GIS and its applications in Health

GIS and its applications in GIS and its applications in HealthHealth

دانشکده بهداشت دانشگاه علوم پزشکی تهراندانشکده بهداشت دانشگاه علوم پزشکی تهران13841384آذر ماه آذر ماه

اکبر حقدوست اکبر حقدوستييارايه دهنده: علارايه دهنده: عل

GIS workshop in TUMSGIS workshop in TUMS A.A. HaghdoostA.A. Haghdoost

Definition

Lilienfeld 1976 in “Fundamental epidemiology”

EpidemiologyEpidemiology: study of distribution of disease, or pathological condition in human populations and factors that influence this distribution

What does mean “distribution” in above definition?

We should explore the distribution in timetime and spacespace

John Snow project


�Example


Poisson DistributionPoisson Distribution

k!

eλk)P(x

λk

Assumptions:Assumptions:

1.1.Constant proportion in areaConstant proportion in area

2.2.Independent distributionIndependent distribution


Flying bombs strikes, south London

آيا بمبهاي آلمان در جنگ جهاني دوم آن گونه كه ادعا مي نمود قدرت تفكيك باال در هدف قرار

دادن مواضع را داشت؟



Number of bombs in cells

# of cells 0 1 2 3 4 5 Total

observed 229 211 93 35 7 1 576

expected 227.5 211.3 98.2 30.4 7.1 1.54 576



How can we test if the distribution of bombs was random?

x

Sm

x

xxobs

dfmwithondistributifollowsmx

S

x

SModelPoissonIn

xm

i

i

i

ii

x

x

2

1

222

22

2

)1()(

exp

)exp(

)1()1(

1:


This method is robust even for small sample numbers

You can estimate variance even based on the group data

Limitation

It does not taking into account distances between events



A

Nearest neighbor method

Minimum distance between observation I and (n-1)

Checks the distances between events

a b

cd


Exponential (e)

nx

n

n

xe

ne

)1lim(

)1

1lim(


Exponential (e)

nqnnn en

nqqp )1()1(


R

r


A

r 2

1

Probability that a random point within the boundaries of the large circle falls within the boundaries of the small circle

Probability that a random point within the boundaries of the large circle does not fall within the boundaries of the small circle

A

r 2

Probability that n random points within the boundaries of the large circle does not fall within the boundaries of the small circle

An

rn

eA

r2

)1(2


Probability that at least one of n random points within the boundaries of the large circle falls

within the boundaries of the small circle

A

rn

e

2

1


Expected median, mean and variance based on these explanations (nearest neighbor analysis)

2068.0)var(5.0)(

)2log(47.047.0)(

n

Arwith

n

Armean

n

Armedian


example

دارد؟ وجود ارتباطي هيچ شكل اين نقاط بين آيا

0

5

10

15

0 5 10 15

x

y


نقاط ضعف روش نزديكترين گزينه

Very large and small area (A) may reduce the significance

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30 35 40 45 50

x

y

0

5

10

0 5 10

x

y


BoundaryBoundary influenceinfluence


Transformed map

Uniform distribution is violated in nearly all situations.

Redraw a map so that the density of the individuals at risk is equalized over the area under study, called a cartogram


Original map


Cartogram


Limitations in using cartogram

1. We need to have an accurate map to illustrate the population density

2. The population at risk is not usually distributed exactly the same as the whole population

3. It needs power full computer to generate a cartogram


Alternative method

Case control approach

It means we can compare the spatial distribution of cases versus controls using Poisson models

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GIS and its applications in Health