EG3246 Spatial Science & Health Introduction to Basic Epidemiology Dr Mark Cresswell

Topics  Definition of Epidemiology  Statistical measures  Ideas of space & time  Modelling and spatial analysis

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

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

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

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

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)

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

Source: Cresswell, 1998

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.

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

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

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

Statistical Measures – Relative Risk If RR=1, there is no association between the risk factor and the disease If RR>1, there is an increased risk of developing the disease if one is exposed to the risk factor (eg. Disease=lung cancer; risk factor=smoking). It suggests that exposure to the risk factor may cause the disease. If RR<1, there is a decreased risk of developing the disease if one is exposed to the risk factor (e.g. disease=colon cancer; risk factor=eating fresh fruit & veg). It suggests that exposure to the risk factor may protect against the disease.

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 (<1 year old) NLB = Number of live births Source: BMJ, 2005

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

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

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

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

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

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

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