PHC6001 Chapter 2: Measures of Disease Frequency
Standardized mortality rate
# observed deaths / # expected deaths 1 = no difference >1 study pop experiencing higher than expected rate <1 study pop experiencey lower than expected rate
population cross section
(lecture) individuals at a point in time
population experience
(lecture) individuals moving through time
summary age adjusted rate
-is the disease/death rate in the study population *IF* the study population had the same distribution as the standard population -can compare (this) from different populations *IF* they have been adjusted to the *SAME* standard population.
incidence
1. measures the occurrence of *new disease* events or *outcome occurrence* 2. measured within a candidate population ("at risk") 3. focuses on the period of *time* of transition from health to disease (or time where new cases develop)
prevalence and Incidence issues
Numerator: 1. What is a case? 2. When is a case a case? 3. What to do with multiple episodes of disease? Denominator: 1. Who is at risk? 2. Are there competing risks? 3. How to treat previously occurring disease in same individual?
health
WHO in 1948: "a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity"
population
a group of people with a common characteristic, such as a place of residence, religion, gender, age, use of hospital services, or life event (such as giving birth); most common way to define is location of residence: country, state, city or neighborhood.
person - time
accrued only among candidates for the disease and only up until he/she is diagnosed with disease; only accrued when person is being followed and stops when person dies or is lost to follow up
denominator
appropriate (this) measures the *population at risk* for outcome of interest
lifetime risk of breast cancer
commonly cited measure of *cumulative incidence* among women where it is estimated at one in eight among US women, so about 12% will develop this over the course of their lives.
age standardization
creates a summary rate which removes the effect of age distribution differences a *fictional* rate that does not reflect the true experience of the population -direct and indirect methods: dependent on types of data available and different interpratations, limitations
length of time
cumulative incidence can be influenced by ___________ ___ ___________: 1. the number of candidate population dwindles over long periods of time 2. if not selected appropriately, may create an assumption of equality when that isn't the case (i.e. migrain med example in book) 3. if a long follow up period, particulary in *dynamic population* is difficult to attain
population at risk or candidate population
cumulative incidence has a denominator of (this)
new cases
cumulative incidence has a numerator of (this)
0 to 1
cumulative incidence has a range of __________
fixed
cumulative incidence is usually reserved for these types of populations becasue the follow up period is short and there are no or few losses to follow up
cumulative incidence
defined as the *proportion* of a candiate population that becomes diseased over a specified period of time or *number of new cases of disease/number of candidate population over a specified time*; *value of 0-1* average *risk of population* getting a disease over a certain period of time major uses of (this) is research on *1. causes 2. prevention 3. treatment of disease*
Objectives of Epidemiology
determine extent of disease in a population, identify patterns and trends in disease occurrence, identify the causes of disease, and evaluate the effectiveness of prevention and treatment activities. Measuring how often a *disease* arises in a population is usually the first step in achieving these goals.
counts
enumeration of an outcome of interest; sufficient for extremely rare situations (i.e. rabies)
disease occurrence
epidemiologists always consider three factors- 1. *number* of people who are *affected* by the disease 2. the *size of the population* from which the cases of disease arise 3. length of *time* that the population is followed; failure to consider all three will give a false impression about the effect of the disease on a population
disease
epidemiologists study the distribution an determinants of (this) frequency in human populations to control health problems.
first
for diseases that occur more than once, incidence usually measures the _____________ occurrence of the disease.
morbidity
general word that can apply to a disease, condition or event.
catchment population
group of people who use a facility's service, like medical a medical facility (doctor's office, clinic or hospital); difficult to define because a person's decision to use a facility may depend on how far it is from home, their medical condition, person's insurance, etc. -if facility offers enhanced services people may come from surrounding regions to utilize for solely that service -socioeconomic status determinant; public v. private hospital use
dynamic/open population
group of people who's membership to the group is changeable or transient in nature; include: groups defined by geographic and hospital catchment areas, religious groups and occupations.
fixed population
group of people who's membership to the group is permanent; always defined by a life event
indirect adjustment
have limited information on the study population and study population is small and consequently the age specific rates are unstable -Final measure is NOT a summary rate, but rather a ratio: SMR -cannot scompare SMRs across study populations-the only appropriate comparison is between the study and standar populations
direct adjustment
identify the study population and identify the standard population (large, established population, one of the study populations, pool the study populations)
incidence
lecture: measure of disease or outcome occurence; number of new events occurring in a specified period in defined population; excludes prevalent cases
incidence density
lecture: number of new events ina specified period divided (most properly) by the accumulated person-time of observation -accounts for variable time to event (# of events / total number of person-time of observation) -accounts for loss to follow up (but not potential bias due to loss) Examples 1. post MI pt. mortality (# deaths/person days) - numerator same as CI 2. work rel injuries per 100,000 work hours among deep mine workers 3. # TB cases / person - years of risk of TB among the homeless TECHNICAL DIFFICULTIES: *SOMETIMES DIFFICULT TO EXPLAIN CONCEPTUALLY* *OFTEN CONVERTED TO RISK LIKE WORDING* *WAITING TIME*: UNDER STEADY STATE CONDITIONS, RECIPROCAL OF THIS SO IF 3.57 CASES/PERSON-YEAR THEN WAIT 3 MONTHS FOR ONE CASE OF OUTCOME
cumulative incidence
lecture: quantified by an incidence proportion; aka "incidence proportion" or *"risk"* proportion of *fixed* population experiencing a new event during a specified period of time Assumes: 1. follow up for entire period 2. static population Examples: a. 30 day mortality of MI patients b. monthly risk of work related injuries among deep mine workers in west virginia c. annual incidence of TB among homeless in Miami TECHNICAL DIFFICULTIES: *COMPETING RISK* *MULTIPLE EVENTS* *DYNAMIC POPULATIONS* *LOSS TO FOLLOW UP*
crude
means a rate is based on raw data
prevalence proportion
measure of disease or outcome status regardless of onset: -the proportion of a specified population that is affected by the outcome of interest at a given time -can be measured on different time scales
prevalence
measure the existence or *frequency* of current disease; focuses on the period of time a person lives with a disease; *proportion* of the total population that is diseased; denominator includes everyone (sick, healthy, risk, not at risk, etc.); dimensionless with possible values rom 0-1 or 0-100% -affected by: *1. number of new cases* *2. duration of health related states* *3. rate of recovery and/or health*
time
more than one scale may exist: calendar, age, employment, since exposure, since conseption
candidate population
new cases of disease/incidence are measured in this type of population - (this) is a population of people who are "at risk" of getting the disease; someone at risk has the appropriate body organ, is not immune, etc.
birth defect rte
number of children born with defects, usually per 10,000 births; numerator usually includes both livebirths and stillbirths.
cause - specific mortality rate
number of deaths from a specific cause per 100,000 population per year.
infant mortality rate
number of deaths of infants less than 1 year of age per 1000 livebirths per year.
case fatality rate
number of deaths per number of cases of disease over a period of time lecture: proportion of people, among those who develop a disease, who then proceed to die from the disease; often no time perriod stated making interpretation difficult; often used for infectious diseases inferring short time period; type of cumulative incidence
crude rate
number of events / total population at risk -used for: 1. international comparisons (i.e. morbidity and mortality) 2. comparisons between states, regions, cities 3. workers vs. general population 4. exposed groups versus unexposed groups -issues: *death and dz usually associated with age* *age structures may be different* *other factors like race may be different*
morbidity rate
number of existing or new cases of a particular disease or condition per 100 population. Time period that is covered and the population size in the denominator vary.
survival rate
number of living cases per number of cases of disease. Is the complement of the case fatality rate and is also a cumulative incidence measure.
gender ratio
number of males divided by number of females expressed as the number of males per 100 females
attack rate
number of new cases of disease that develop (usually during a defined and short time perriod) per the number in a healthy population at risk at the start of the period. This cumulative incidence measure is usually reserrved for infectious disease outbreaks. LECTURE: incidence proportion of becoming afflicted with a condition during an epidemic period; often used in infectious disease circles; type of cumulative incidence
incidence rate
occurrence of new cases of disease that aris during *person-time* of observation; can be calculated for either a fixed or dynamic population because it takes into account population changes and is especially useful for measuring the transition between health and disease in *dynamic populations*. lecture: aka *incidence density*
rate
one number divided by another, with *time* being an integral part of denominator. Example: 55miles/hour.
proportions (fractions)
one related number divided by another; numerator is always a "subset" of the denominator. usually expressed as percentages.
ratios
one unrelated number divided by another. i.e. number of individuals in numerator can be different than the denominator. *typically used in epidem. - account for the population at risk and desireable statistical properties
subgroups
populations can be further divided into (this) con the basis of any characteristic, i.e. age, gender
livebirth
pregnancy that results in a child who ater separation breathes or shows any other evidence of life.
age specific rates
problem: too many rates to draw meaningful conclusions
odds
ratio of the probability of an event occurrence to the probability of non-occurrence (or ratio of two different probabilities)
period prevalence
refers to the proportion of the population that is diseased during a specified duration of time, such as during the year 2017; includes number of case that were present at any time over the course of the year. equation: number of cases of disease or other outcome during a specified period of time, divided by (usually) the total specified population at the mid-point of the time period
point prevalence
refers to the proportion of the popultion that is diseased at a single point in time and can be thought of as a single sapshot of the population; can be a calendar date (i.e. birthday) or point in someone's life (i.e. graduation) lecture: the number of cases of the outcome of interest at a given time, divided by the total specified populationofthe time of outcome ascertainment
steady state
situation where the number of people entering the population is equal to the number leaving.
static population
size of cohort and composition stay the same
adjusted (standardized) rate
the hypothetical event rate that would have occurred had the observed rates prevailed in a population with a distribution of that of the standard population. - adjusted rates not from real population, can mask important differences
neonatal deaths
those deaths occurring during the first 27 days following birth
postnatal deaths
those occurring from 28 days through 12 months
ratios, proportions and rates
three types of calculations used to describe and compare measures of disease occurrence
years of potential life lost
tota number of years that an individual was expected to live beyond his or her death; reflects both the number of individuals who died of a particular cause and the age at which the death occurred.
age - specific mortality rate
total number of deaths from all causes among individuals in a specific age category per 100,000 population per year in the age category. *excellent measure for comparisons because it removes the effect of age across populations*
crude mortality (or death) rate
total number of deaths from all causes per 100,000 population per year.
livebirth rate
total number of livebirths per 1000 population per year
incidence and prevalence
two basic measures of disease frequency in epidemiology are .....
mortality rate
type of incidence rate; total number of deaths from ll causes per 100,000 population. Usually expressed for a 1-yr period.
proportion
type of number of cumulative incidence
incidence rate formula
units expressed as 1/time