1,000.
Assuming the data is age-adjusted,
is it correct to assume that women have twice the risk of Sargentitis compared with men?
No, there could be a lot more women in the total population compared to men
Changing Patterns of Mortality
until the 20th century the major causes of death in all parts of the world combined were infectious diseases
in the 20th century, control of infectious diseases is paralleled by the emergence of chronic diseases as major causes of mortality
Characteristics of Infectious Diseases
relatively short latency period from exposure to disease
Characteristics of Chronic Diseases
long latency period, 10-20+ years
the length of latency period impacts public health approaches to reduce disease rates within a population
health care and public health care systems need to adapt
Risk factor
a characteristic or agent whose presence increases the probability of occurrence of a disease
Social determinants of a health fact file
fact file: social determinants of health
poverty, social exclusion, poor housing, and poor health systems are among the main social causes of ill health
Objectives of Epidemiology
1) the extent of disease in a population
2) identify patterns and trends in disease
3) identify causes/risk factors of disease
4) evaluate effectiveness of prevention and treatment activities
Fixed population
a population whose membership is permanent
ex) population of Hiroshima, Japan during atomic bomb explosion in WWII
Dynamic or open population
membership of this population is defined by a changeable state or condition
ex) resident of Boston
Steady state
the number entering a population is equal to the number leaving
ex) BU student population
Count
simplest and most frequently performed quantitative measure
refers to the number of cases of a disease or other health phenomenon
for rare diseases or symptom presentations (cases of Ebola virus)
ex) 5000 cases of influenza in Suffolk county in Jan. 2001
ex) 3000 traffic fatalities in Springfield, IL in 2001
Proportion
for a count to be descriptive of a group, it must be seen relative to the size of the group
a fraction where the numerator is part/subset of the denominator
ex) male births/all births
Ratio
a relative relationship
like a proportion or a fraction
numerator and denominator are mutually exclusive/not related - different from a proportion
Rate
a fraction that consists of a numerator and a denominator and in which time forms part of the denominator
the numerator is a subset/part of the denominator
involved a measure of time
the numerator is the frequency of disease
the denominator is a unit size of population, over a specified time period
Rate purpose
Contain the following 3 elements:
1) disease/health event frequency
2) unit size of population
3) time period
Crude rates
summary rates based on the actual number of events in a population over a given time period
Crude birth rates
number of live births within a given period / population size at the middle of that period
General fertility rate
of live births within a year / # of women aged 15-44 years during the midpoint of the year
Infant mortality rate
number of infant deaths among infants aged 0-365 days during the year / number of live births during the year
Neonatal period
neonatal period: birth - 28 days of age
Neonatal mortality rate
number of infant deaths under 28 days of age / number of live births
reflects consequences of perinatal events, primarily:
- congenital malformations
- prematurity (birth before gestation week 37)
- low birth weight (birth weight less than 2,500 g)
Postneonatal mortality rate
reflects environmental events, control of infectious diseases, and improvement in nutrition
Fetal death rate
used to estimate the risk of death of the fetus associated with the stages of gestation
number of fetal deaths after 20 weeks or more gestation / number of live births and number of fetals deaths after 20 weeks or more gestation
Late fetal death rate
number of fetal deaths after 28 weeks or more gestation / number of live births and number of fetal deaths after 28 weeks or more gestation
Perinatal mortality rate
reflects environmental events that occur during pregnancy and after birth; it combines mortality during the prenatal and postnatal periods
Maternal mortality rate
reflects health care access and socioeconomic factors; it includes maternal deaths resulting from causes associated with pregnancy and puerperium (during and after childbirth)
(number of maternal deaths assigned to causes related to childbirth / number of live births (during a year)) x 100,000
per 100,000 live births, including multiple births
Crude rates
use crude rates with caution when comparing disease frequencies between populations
observed differences in crude rates may be the result of systematic factors within the population rather than true variation in rates
Different types of rates
1) crude rates
2) specific rates
3) adjusted rates
Who started natural experiments
John Snow (mid 19th century)
cholera outbreak
John Snow’s natural experiment
linked the cholera epidemic to contaminated water supplies
used a spot map of cases and tabulation of fatal attacks and deaths
observe and record data (quantitative approach)
two different water companies supplied water from the Thames River to houses in the same area
The Lambeth Company moved its source of water to a less polluted portion of the river
during the next cholera outbreak those served by the Lambeth Company had fewer cases of cholera than in prior outbreaks
Natural experiment
the epidemiologist does not manipulate a risk factor but rather observes the changes in an outcome as the result of natural occurring situation
City A: population 500,000 reports 1,000 new cases of prostate cancer in 1-year period
City B: population 50,000 reports 750 new cases of prostate cancer in a 3-year period.
Which city has the higher rate of new prostate cancer cases?
1000/500000 = 0.002
0.002 x 1000 = 2
750/50000 = 0.015
- 015 / 3 = 0.005
- 005 x 1000 = 5
City B has a higher rate of new prostate cancer cases
Limitations of Crude Rates
observed differences may be due to systematic differences (age, gender, race) between the populations rather than true variations in rates
How to correct crude rates
specific rates and adjusted rates
Specific rates
refer to a particular subgroup of the population defined in terms of race, age, sex, or single cause of death or illness
may also examine cause specific morbidity and mortality rates within a subgroup of the population
ex) mortality from HIV among 25-34 year olds
Age specific death rate
(number of deaths in age group / number of persons in age group) x 100,000
Cause-specific rate
(mortality (or frequency of a given disease) / population size at midpoint of time period) x 100,000
Cause Specific Mortality
the number of deaths from a disease or cause divided by the population size at the mid-point of the time period
shows the risk of mortality from a specific cause within that population
IS a measure of risk of dying of a specific disease/cause
Proportional mortality ratio
(mortality due to a specific cause during a time period / mortality due to all causes during the same time period) x 100
the number of deaths within a population due to a specific disease or cause divided by the total number of deaths in the same population during a specific time period
indicates relative importance of a specific cause of death; NOT a measure of the risk of dying of a particular cause
Country A and B have identical populations (1 million) and cause specific mortality for cardiovascular disease (CVD): 5 per 100,000.
Risk of dying from cardiovascular disease is the same for both countries
Yet PMR from CVD is:
16.6% in Country A
50% in Country B
A: overall mortality rate = 30 per 100,000
B: overall mortality rate = 10 per 100,000
How can this be?
Need to know each country’s mortality rate to calculate total number deaths in each community:
A: overall mortality rate = 30 per 100,000
B: overall mortality rate = 10 per 100,000
Need to know total deaths for each country to calculate each country’s PMR
A: 30/100,000 X 1,000,000 (total population)
total deaths = 300
B: 10/100,000 X 1,000,000 (total population)
total deaths = 100
Cause specific mortality for cardiovascular disease is 5 per 100,000. (i.e. 50 per 1,000,000, total number of deaths from CVD in each country.
A: PMR = 50/300 = .166 X 100 = 16.6%
B: PMR = 50/100 = .5 X 100 = 50%
Uses of PMR
determine priorities in health care planning within a population (administrator)
indicate an area for further study (epidemiologist)
Adjusted rates
rates that have been modified statistically to remove the effect of differences in population composition, such as differences in age, sex, or income across various populations to allow comparisons
Direct Method for Age Adjustment
may be used if age-specific death rates in a population to be standardized are known and if a suitable standard population is available
Direct Standardized Mortality Ratio (DSMR)
(age adjusted mortality rate of test communiity / crude mortality rate of standard pop) x 100
ex) (17.0/12.5) x 100 = 136
“the mortality experience of Community C was thirty six percent (36%) higher than that of Community A)
When to age adjust
1) age must be related to mortality (or morbidity)
2) age distribution of the 2 populations being studied must be different
Standardization for mortality rates may also be appropriate for other factors (sex, ethnic group) … IF both conditions apply
1) sex is related to mortality
2) sex distribution of both populations being studied is known and differs
Why should we collect population data?
Estimating parameters of:
- health
- morbidity
- mortality
Census data purpose
provides information on the general, social, and economic characteristics of the US population
administered every 10 years
attempts to account for every person and his or her residence
characterizes population according to sex, age, family relationships, and other demographic variables
Vital statistics
births, deaths, fetal deaths, marriages, divorces
Statistics derived from the vital registration system
mortality statistics
birth statistics: certificates of birth and fetal deaths
Limitations of mortality data
overall death certificates are accurate BUT
stigma associated with certain diseases may lead to inaccurate reporting (underreporting)
Where errors occur:
errorrs in coding
lack of standardization of diagnostic criteria
changes in coding
