4. Health information intro Flashcards
what is EPIDEMIOLOGY the study of
the study of the DISTRIBUTION and DETERMINANTS of DISEASE FREQUENCY in HUMAN populations
and the APPLICATION of this study to control health problems
EPIDEMIOLOGY 5 GOALS
- Identify the CAUSES of disease (aetiology)
- Determine the EXTENT/BURDEN of disease (assess healthcare needs and plan services)
- study the NATURAL HISTORY of disease (prognosis)
- Evaluate EFFECTIVENESS of interventions (prevention and treatment)
- Develop PUBLIC POLICY
4 components of MEASURING DISEASE FREQUENCY
- POPULATION (which group of people)
- NUMBER OF CASES (numerator)
- SIZE OF POPULATION (denominator)
- MEASURE of TIME
what is a POPULATION
the base group from which we count disease frequency
Group of people with a COMMON CHARACTERISTIC
eg
- place of residence
- gender, age
- use of hospital services (catchment population)
- life event (ie surgical procedure, giving birth)
what are the 2 Types of POPULATION and what do they mean
- FIXED
membership based on an EVENT and is PERMANENT
eg hiroshima atomic bomb survivors - DYNAMIC (open)
membership based on a CONDITION and is TRANSIENT
eg. residents of Liverpool, hospital patients
NUMBER OF CASES is the NUMERATOR for all measures of frequency.
what are the PROBLEMS with NUMERATORS:
- WHO has the disease? (CASE DEFINITION)
- Symptoms; subjective, reported by patient
- Signs; objective, observed by clinician
- Tests - HOW do we find the cases? (DISEASE ASCERTAINMENT)
- Routinely collected data
- Specially commissioned data collections
different criteria used to define a case can give different results
SIZE OF POPULATION is the
DENOMINATOR for all measures
FULL or SAMPLE of population
necessary for COMPARISON of disease across populations
what is NECESSARY for ALL measures of disease frequency
TIME
MEASURE OF TIME
how can DISEASE OCCURANCE be measured
- at SINGLE POINT in time
- OVER a PERIOD of time
what is RATIO, PROPORTION and RATE
RATIO : division of two UNRELATED numbers
PROPORTION: division of two RELATED numbers
numerator is a SUBSET of denominator
expressed as %
RATE: division of two numbers
TIME is ALWAYS part of the DENOMINATOR
what is PREVALENCE and what is the calculation
EXISTING CASES
PROPORTION of population who have the disease
PREVALENCE = EXISTING CASES / TOTAL POPULATION
how is TIME in PREVALENCE
- POINT : at a SINGLE point in time (MOST common) or
- Period: at any time during a period
relevant time does not appear in formula but MUST BE STATED using words
what is INCIDENCE
NEW CASES of disease
during a SPECIFIED TIME PERIOD
- DENOMINATOR includes POPULATION AT RISK (initially disease free)
- involves TRANSITION from health to disease,
TIME MUST PASS
what does the DENOMINATOR for INCIDENCE (New cases) include
the POPULATION AT RISK
- excludes people who already have the disease or are immune
what does the DENOMINATOR for INCIDENCE (New cases) include
the POPULATION AT RISK
- EXCLUDES people who already HAVE the DISEASE or are IMMUNE
eg post-natal depression population at risk is females that give birth,
prostate cancer - all adult males who have not removed their prostate
how are PREVALENCE STUDIES vs INCIDENCE STUDIES measured
- Prevalence studies: CROSS-SECTIONAL
people are studied for the presence of a condition at a ‘cross-section’ of time
also called SURVEY if the main measurement is a questionnaire - Incidence studies: COHORT
a sample of people FREE of the OUTCOME of interest is identified and observed OVER TIME to see whether an outcome event occurs
how TIME is incorporated is different in INCIDENCE:
(2 types)
- CUMULATIVE INCIDENCE
Time is DESCRIBED in WORDS along with the number - INCIDENCE RATE
Time is an intrinsic part of the DENOMINATOR
what is CUMULATIVE INCIDENCE and what is the calculation
aka RISK
proportion of a population AT RISK that develops disease over a specified time period
CUMULATIVE INCIDENCE =
NO. NEW CASES during time period /
NO. POPULATION AT RISK at START of time period
time must be stated in words
PROBLEMS with CUMULATIVE INCIDENCE
ASSUMES ALL PEOPLE in the study have been followed for the ENTIRE TIME PERIOD
(population could have changed by some moving away or death from other causes)
NOT a good measure in a DYNAMIC POPULATION or in a FIXED POPULATION that LOSES MEMBERS over time
what does INCIDENCE RATE measure and what is the calculation
measures SPEED at which NEW CASES of disease occur
INCIDENCE RATE =
No. NEW CASES during time period /
TOTAL PERSON-TIME of observation in POPULATION AT RISK
Unit of denominator is ‘PERSON TIME’
What is PERSON-TIME
counting the AMOUNT OF TIME people were AT RISK for (instead of counting people at risk)
when someone develops DISEASE they are no longer ‘at risk’ so NO LONGER contribute PERSON-TIME
each person contributes different amounts of person-time to the denominator
DENOMINATOR for INCIDENCE RATE
HIGH PREVALENCE may result from
- HIGH INCIDENCE (new cases)
- LONG DISEASE DURATION
RELATIONSHIP between PREVALENCE and INCIDENCE
PREVALENCE approx =
INCIDENCE RATE X AVERAGE DURATION
the EPIDEMIOLOGIST’S BATHTUB
flow of water from tap: INCIDENCE
tub of water: PREVALENCE
flow of water through drain: DEATH (MORTALITY)
evaporation of water: RECOVERY or EMIGRATION
what is MORTALITY RATE and how to calculate
INCIDENCE OF DEATH
MORTALITY RATE =
TOTAL NUMBER OF DEATHS /
TOTAL POPULATION AT RISK OF DEATH in the same period
eg still births mortality rate = stillbirths / live births and still births
(denominator: those who have disease and those who don’t)
(technically a proportion)
why look at mortality?
- Reliable data
- Deaths classified according to standardised rules
- Index of Severity of disease
- Easier to obtain than Incidence data
- Proxy (representative) for incidence (when disease is fatal and has short duration)
time scale for Perinatal, Neonatal, Post-Neonatal and Infant MORTALITY
Perinatal: 28 weeks gestation - 7 days after birth
Neonatal: birth - 28 days
Post-Neonatal: 28 days - 1 year
Infant mortality: birth - 1 year
a study that compares different populations is called
ECOLOGICAL STUDY
what is CASE FATALITY RATE* (CFR)
- What % of people with a DISEASE DIE within a certain time period after the disease was DIAGNOSED
CFR =
no. PEOPLE DYING during specified period AFTER DISEASE ONSET OR DIAGNOSIS /
No. PEOPLE DIAGNOSED with DISEASE
- risk of death among those diagnosed
- measure of disease severity
(technically a proportion)
difference between MORTALITY RATE and CFR
mortality rate: denominator is entire population at risk (of disease/death)
CFR: denominator is only those DIAGNOSED with the disease
ie. population at risk = 15
people with disease = 5
death from disease = 2
CFR = 2/5 = 40%
MORTALITY rate = 2/15 = 13.3%
(cumulative incidence = 5/15 = 33%)
what is LIFE EXPECTANCY
AVERAGE LENGTH OF TIME AN INDIVIDUAL IS EXPECTED TO LIVE
if TODAY’S AGE AND SEX-SPECIFIC MORTALITY RATES CONTINUE
- can be calculated from any age, most common at birth
- CANNOT be used to predict individual lifespan
what is HEALTHY LIFE EXPECTANCY
AVERAGE LENGTH OF TIME A PERSON CAN EXPECT TO LIVE FREE OF DISEASE
combines measures of MORBIDITY with MORTALITY
DATA SOURCE for MORTALITY
DEATH REGISTRATIONS (ONS)
DATA SOURCES for MORBIDITY
- Primary care
- Secondary care
- Notifications of infectious diseases & Surveillance
- Disease registers
- Population based Surveys
CRITIERIA for the Quality and Utility of HEALTH DATA
- NATURE of data
- AVAILABILITY of data
- COMPLETENESS of POPULATION COVERAGE
representativeness,
generalizability (external validity)
thoroughness - STRENGTHS vs LIMITATIONS
