Effective graphs and maps

The visual display of quantitative and geographical information

EPIET/EUPHEM introductory course

Chris Williams, Adapted from slides Y. Hutin

Communicating quantitative information

• Analogue communication (graphs, maps)– Graphic overview

– “hand waving”

– Less precise

• Digital communication (e.g. Tables)– Precise

– Numeric (text also?)

– Provides detailed and exact description 7:00 am

Digital and analogical information

Table, graph or map?(or none of the above)

Vaccine coverage in Yamoussoukro district,

Ivory Coast, 1995

33

42

52

75

79

0 20 40 60 80 100

Y. fever

Measles

DTP3

DTP1

BCG

Coverage (%)

The exact % does not add anything

and may be distracting

EPIGEPS course of field epidemiologyDigital and analogical information

Graphs: Key areas

• What is the message?– Note the singular

• Choose the right graph for the right data, that demonstrates the message clearly

• Style and etiquette– Message:ink ratio

Decide on a message

• Don’t use a graph if there is nothing to say

• Frame the idea to communicate

• Identify the graph that matches this idea

• Eliminate unnecessary information

• If there are two ideas, use two slides

Framing the message

No clear message

Symptom surveillance by region

Two messages- too many?

Weekly norovirus tests by result

One message- varying proportion positive

Choosing the right graph

Frequency distribution

• Histogram– Graphic representation of the frequency distribution of a

continuous variable

– Rectangles drawn in such a way that their bases lie on a

linear scale representing different intervals

– Areas are proportional to the frequencies of the values

within each of the intervals

• Epidemic curve is an example of histogram

Analog information

0

20

40

60

80

0-19.9 20-49.9 50-99.9 100-300 > 300

Urinary Iodine Excretion levels (µg/L)

Pe

rce

nta

ge

HistogramUrinary iodine excretion status, 24 N Parganas, West Bengal, India, 2004

Epicurve: gastroenteritis at a music festival

updated 29 JulyGastroenteritis meeting case definition (n=19)

Quarter 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3

Date

15-Jul

16-Jul

17-Jul

18-Jul

19-Jul

20-Jul

Day quarters are of 6 hours (0:00-05:59, 06:00 - 11:59, 12:00 - 17:59, 18:00 - 23:59 )

COLD/FLU CALLS:Daily ‘cold/flu’ calls as a proportion of total calls (7-

day moving average) by region(2011- 2012).

Presentation of time series data

Proportions of a total presenting selected characteristics

• Breakdown of a total in proportions:– Pie chart

• Breakdown of more than one total into proportion:– Stacked bar charts adding to 100%

Analog information

Respiratory virus isolates by type: Pie chart

Estimated and projected proportion of deaths due to non-communicable

diseases, India, 1990-2010

0%10%20%30%40%50%60%70%80%90%

100%

1990 2000 2010

Year

Pro

port

ion (

%)

Injuries

CommunicablediseasesNon communicablediseases

Cumulated bar chart for the breakdown of many totals in proportions

Alignment allows comparing proportions

across groups

Comparing proportions across groups

• No logical order: Horizontal bar chart – Sort according to decreasing proportions

• Logical order: Vertical bar chart– Not a continuous variable : Do not display axis

– Continuous variable: Display axis

Analog information

Causes of non vaccination as reported by the mothers, Bubaneshwar, Orissa, India,

2003

Analog information

0% 20% 40% 60% 80% 100%

Lack of money

Lack of facility

Lack of time

Lack of motivation

Irregularity by health staff

Child sick

Lack of awareness

India FETP

Level of education

Fre

qu

ency

(%

)

0

10

20

30

40

50

60

70

None Prim Sec Sup

Maternal education of mothers, vaccine coverage survey, Yamoussoukro,

Ivory Cost, 1995

Analog informationEPIGEPS course of field epidemiology

Vertical bar chart: Gradient, but not quantified (No x axis)

Prevalence of hypertension by age and sex, Aizawl, Mizoram, India, 2003

Vertical bar chart: Quantified gradient (x Axis)

0

10

20

30

40

50

60

30-39 40-49 50-59 60-69 70 +

Age group (years)

%

Deciding the type of graph that is needed (for reference)

Y es!V ertica l ba r cha rt

(no in te rva ls)

N o !L ine g raph

(T im es series)

Y es!E p idem ic cu rve?

Y es!H is togram m e

Y es!P ie cha rt

N o !Juxtapozed ba rs

cum u la ting to 100%

Y es!O n ly one to ta l?

Q uan tified g rad ientx ax is

N o quan tified grad ientno x ax is

Y es!V ertical ba rs ranked

by ca tego ries in log ica l o rder

N o !H orizon ta l ba rs ranked

by m agn itude o f p roportions

Y es!Log ica l o rde r inthe ca tegories?

N o !Is is repo rting proportions

in va rious ca tego ries?

N o !B reak dow n o f 1 or

m ore to ta ls in propo rtions?

N o !F requency d is tr ibu tion?

E ven t follow ed ove r tim e?

Putting it on the map:using maps in epidemiology

Learning objectives

Understand the principles of mapping

Understand maps of counts and rates

Understand how maps can generate hypotheses

How to create maps

Two basic tips

• Maps in background– Include if location is

relevant to your key message

• Politics– Some maps/borders/

comparisons are sensitive

Using maps in field epidemiology

• Principals of case mapping – spot/count and population-adjusted maps

• Using maps to generate hypotheses

• Practical and analytic aspects of maps

First, catch your hare

• Where are the cases?– What do we mean by “where”

• Once we know where they are, we can:– Go back to them for case management/investigation

– Create a map to generate hypotheses or convey a message

Geolocation

Case Onset Other Geolocation

Case 1 24-May ....

Case 2 25-May ....

Case 3 26-May ....

Case 4 27-May ....

Case 5 28-May ....

Case 6 29-May ....

Case 7 30-May ....

Case 8 31-May ....

Case 9 01-Jun ....

Case 10 02-Jun ....

Case 11 03-Jun ....

Case 12 04-Jun ....

Case 13 05-Jun ....

Case 14 06-Jun ....

Case 15 07-Jun ....

Choosing a geo-locator

• Cases are people (or animals!)

• People move around

• Options:– Place of residence (temporary residence)

– Place of work or study

– Healthcare provider location

– Bed/room?

– Merging into exposure locations e.g. Pools, water sources

• Note time also – location when?

General points on mapping

• Two types of maps:– Spot maps

– Incidence maps

• Use a key

• Add a title with time, place and person information

Field epi map

Techniques for mapping

• Pen and paper

• Drawing packages (including Excel)– Transparency method

• Simple mapping- EpiMap, HPZone

• GIS packages- ArcView, MapInfo– Can generate derived geographical variables

Geographic information system (GIS)

• A database linked to geographical information

• Location obtained:– Directly via GPS devices or similar

– Indirectly from information on address/GP/lab

• Can serve as case database

• Also can generate maps and test hypotheses

GPS

Drawing a spot map during an outbreak investigation

• Rough sketch of the setting of an outbreak

• One dot = One case

• Other locations of potential importance are also recorded

• Does not adjust for population density (OK in small places)

Field epi map

Using simple packages for maps: Excel

SR2 02-Mar

SR1 04-Mar 01-MarPedlars 02-Mar

02-Mar

Rix

14-FebVictoria 28-Feb

23-FebChapman 04-Mar

05-Mar Nursing station

Spot map: ornithosis cases by place of residence, East of

England 2008, n=3

Spot map: ornithosis cases by place of residence, East of England 2008

(n=10)

Spot map: ornithosis cases by place of work, East of England

2008 (n=10)

An incidence map adjusts for population density

• List the cases

• Regroup cases by location for which population denominator is available– Look up census data

• Divide the number of cases by the population denominator

• Choose gradients of colours to represent increasing incidence

Field epi map

Incidence mapInvasive meningococal disease, East of England

Using colours in maps

• The cold / warm scale– Represents violent contrasts

• Increasing density in one colour– Represents increasing levels of magnitude

• Complementary colours – Use equivalent intensity

– Represents unrelated notions

Tips

Generating hypotheses

• Brainstorm likely geographical links

• Map cases plus other relevant features– Water sources

– Roads

– Cooling towers

• Can also map epidemiological indicators by area– Type of ground

– Deprivation

– Prevalence of another infection/disease

Mapping an area exposure/determinant

Q fever: incidence and spot map

Take home messages

• Know how to obtain geolocator information and link to cases

• Use spot maps and incidence maps to generate hypotheses– Adjust for population size with rates when needed

• Communicate efficiently the spatial distribution of health information

• Practice map preparation to produce them rapidly when needed in practice

Graph exercise: reported reasons for not swimming at Lazareto (% responses)

2008 2009 2010 2011 2012Can't swim 10 8 12 9 11Too cold 30 34 29 26 32No swimming costume 22 23 18 19 25Fear of jellyfish 20 24 27 22 80Fear of sharks 5 3 1 7 2

Questions

• Message

• Graph type

• Style

First attempt: Select, Insert graph

Message: comparing proportions of responses

Type: Bar chart showing proportion by category

Style: reduce unnecessary ink

Don’t forget the jellyfish

QUESTIONS?

A quick and not-so-dirty electronic map in three steps

• A hardcopy of your map

• A transparency

• Cello tape

• Permanent markers

• Computer

• Drawing software

Tips

Step 1: Place transparency on the hardcopy of map

to draw map with permanent marker

Tips

Step 2: Stick the transparency on the screen with cello tape and follow the guide to draw map with the mouse in a drawing

software

Tips

Step 3: Remove the transparency and edit the map in the drawing software

Tips

Your map is ready for use! Edit lines and fill, add title, legends and footnotes

Tips

Fraction of HBV infections attributable to sharps injuries among health care

workers, by region, 2000

North America

Latin America Africa D Africa E

W Europe

Eastern Europe

M. EastSouth Asia

S. E. Asia

Western Pacific

AustraliaJapan

Gulf

World Health Report, 2002

< 1%

1-9%

10-19%

20-29%

30-49%

> 50%

Former USSR

Tips

Further functions of GIS

• Point in polygon

• Distance to vector (e.g. river, road)

• Raster – defining exposure areas – Satellite images

• ”Hotspot” maps

• Interactive maps : http://www.apho.org.uk/addons/_118371/atlas.html