EG3246%20Spatial%20Science%20

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EG3246Spatial Science Health

• Introduction to Basic Epidemiology
• Dr Mark Cresswell

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Topics

• Definition of Epidemiology
• Statistical measures
• Ideas of space time
• Modelling and spatial analysis

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Definition of Epidemiology

• Epidemiology is the study of the distribution and
  determinants of diseases within human
  populations. Research in this field is based
  primarily upon observing people directly in their
  natural environments.
• Greenberg et al. 2005

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Definition of Epidemiology

• Epidemiologists often refer to a population at
  risk
• These are people who (regardless of their state
  of health) would be regarded as a new case if
  they contracted the disease being studied
• Can be country, region, town or GP list

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Definition of Epidemiology

• A target population may be regarded as a group
  studied who are thought to be at a higher risk
  e.g. people living within a 10 mile radius of a
  nuclear power station
• A study sample is (usually) a randomly selected
  sample whose characteristics are often
  extrapolated to be representative of the larger
  population

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Definition of Epidemiology

• The aetiology of a disease is the study of the
  agent(s) of causality that causes that disease
• Aetiology studies often examine levels of
  exposure of some agent (e.g. incidence of cancers
  between people who have lived close to nuclear
  power stations compared to those who have not)

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Definition of Epidemiology

• Epidemiology also looks at temporal change
• How have incidence levels changed over a period
  of time?
• Inherent in this type of analysis is discovery of
  a rising or falling trend
• This is achieved by active or passive
  surveillance techniques

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Statistical Measures - Incidence
This is the number of new cases in a particular
time period
I Incidence N Number of new cases in a given
time period P Person years at risk during same
time period Note that person years at risk means
the total amount of time (in years) that each
member of the population being studied (the study
population) is at risk of the disease during the
period of interest.
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Statistical Measures - Prevalence
This is the proportion of current cases in a
population at a given point in time
P Prevalence Nc Number of cases in the
population at a given point in time P Total
population at the same point in time
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Statistical Measures Absolute Risk
The probability of having a disease, for those
individuals who were exposed to a risk factor.
Ra Absolute Risk Ne Number of cases of
disease in those exposed Ie Number of
individuals exposed
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Statistical Measures Relative Risk
This is an indication of the risk of developing a
disease in a group of people who were exposed to
a risk factor, relative to a group who were not
exposed to it.
RR Relative Risk Ie Disease incidence in
exposed group In Disease incidence in
non-exposed group
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Statistical Measures Relative Risk
If RR1, there is no association between the risk
factor and the disease If RRgt1, there is an
increased risk of developing the disease if one
is exposed to the risk factor (eg. Diseaselung
cancer risk factorsmoking). It suggests that
exposure to the risk factor may cause the
disease. If RRlt1, there is a decreased risk of
developing the disease if one is exposed to the
risk factor (e.g. diseasecolon cancer risk
factoreating fresh fruit veg). It suggests
that exposure to the risk factor may protect
against the disease.
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Statistical Measures Other Rates
FR FERTILITY RATE NLB Number of live
births NW Number of women aged 15 44
years BR BIRTH RATE NLB Number of live
births MYP Mid-year population IMR INFANT
MORTALITY RATE NI Number of infant deaths (lt1
year old) NLB Number of live births

Source BMJ, 2005
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Ideas of Space Time

• Diseases may typically exist within geographical
  limits
• If the exposure to a hazard is determined by
  distance from the source OR
• If exposure to the hazard is dependent upon
  environmental/climatic factors

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Ideas of Space Time
Effects of time age (source BMJ, 2005)
Nuclear power station cancers
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Ideas of Space Time

• For a disease to be regarded as endemic, it must
  be habitually present in a community of
  individuals
• A sudden and great increase in the occurrence of
  a disease within a population is referred to as
  an epidemic.
• A rapidly emerging outbreak of a disease that
  affects a wide range of geographically
  distributed people is described as a pandemic

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Ideas of Space Time

• Hypoendemic means little transmission where
  effect on population is negligible
• Mesoendemic means disease is found in small
  isolated communities of varying intensity
• Hyperendemic refers to intense but seasonal
  transmission not affecting all age groups
• Holoendemic refers to perennial and intense
  transmission leading to adult immunity

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Modelling Spatial Analysis

• Epidemiological data may be modelled spatially,
  temporally or spatio-temporally
• Disease may be visualised using a GIS
• Co-factors affecting epidemiological
  characteristics (exposure or seasonal cycles such
  as weather) may be modelled to ascertain risk

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Temporal change in Standardised Death Rate (cases
per 100,000) due to Car Accidents Source
WHO/Europe European Mortality Database, 2005
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Temporal change in Standardised Death Rate (cases
per 100,000) due to Heart Disease Source
WHO/Europe European Mortality Database, 2005