Lecture 2:Population Health and Disease Measures

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Epidemiology

• Lecture 2 Population Health and Disease Measures

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Learning outcomes

• Describe measures of disease occurrence and
  frequency of health events
• Calculate measures of disease frequency
• Interpret epidemiologic information related to
  these measures

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Measurement Comparison

• To find out whether a community is healthy or
  unhealthy
• first measure one or more indicators of health
  (deaths, new cases of disease, etc)
• compare the results with another community or
  group.

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Measurement Comparison

• Epidemiological principles are used to ensure
  that the study is conducted in an accurate and
  reliable way by
• Selecting the best study population and
  comparison group
• Reducing or controlling for errors in measurement
  of the exposure and the outcome

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Types of Measures

• Epidemiology is concerned with presence of health
  problems or the occurrence of new health events
  in a population.
• The most common types of measures are
• counts
• ratios
• proportions
• rates

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Types of Measures

• Count - number of persons who have a particular
  disease
• Around 171,000 people in NSW have diabetes
• An estimated 13,120 people were living with
  HIV/AIDS in Australia in 2002.
• Ratio - (includes proportion, percentage and
  rate) - is obtained by dividing one quantity by
  another. There is no specific relationship
  between the numerator and the denominator.
• eg Number of hospital beds per 1000 population

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Types of Measures

• Proportion
• a type of ratio in which the numerator is
  included in the denominator
• The ratio of a part to the whole, expressed as a
  decimal fraction'' (e.g., 0.2)
• as a fraction (1/5) or
• as a percentage (20). eg 3.6 NSW population
  have diabetes

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• Rate
• An expression of the frequency with which an
  event occurs in a defined population.
• There is a relationship between the numerator and
  the denominator and a specified time period.
• Example 0.36 per 1,000 people reported having
  diabetes in 1997-98

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Measures of Disease Frequency

• When measuring it is important in epidemiology to
  define
• What is being measured?
• Person ie individuals included
• Place or location of the study population
• Time period of the study

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Measures of Disease Frequency

• Example
• What Vaccination status
• Person School children 7- 10yrs
• Place or location live in Western
  Sydney
• Time period 2003

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Measures of Disease Frequency

• This helps to
• define the population at risk
• clarify relationships between the numerator and
  denominator for the calculation of rates and
  ratios
• identify potential associations between outcome
  and the factors being studied
• Do stroke rates differ by geographical location?
• Does the prevalence rate of AIDS differ by
  geographical location?

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The key measures of disease occurrence and health
events

• Incidence
• Cumulative Incidence
• Incidence Rate or Incidence Density
• Prevalence
• Point Prevalence
• Period Prevalence

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Incidence

• Incidence Number of new cases/events in a
  population, over a period of time.
• Example
• the incidence of AIDS in the Australian
  population was 178 in 2001
• ie 178 new cases of AIDS were diagnosed in
  Australia in 2001.

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Incidence

• There are 2 main ways incidence is reported
• Cumulative Incidence or attack rate
• Incidence rate or incidence density

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Cumulative Incidence

• Cumulative incidence (CI) is the proportion of
  people in a population who became diseased or ill
  or experienced an event during the specified
  period of time.
• CI No. new cases of disease or events
  during time period Total population at risk at
  the beginning of the time period

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Cumulative Incidence

• Two assumptions when calculating Cumulative
  Incidence
• entire population at risk has been followed from
  the beginning of the study till the end
• all participants are at risk of the outcome of
  interest

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Cumulative Incidence

• Example 1
• The daily incidence of chickenpox in first grade
  children at Booral primary school during the 1998
  epidemic was 10 new cases per 100 children.
• If there were 200 children in the first grade,
  how many new cases would there be each day?
• 20

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Cumulative Incidence

• Example 2
• 818 women had in vitro fertilization in NSW
  during 2000
• 80 developed a clinical pregnancy within one
  month of follow-up after the first embryo
  transfer procedure. The CI of pregnancy was??
• CI 80/818
• 9.8 cases per hundred women on the program

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Attack Rate

• Attack rate is a type of cumulative incidence
  applied to a narrowly defined population observed
  for a limited period of time, such as during an
  epidemic.
• Attack rate
• No new cases of illness during a specified
  time period
• Total population at risk during that
  specified period

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Attack Rate - Example

• The Public Health Unit was called in to
  investigate more than 20 reports of people being
  ill with gastroenteritis after eating at a large
  restaurant in Parramatta during the first week of
  April 2002.
• An investigation was conducted interviewing all
  patrons who ate at the restaurant during that
  week
• They found 2000 patrons ate at the restaurant
  that week and 400 became sick.
• What was the attack rate?
• Attack rate 400/2000
• 20 ill per 100 patrons

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Incidence Rate (Incidence Density)

• The incidence rate or incidence density is the
  number of new cases in a population divided by
  the total time units each individual in the
  population at risk was observed.
• Incidence Rate
• No new cases of disease/events during the
  specified time period
• Sum of the length of time during which each
  person in the population is at risk

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Incidence Rate - Example(Incidence Density)

• In investigating the incidence of duodenal ulcer
  following the use of a specific drug in 14
  subjects.
• 4 subjects started the study in Jan 1990 and all
  finished the study in Dec 1999.
• Ten subjects joined the study in Dec1995 and
  finished the study in November 1996.
• During the period of observation 5 people
  developed duodenal ulcer while taking the drug.

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Incidence Rate (Incidence Density)

• What is total length of time that persons were
  in the population is at risk (denominator)?
• 4 X 10 years 40 years
• 10 x 1 year 10 years
• Total 50 person-years
• New cases 5
• What is the incidence rate of duodenal ulcers
  after taking the drug?
• Incidence rate 5 / 50
• 10 cases per 100 person-years

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Incidence Rate

• Can be presented in many different ways
• 10 cases/1000 person-years
• 1 case/100 person-years
• 0.1 cases/10 person-years
• 0.01 cases/1 person-year
• These are all the same

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Incidence Rate

• In many circumstances, you can assume
• that entry and exit from the population occurs
  evenly over the time period, or
• you only know the average population at risk,
  an approximate incidence density rate can then be
  estimated as
• Incidence Rate
• No new cases of disease/events during the
  specified time period
• (Initial population at risk final population)
  /2 in the time period

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Mortality Rate

• Mortality Rate the incidence of deaths from all
  causes (all cause mortality rate) for the
  Australian population in one year
• All Cause Mortality Rate 2001
• No. new deaths during 2001
• Total Aust.population at risk
• midyear 2001

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Mortality Rate - Example

• If there were 50,000 deaths in one year and
  20,000,000 people living in Australia what will
  be the mortality rate for Australia in that year?
• Mortality rate 50,000/20 millio
• 2.5 deaths per 1000 pop
• 250 deaths per 100,000 pop

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Mortality Rate - Example

• Annual Mortality Rate for Lung Cancer per 1,000
  population
• No. new deaths from lung cancer per year
• No. of persons in the population at mid year
• X 1,000

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Mortality Rate - Example

• We can place restrictions on more than one
  characteristic simultaneously eg age and cause of
  death
• No. new deaths from leukemia per year in children
  under 10 years
• No. of children under 10 years in the population
  at mid year
• X 1,000
• Note Time must be specified in a mortality rate.
  
• Mortality can be calculated over 1 year, 5 years,
  or longer.

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Case fatality rate (percent)

• Case fatality rate
• No of individuals dying during a specified
  period of time after disease onset or diagnosis
  X 100
• Total number of individuals with the disease
  during that specified period

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Case fatality rate - Example

• Assume a population of 100,000 people of who 20
  are sick with disease X, and in 1999, 18 die
  from the disease.
• What is the mortality rate?
• 18/100,000 died of disease X in 1999
• 0.018 per 1,000 pop in 1999
• What is the case-fatality rate?
• 18/20
• 90 or 90 per 100 people with X

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Prevalence

• Prevalence is the proportion of a defined
  population with the disease/event of interest at
  a specified time period.
• Prevalence is a snap shot of the disease
  frequency at a point in time
• Point prevalence is the most common measure of
  prevalence
• Prevalence is often established by
  cross-sectional surveys

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Prevalence

• Point Prevalence Total number of the population
  with the disease/event at a particular time
• Total population at that time

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Prevalence

• An incident case becomes a prevalent case and
  remains a prevalent case until recovery or death.
• Where a population is in a steady state,
  prevalence depends on incidence and duration of
  disease.
• Thus prevalence of a disease may increase when
  incidence remains stable but survival of cases
  improves

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Period Prevalence

• Period Prevalence
• Number of the population with the disease/event
  at any time during a specified period
• Total population during that period

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Factors influencing prevalence rate
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Relationship between Incidence and Prevalence

• In a steady-state population, prevalence depends
  on
• incidence of the condition (as all incident cases
  become prevalence cases) and
• duration of condition
• The relationship can be can be expressed as
  follows
• Point prevalence
• Incidence density x Duration of
• condition

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Use of Incidence and Prevalence

• If one wishes to look at a change in disease (eg
  studies of causality, acute conditions or events,
  outbreak investigation) ? use incidence.
• For example Looking at the change in the
  incidence of cancer is important to know whether
  current prevention, screening and treatment
  activities are working.

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Use of Incidence and Prevalence

• Prevalence is used when looking at the magnitude
  of existing diseases usually chronic disease like
  diabetes where change does not occur rapidly
• Often both measures are used

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Diabetes Prevalence, US (CDC)
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Trends in Diabetes Prevalence, US (CDC)
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Comparing disease and health event rates

• Disease and mortality rates can be used to
  compare
• disease/mortality rates between populations
• the same population over time.
• When comparing rates we should be aware of the
  characteristics of the population(s) that may
  affect the rates.
• The age and sex structure of two populations
  being examined may differ and these differences
  will affect the rates.

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Comparing disease and health event rates
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Australian population pyramid
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Australian indigenous population pyramid
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Comparison of crude death rates for indigenous
Australians and all Australians in 1995-97

• How do the population structure of the indigenous
  Australian population and the total Australian
  population compare?

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Comparison of crude death rates for indigenous
Australians and all Australians in 1995-97

• Are there differences between the crude deaths
  rates for indigenous Australian compared with all
  Australians?

(AIHW, 2001)
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Comparison of crude death rates for indigenous
Australians and all Australians in 1995-97

• However we know
• death is closely related to age
• the age structure of the Indigenous population is
  very different to that of the total population
• Therefore we need to adjust for the effect of the
  age structure to make a meaningful comparison.
• This can be done using standardization

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Standardization

• There are 2 main forms of standardisation
• Direct
• Indirect and the calculation of standardised
  mortality ratios (SMRs)

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Direct Standardisation

• is used to compare large populations
• uses a standard reference population to compare
  both populations
• applies the age-specific disease/death rates of
  the population of interest to the standard
  population
• allows us to compare death rates, by calculating
  what their death rates would be if the
  populations of interest had the same age
  population structure as the reference population.

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Applying Direct Standardisation

• Example Comparison of Death rates in NZ between
  the Maori population and the non-Maori population.

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Steps on Direct Standardisation

• 1. Select standard population NZ Pop. 1966 (2)
• 2. Identify Populations of Interest Maori
  population and the non Maori Population
• Crude death rates are 6.37 and 9.07
• 3. Identify age groups breakdown (1)
• 4. Calculate the age-specific death rates for
  each of the populations you wish to compare
  (Maori population and the non Maori
  Population)no. of deaths /population for the age
  group

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Steps on Direct Standardisation

• 5. Calculate the expected deaths for the Maori
  and non-Maori populations by multiplying the
  age-specific death rates of each of the Maori and
  non-Maori populations by the standard population
  (5) x (2) (6), (7) x (2) (8)
• 6. Standardised death rate the population of
  interest is calculated by summing the expected
  number of deaths for that population and divide
  it by the total standard population. (d) / (a)
  (e) / (a

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Indirect standardization and SMRs

• Adjusts for differences in age by calculating the
  number of deaths expected in a population,
  based on its age structure, if it had the same
  mortality experience as a reference population.

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Applying Indirect Standardisation

• Example Comparison of Death rates in NZ between
  the Maori population and the non-Maori
  population.
• 1. Select standard population NZ Pop. 1966 (2)
• 2. Identify Populations of Interest Maori
  population and the non Maori Population
• Crude death rates are 6.37 and 9.07
• 3. Identify age groups breakdown (1)
• 4. Calculate the age-specific death rates for the
  standard population

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Applying Indirect Standardisation

• 5. Calculate expected deaths for the Maori
  population by multiplying the age-sex specific
  rates for the reference population by the
  corresponding age-sex specific populations for
  Maori population
• 6. Calculate total expected deaths by summing the
  expected deaths for Maori population

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Applying Indirect Standardisation

• 7. The Standardised Mortality Ratio (SMR) for the
  Maori population is calculated by dividing the
  actual number of deaths for Maori population by
  the expected number of deaths for Maori
  population x 100.
• 8. Repeat 5-7 using the Non Maori population as
  the population of interest

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Applying Indirect Standardisation

• Note If the SMR is greater than 1.0, there were
  more deaths than expected if the ratio is less
  than 1.0, there were fewer than the expected
  number of deaths.
• An SMR value of 1.67 indicates death rates 67
  above that of the reference group or 1.67 times
  higher than expected, whereas an SMR of 97.7
  indicates separation rates 2.3 below that of the
  reference group.
• Standardisation- allows valid comparisons between
  populations with different age and sex structures

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Comparison of all death rates for indigenous
Australians all Australians in 1995-97