Chapter 2 Notes

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20 Feb 2011
ANTC67: Midterm Notes
Chapter 2: Measuring Health and Disease
Defining Health and Disease
¾ The most ambitious definition of health is that proposed by WHO in 1948: ^ZoZ]}(}uoZÇ]oU mental, and social well-
¾ This definition t criticized because of the difficulty in defining and measuring well-being.
¾ A specific cut-off point for an abnormal value is based on an operational definition and not on any absolute threshold.
¾ Diagnostic criteria are usually based on symptoms, signs, history, and test results.
¾ Diagnostic criteria may change quite rapidly as knowledge increases or diagnostic techniques improve; they also often change according
to the context in which they are being used.
Measuring Disease Frequency
¾ Several measures of disease frequency are based on the concepts of prevalence and incidence.
¾ ]u]}o}P]ZÀv}ÇZ}uoPuv}v(]v]]}vuVÁ[µ]vP>[Dictionary or Epidemiology.
Population at Risk
¾ An important factor in calculating measures of disease frequency is the correct estimate of the numbers of people under study.
¾ Ideally these numbers should only include people who are potentially susceptible to the diseases being studied.
¾ The people who are susceptible to a given disease are called the population at risk, and can be defined by demographic, geographic, or
environmental factors.
Incidence and Prevalence
¾ The incidence of disease represents the rate of occurrence of new cases arising in a given period in a specified populations, while
prevalence is the frequency of existing cases in a defined population at given point in time.
¾ These are fundamentally different ways of measuring occurrence.
¾ If incident cases are not resolved, but continue over time, then they become existing (prevalent) cases. In this sense, prevalence =
incidence x duration.
¾ Measuring prevalence and incidence involves the counting of cases in defined populations at risk. Reporting the number of cases without
the reference to the population at risk can be used to five an impression of the overall magnitude of a health problem, or of short-term
trends in a population.
¾ dZu^l_]}(vµ]v}( incidence during a disease outbreak in a narrowly-defined population over a short period
of time.
¾ The attack rate can be calculated as the number o f people affected divided by the number exposed.
¾ A rate is calculated by dividing the number of the cases by the corresponding number of people in the population at risk and is expressed
as cases per 10n people.
¾ Prevalence (P) of a disease is calculated as follows:
P= #of people with the disease/condition at a specified time
# of people in the population at risk at the specified time
¾ Data on the population at risk are not always available and in many studies the total population in the study area is used as an
¾ Prevalence is often expressed as cases per 100 (percentage), or per 1000 population.
¾ P ]Z^}]v}(ÀovX_
¾ /]}u]uu}}vÀv]v}µZ^]}}(ÀovU_oµoZ}ovµu}(vÇ]uµ]vP a specified
period, divided by the population at risk midway through the period.
¾ ^]u]ooÇU^o](]uÀov_]Z}ovµu}(}vlv}Áv}ZÀZZ](}o}u}(Z] lives.
¾ Apart from age, several factors determine prevalence. In particular:
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ANTC67: Midterm Notes
o The severity of illness (if many people who develop a disease die within a short time, its prevalence is decreased)
o The duration of illness (if a disease lasts a short time its prevalence is lower than if it lasts a long time)
o The number of new cases (if many people develop a disease, its prevalence is higher than if a few people do so).
¾ Prevalence studies do not usually provide strong evidence of causality.
¾ Measures of prevalence are, however, helpful in assessing the need for preventive action, healthcare and planning of health services.
¾ Incidence refers to the rate at which new events occur in a population.
¾ In the calculation of incidence, the numerator if the number of new events that occur in a defined time period, and the denominator is
the population at risk of experiencing the even during this period.
¾ dZu}µÁÇ}(oµo]vP]v]v]}oµoÁZ>ooZ^}v-]u]v]vX_Z}v]vZµÇ
population contributes one person-year to the denominator for each year (or day, week, month) of observation before disease develops,
or the person is lost to follow-up.
¾ Incidence (I) is calculated as follows:
I= # of new events in a specified period (x10n)
# of persons exposed to risk during this period
¾ The numerator strictly refers only to first events of disease.
¾ The units of incidence rate must always include a unit of time (cases per 10n and per day, week, month, year, etc).
¾ For each individual in the population, the time of observation is the period that the person remains disease-free.
¾ Since it may not be possible to measure disease-free periods precisely, the denominator is often calculated approximately by multiplying
the average size of the study population by the length of the study period.
Cumulative Incidence
¾ Cumulative incidence is a simpler measure of the occurrence of a disease of health status. Unlike incidence, it measures the denominator
only at the beginning of a study.
¾ The cumulative incidence can be calculated as follows:
Cumulative Incidence= # of people who get a disease during a specified period
# of people free of the disease in the population at risk at the beginning of the period
¾ Cumulative incidence is often presented as cases per 1000 population.
¾ In a statistical sense, the cumulative incidence is the probability that individuals in the population get the disease during the specified
¾ The period can be of any length but is usually several years, or even the whole lifetime.
¾ The simplicity of cumulative incidence rates makes them useful when communicating health information to the general public.
Case Fatality
¾ Case fatality is a measure of disease severity and is defined as the proportion of cases with a specified disease or condition who die within
a specified time.
¾ It is usually expressed as a percentage.
Case fatality = # of deaths from diagnosed cases in a given period .
# of diagnosed cases of the disease in the same period
Interrelationships of the Different Measures
¾ Prevalence is dependent on both incidence and disease duration. It can be calculated approximately as:
P= incidence x average duration of disease
¾ Since incidence usually changes with age, age-specific incidence rates need to be calculated.
¾ The cumulative incidence rate is a useful approximation of incidence when the rate is low or when the study period is short.
Using Available Information to Measure Health and Disease
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