Effective graphs

The visual display of quantitative information

FETP India

Competency to be gained from this lecture

Present quantitative data effectively with graphs

Key areas

• Digital and analogical information

• Framing the message• Choosing the right graph for the right data

Communicating quantitative information

• Analogical communication Less precise More graphic Provides overall understanding

• Digital communication Precise Numeric Provides detailed and exact description

7:00 amDigital and analogical information

Cases of Monkeypox by month of onset, Katako-Kombe, Zaire,

1996-1997Primary cases Secondary cases

Feb-96 2 0Mar-96 0 3Apr-96 2 2May-96 5 6Jun-96 0 5Jul-96 4 9Aug-96 3 23Sep-96 1 5Oct-96 1 2Nov-96 3 1Dec-96 0 0Jan-97 2 0Feb-97 1 9Total 24 65

Example: Digital display of informationWHO-CDC

Digital and analogical information

Nu

mb

er

of c

ase

s

Month of onset

Cases of monkeypox by month of onset, Katako-Kombe, Zaire,

1996-1997

0

5

10

15

20

25

30

Feb Mar Apr May Jun1996

Jul Aug Sep Oct Nov Dec Jan1997

Feb

Secondary cases

Primary cases

Example: Analog display of information

WHO-CDC

Digital and analogical information

Do not add digital information on graphs

• Use graphs to communicate analogical data

• Addition of digital information: Will attract the eye of some of the audience

May distract from the key analogical idea

• Exact figures may be provided in the script

Digital and analogical information

Vaccine coverage in Yamoussoukro district,

Ivory Coast, 1995 The exact % does not add anything and

may be distracting

33

42

52

75

79

0 20 40 60 80 100

Y. fever

Measles

DTP3

DTP1

BCG

Coverage (%)

EPIGEPS course of field epidemiologyDigital and analogical information

Use one graph to get one idea across

• 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

Proportion of eligible patients whose blood slides were examined for malaria, Dhenkanal district, Orissa,

1996-2002

0

2

4

6

8

10

12

14

16

1996 1997 1998 1999 2000 2001 20002

Years

Pro

po

rtio

n (

%)

Is the graph useful?

Sources of hepatitis A virus infection among case-patients reporting at least one exposure, USA, 1983-1995

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

Year

Pro

port

ion

of c

ases

Day care related

Contact with a case

Common source outbreakForeign travel

Injecting drug useMen who have sex with men

Information on irrelevant risk

factors

Dilution of the information on injection drug

useCDC

Proportion of hepatitis A cases-patients reporting injection drug use*,

USA, 1983-1995

0%

2%

4%

6%

8%

10%

12%

14%

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

Year* Among case-patients reporting at least one exposure

Proportion

Information on other risk factors

eliminated

The two waves of appear

clearly in the absence of

background data

CDCFraming the message

Keep the ink-to-data ratio low

• All elements on a graph must be justified

• Eliminate distracting, non-essential elements Secondary axis Gridlines 3-D effects Bordering lines Etc…

Framing the message

Dracunculiasis incidenceBam and Oubritenga, Burkina, Faso,

1989-1992

0

20

40

60

80

100

1989 1990 1991 1992

Years of the project

Inci

denc

e p.

100

00

Bam

Oubritenga

OCCGE

1989 1990 1991 1992

BamOubritenga

020406080

100

Incidence p. 10000

Years of the project

Bam Oubritenga

Framing the message

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

Cases of acute diarrhea by date of onset, Parbatia,

Orissa, India (n= 41)

0

2

4

6

8

10

12

14

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Date of onsetNovember 2003

Num

ber

of c

ases

Analog informationIndia FETP

Epidemic curve

Malaria in Kurseong block, Darjeeling District, West Bengal, India, 2000-2004

0

5

10

15

20

25

30

35

40

45

Janu

ary

Feb

ruar

y

Mar

chA

pril

May

June

July

Aug

ust

Sep

tem

ber

Oct

ober

Nov

embe

r

Dec

embe

rJa

nuar

yF

ebru

ary

Mar

chA

pril

May

June

July

Aug

ust

Sep

tem

ber

Oct

ober

Nov

embe

r

Dec

embe

r

Janu

ary

Feb

ruar

yM

arch

Apr

ilM

ayJu

neJu

lyA

ugus

t

Sep

tem

ber

Oct

ober

Nov

embe

r

Dec

embe

rJa

nuar

y

Feb

ruar

yM

arch

Apr

ilM

ay

June

July

Aug

ust

Sep

tem

ber

Oct

ober

Nov

embe

rD

ecem

ber

Janu

ary

Feb

ruar

yM

arch

Apr

il

May

June

July

Aug

ust

Sep

tem

ber

Oct

ober

Nov

embe

rD

ecem

ber

2000 2001 2002 2003 2004

Months

Inci

denc

e of

mal

aria

per

10,

000 Incidence of malaria

Incidence of Pf malaria

Line graph for time series

Proportions of a total presenting

selected characteristics• Breakdown of a total in proportions: Pie chart

• Breakdown of more than one total into proportion: Juxtaposed bar charts cumulated to 100%

Analog information

Road10%

Fall32%

Bites16%

Burns7%

Minor injuries35%

Types of unintentional injuries, Tiruchirappalli, Tamil Nadu,

India, 2003

Incidence: 9.6 per 100 person-month

(95% C.I. 8-11

Pie chart for the breakdown of a total in proportions

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

Proportion (%)

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)

%

Avoid juxtaposition of bars

• Juxtaposed bars charts are confusing

• Three options: Eliminate one category of data if not relevant

Split the graph in two Use cumulated bars when applicable

Analog information

Nu

mb

er

of c

ase

s

Month of onset

Cases of monkeypox by month of onset, Katako-Kombe, Zaire,

1996-1997

0

5

10

15

20

25

Feb Mar Apr May Jun1996

Jul Aug Sep Oct Nov Dec Jan1997

Feb

Primary cases

Secondary cases

The juxtaposed bars break the continuity and

make it difficult to follow the

dynamic of the outbreak on the

graph

Analog informationCDC-WHO

Nu

mb

er

of c

ase

s

Month of onset

Cases of monkeypox by month of onset, Katako-Kombe, Zaire,

1996-1997

0

5

10

15

20

25

30

Feb Mar Apr May Jun1996

Jul Aug Sep Oct Nov Dec Jan1997

Feb

Secondary cases

Primary cases

Cumulated bars respect the

representation of the dynamic of the outbreak and illustrate the break down

of cases

Analog informationCDC-WHO

Deciding the type of graph that is needed

Yes!Vertical bar chart

(no intervals)

No!Line graph

(Times series)

Yes!Epidemic curve?

Yes!Histogramme

Yes!Pie chart

No!Juxtapozed bars

cumulating to 100%

Yes!Only one total?

Quantified gradientx axis

No quantified gradientno x axis

Yes!Vertical bars ranked

by categories in logical order

No!Horizontal bars ranked

by magnitude of proportions

Yes!Logical order inthe categories?

No!Is is reporting proportions

in various categories?

No!Break down of 1 or

more totals in proportions?

No!Frequency distribution?

Event followed over time?

Take home messages

• Know when you want to use a graph • Know what your graph will say• Tailor your graph to your message