Measures of disease frequency

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Measures of disease frequency

• PH2610 Introduction to Epidemiology
• Spring 2001
• Stephen Waring, DVM, PhD

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Epidemiology basic assumption

• Disease does not occur randomly
• Disease has identifiable causes which can be
  altered, thereby preventing disease from
  developing

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Definition of Epidemiology

• The study of the distribution and determinants
  of health-related states or events in specified
  population, and the application of this study to
  control of health problems.
• source Last (ed.) Dictionary of Epidemiology,
  1995

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Definition of Epidemiology

• Frequency
• quantification of the presence of disease
• prerequisite for any systematic investigation of
  the patterns of occurrence

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Definition of Epidemiology

• Frequency in populations
• Rates and proportions
• Averages
• Dynamics demography, environment, lifestyle

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Definition of Epidemiology

• Distribution
• analysis by time, place, and person
• who, when, where
• how many - counts, ratios, differences

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Descriptive Epidemiology

• Calculating rates of disease occurrence
  (frequency) requires
• COUNT number of individuals experiencing the
  health event (numerator)
• ESTIMATE of the population at risk (denominator)
• Definition and ascertainment of CASES

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Defining case

• Case
• an individual with the outcome of interest to be
  studied (i.e., disease, exposure, event)
• Criteria
• clinical vs. research
• Definition must be clear, concise to afford
  accurate comparison

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Defining case

• Definition
• may be restrictive or broad depending on the
  scope of the study
• Detection and diagnosis
• may be difficult in latent or subclinical
  disease, so diagnostic criteria may be included
  in case definition criteria

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Defining case

• Natural history
• the course (stages) of disease from exposure to
  ultimate resolution without intervention
• critical in detection, diagnosis, establishment
  of causality, and prevention/control

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Defining case

• Natural course of disease
• Exposure ? onset ? symptoms ? dx ? outcome

Incubation period
Recovery Death Chronic disease
Clinical Stage
Subclinical Stage
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Natural History of Disease
Natural History of Disease
The course of disease in an individual over time
The course of disease in an individual over time
Cure
Cure
Control
Control
Disability
Disability
Seek care
Death
Death
Common sources of data
Common sources of data
Source http//www.ph.ucla.edu/epi
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Steps of epidemiologic research

• The research question
• describe EXPOSURE and OUTCOME
• in terms of
• Person
• Place
• Time

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Steps of epidemiologic research

• Example
• Exposure exposure to dirty needles
• Outcome HIV
• Person IV drug users
• Place Houston, Texas
• Time 1/1/2000 to 12/31/2001
• What is the incidence of HIV among IV drug users
  in Houston, TX that is attributable to use of
  dirty needles?

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Steps of epidemiologic research

• To describe and measure the distribution of
  disease in the population
• Define a case / ascertain (count) cases
• Describe who has the disease
• Describe when and where the disease occurred
• Finally, compare outcomes among groups

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Epidemiologic Data Sources

• Mortality Statistics
• Birth Records
• Reportable Disease Statistics
• Screening Surveys
• Disease Registries
• Morbidity Surveys

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Epidemiologic Data Sources

• Health Insurance Statistics
• Life Insurance Statistics
• Hospital Inpatient Statistics
• School Health Programs
• Armed Forces Morbidity
• Veterans Statistics
• Social Security Statistics

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Epidemiologic Data Sources

• US Census Data
• www.census.gov
• State of Texas
• www.tdh.state.tx.us/dpa/ popdata
• City of Houston
• www.ci.houston.tx.us/census

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Birth Statistics

• Certificates of Birth and of fetal death
• details of pregnancy
• details of delivery
• demographic information
• congenital malformations,
• birth weight
• length of gestation

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Birth Statistics

• Depend on maternal recall
• Certain malformations affecting the neonate may
  not be detected at birth
• 24 hour stays decrease likelihood many anomalies
  will be identified

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Fetal Death

• Variation from state to state on what is required
• Differences limit comparability across states

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Mortality Statistics

• Almost complete
• Death certificate data in US includes
• demographic information
• cause of death
• immediate
• contributing

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Mortality Statistics

• Limitations
• Cause of death may be unclear
• Cause of death may be arbitrary
• Lack of standardization of diagnostic criteria
• Stigma associated with certain diseases
• AIDS, suicide, alcoholism

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Mortality Statistics

• Words are transcribed into codes
• International Classification of Diseases and
  Related Health Problems (ICD)
• If disease spans more than one version of the ICD
  - codes and groupings may change

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Mortality Statistics

• Sudden increases or decreases in a death rate may
  not be real
• May reflect a change of coding systems

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Measures of disease frequency

• Counts
• Measures of morbidity
• Prevalence
• Cumulative incidence
• Incidence rate
• Measures of mortality
• Mortality rate
• Standardized mortality rate (SMR)

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Why study rates / ratios

• Epidemiology is the study of the distribution and
  determinants of disease in human population
• We describe distribution by
• Person, Place, Time
• We do this to try to understand WHY disease
  development is not random

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Counts

• Prerequisite for epidemiologic investigation
• Simplest measure of disease frequency
• Frequency of affected individuals
• Useful for planning adequacy of health care
  allocation at a particular level (local,
  regional, national, international)

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Ratio

• A fraction with no specified relationship between
  numerator and denominator
• Range 0 to ?
• A/B
• Examples
• sex ratio (MF) wt-ht index
• relative risk (affectedunaffected)
• risk ratio
• odds ratio

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Proportion

• Type of ratio
• Numerator included in denominator
• May be expressed as percentage
• Range 0 to 1
• A/(AB)
• Example
• Prevalence (always a proportion)
• Cumulative Incidence (proportion at risk)

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Proportions

• Ratio
• numerator is always part of the denominator
• Common proportions used are percentages
• Assess the relative importance of an attribute in
  the population
• Prevalence
• proportion of population with the attribute

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Prevalence (a proportion)

• Proportion of the population at a given time that
  have the factor of interest
• Prevalence of an exposure
• what proportion of this class grew up drinking
  fluoridated water
• Prevalence of outcome
• what proportion of this class have dental caries

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Rate

• A special type of proportion
• Unit of time in denominator
• Best for comparison of disease frequencies among
  subsets of population
• Common denominator population from which cases
  came

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Population Measures

• Cases
• Deaths
• Events

Numerator
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Prevalence

• All individuals with a disease at a given point
  in time
• A proportion
• Dimensionlessshould not be described as a rate
• number of cases (A)
• P
• total population (AB)

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Prevalence

• Types
• Point prevalence proportion of all cases at a
  specific point in time
• Period prevalence proportion of all cases
  during a period of time
• Frequency of disease at autopsy all cases among
  those autopsied
• Birth defect rate number of new births with
  abnormality among all live births

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Prevalence

• Utility
• Describe health burden of a population
• Status of disease in a population
• Estimate the frequency of exposure
• Project health care needs of affected individuals

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Prevalence

• Limitations
• All cases with a disease at a given point in time
  are a product of
• Factors influencing development of disease
• Factors influencing survival (duration, severity)
• Difficulty in establishing antecedent events

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Incidence

• Rate of development of disease during a given
  period of time
• Three key elements
• Only new cases included in numerator
• Total population at risk in the denominator
• Time element period over which new cases
  developed

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Incidence

• Types
• Cumulative Incidence estimate of probability
  (risk) that an individual will develop disease
  during a specified period of time
• new cases in a given period of time
• IR (CI)
• total population at risk

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Incidence

• Types
• Incidence Density (ID) measure of the true rate
  of disease development
• True force of morbidity or mortality
• new cases in a given period of time
• ID (IR)
• total person-time of observation

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Incidence

• Measuring occurrence of disease in a population
  over time either by
• Measure number of persons at risk - CI
• Measure time spent at risk IR (ID)

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Calculating person-time
x
x
ID 2/17 11.8/100 person-years of observation
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Crude rates

• Overall measurement of the event
• does not take into consideration heterogeneity of
  population (variation due to age, gender,
  race/ethnicity, residence, socioeconomic status)
• cases in a specified time period
• CIoverall
• population at risk in that time period

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Crude rates

• Birth Rate total live births / total population
• to project population changes
• affected by the and age composition of women of
  childbearing age
• Fertility Rate - live births / women 15-44 yrs
• for comparisons of fertility among age, racial,
  and socioeconomic groups
• Infant mortality rate infant deaths / live
  births
• for international comparisons a high rate
  indicates unmet health needs and poor
  environmental conditions

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Crude rates

• Fetal Death Rate
• fetal deaths (gt20 wks)/live birthsfetal deaths
• to estimate the risk of death of the fetus
  associated with the stages of gestation
• Neonatal Mortality Rate
• Deaths (lt28 days of age)/ number of live births
• Risk of dying among infants lt28 days
• reflects events present at birth
• 1. congenital malformations
• 2. prematurity (birth before 28 weeks gestation)
• 3. low birth weight (weight less than 2,500 g)

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Crude rates

• Cause Specific Mortality Rate
• reflects the burden one particular disease has on
  total mortality
• Accidents
• HIV infection
• Cancer
• Suicide

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Crude Rates

• Perinatal Mortality Rate
• reflects events that occur during pregnancy and
  after birth
• it combines mortality during the prenatal and
  postnatal periods

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Crude Rates

• Maternal Mortality Rate
• reflects health care access and socioeconomic
  factors
• it includes maternal deaths resulting from causes
  associated with pregnancy and during and after
  childbirth

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Strata Specific Rates

• If we wanted to compare mortality or morbidity
  among specific sub-groups of the population
• we would need to use stratified rates
• so that we take into account the differences in
  the proportion of the population each subgroup
  represents
• Strata specific rates are better indicators of
  risk than crude rates

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Strata Specific Rates

• Rather than the whole population, rates are
  presented by categories
• Outcome is grouped by a particular trait
• CRUDE RATE may hide information
• without examining the strata specific rates, one
  cannot appreciate that risk of an adverse event
  is different in specific segments of the
  population

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Category-specific rates

• Analysis based on characteristics of PERSON
• Age
• Sex
• Race/ethnicity
• Marital status
• Nativity and migration
• Religion
• Socioeconomic Status

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Category-specific rates

• Stratifying on PERSON characteristics provides
  better description of who is at risk
• plan interventions
• prescribe program needs

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Incidence of Guam neurodegenerative disease
(ALS/PDC)
Amyotrophic lateral sclerosis/parkinsonism-dement
ia complex
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Age-specific death rates (U.S.)
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Syphilis Ethnicity and gender
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Category-specific rates

• Analysis based on characteristics of PLACE
• International
• Geographic Variations
• Urban/Rural differences
• Localized occurrence of disease

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Category-specific rates

• Utility of analysis by PLACE
• Serve as an aid in establishing priorities for
  allocation of resources
• Serve as an aid in developing policy for
  community health and social programs
• Identifies high- and low-risk areas
• Provide a reliable method of presenting data to
  concerned groups

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Category-specific rates

• Utility of analysis by PLACE
• Provide a basis for determining the health status
  of the population
• Aid in establishing priorities for allocation of
  resources
• Aid in developing policy for community health and
  social programs

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Geographic Stratification Heart Disease
Mortality, 1998
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Geographic and Race/ethnicity Heart Disease
Mortality, 1998
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Chlamydia Percent Infected at Screening
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Category-specific rates

• Analysis based on characteristics of TIME
• Cyclic fluctuations
• Point epidemics
• Secular time trends
• Cohort effects
• Clustering

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Chlamydia TIME number of cases and rates
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Category-specific rates

• Limitations
• Category-specific rates may be confusing when
  stratifying on multiple variables such as age,
  race, gender
• Tables become incomprehensible
• Use of adjusted rates preferred in these
  situations

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Proportionate Mortality

• Number of deaths due to a specific disease
  proportionate to all deaths in a population
• Not a measure of the risk of dying from a
  particular disease
• Relative importance of a particular disease
• Useful for health care planning and indicating
  areas for further study

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Proportionate Mortality
Diseases of the Heart US, 1998
724,859 DOH deaths
0.31
Proportionate Mortality proportion of deaths due
to a specified cause
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Interrelationship between prevalence and incidence

• Proportion of individuals with disease and rate
  of occurrence of new cases are closely correlated
• Prevalence (P) is a product of both Incidence (I)
  and duration/survival (D)
• If know two, can estimate the third
• P ID

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Interrelationship between prevalence and incidence

• Example
• Prevalence 23/100,000
• Incidence 6/100,000
• Duration (survival) 3.8 yrs

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Interrelationship between prevalence and incidence

• Relationship

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Descriptive Studies-example

• Outbreak investigation
• Food-borne disease associated with church picnic
• illness 6 to 24 hours after a church picnic
• vomiting
• diarrhea
• chills
• nausea
• fever

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Descriptive Studies-example

• Case definition
• vomiting (more than 2 times in 24 hours) 6 or
  more hours after picnic
• nausea, chills and
• diarrhea (more than 3 bowel movements in 24
  hours) 6 or more hours after picnic

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Descriptive Studies-example

• Who, When, Where, What
• How many attended picnic?
• How many got sick?
• When did the people arrive, eat, drink?
• What did the people eat/drink?

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Descriptive Studies-example

• 80 attended
• 55 got sick within 6 to 10 hours
• Ill
• potato salad 55/55
• ham sandwiches 10/55
• hamburgers 4/55
• chicken salad 2/55

This can help us determine what the sick
individuals ate, but can it help us identify the
probable cause of the illness? What is missing?
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Descriptive Studies-example

• 80 attended
• 55 got sick within 6 to 10 hours
• Ill Not Ill
• potato salad 55/55 25/25
• ham sandwiches 47/55 8/25
• hamburgers 4/55 13/25
• chicken salad 2/55 2/25

Would percentages or rates be easier to compare?
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Descriptive Studies-example

• 80 attended
• 55 got sick within 6 to 10 hours
• Ill Not Ill
• potato salad 55/55 100 25/25
  100
• ham sandwiches 47/55 85 8/25
  32
• hamburgers 4/55 7 13/25
  52
• chicken salad 2/55 4
  2/25 8

What was the culprit?
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Incidence, Prevalence
X
X
What was prevalence of disease in 1992?
Prevalence 1 case/ 4 subjects 25
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Incidence, Prevalence
X
What is risk of developing disease within 2
years?
Cumulative incidence 1/6 17
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Incidence, Prevalence
X
X
What is the incidence rate?
Incidence rate 2/15 person-years 0.13
13 13/100 person-yrs 133/1000 person-yrs
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Incidence denominator issues
Uterine cancer rates, 1975-1990
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Incidence denominator issues
Uterine cancer rates, 1975-1990

• Population at risk?
• Women with uterus
• Denominator should only include women without
  hysterectomy

Corrected
Uncorrected
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Conclusion

• To measure the occurrence of disease in the
  community, you need to
• understand the natural history of disease
• use concepts of prevalence and incidence to
  describe disease patterns in communities
• locate sources of disease and death
• appreciate effects of denominator changes on
  population-based rates