Visualising Variables – Validly! Damien Jolley School of Health & Social Development Deakin University, Victoria Originally presented at: ASCEPT Workshop: “Extracting the real significance from biological data” AHMRC, Thursday 28 November 2002 AHMRC Posters Department of Human Physiology & Anatomy School of Human Biosciences La Trobe University 23 April 2004
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Visualising Variables – Validly! Damien Jolley School of Health & Social Development Deakin University, Victoria Originally presented at: ASCEPT Workshop:
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Visualising Variables – Validly!
Damien JolleySchool of Health & Social
DevelopmentDeakin University, Victoria
Originally presented at:
ASCEPT Workshop:“Extracting the real significance from biological data”
AHMRC, Thursday 28 November 2002
AHMRC Posters
Department of Human Physiology & AnatomySchool of Human Biosciences La Trobe University23 April 2004
Download slides from: http://www.jolley.com.auAverage daily retail petrol price,Melbourne, Oct-Nov 2002
Note the exception for X-Y orientation: because predictor is qualitative (unordered)
Download slides from: http://www.jolley.com.au
Software for scientific graphics
Dedicated programs – thousands!
DeltaGraph (SPSS)
Prism ViSta
Business graphics MS Excel many other
spreadsheet programs
Graphics in statistical packages
S-Plus, R powerful, difficult
SPSS interactive graphics
easy, expensive Systat
good reputation SAS GRAPH language
expensive, powerful Stata
simple, limited
Advice: Avoid “default” choice in all programs (almost always wrong).Avoid programs with “Chart Type” menus – wrong approach.
Download slides from: http://www.jolley.com.au
Graph formats
Object-oriented lines, shapes, etc
can be identified within graph
each object has attributes (eg size, colour, font)
editable using selection and “grouping”
Common formats: Postscript (ps,eps) Windows metafile
(wmf,emf)
Bit-mapped image exists as a
collection of pixels each pixel is light or
dark, coloured can edit only pixels not
objects often “compressed” to
save disk space, bandwidth
Common formats graphics interchange
(gif) Windows bitmap (bmp) JPEG interchange (jpg)
Advice: Use WMF format where possible. Paste WMF into PowerPoint, “ungroup”, then edit objects for publication quality.
Download slides from: http://www.jolley.com.au
References, further reading
Tufte ER.
The Visual Display of Quantitative Information
Cheshire, CT: Graphics Press 2001
www.edwardtufte.com
Cleveland WS.
Visualizing Data
Summit NJ: Hobart Press, 1993
Wainer H.
Visual Revelations. Graphical Tales of Fate and Deception from Napoleon Bonaparte to Ross Perot
Mahwah, NJ: Lawrence Erlbaum Associates, Publishers. 1997
www.erlbaum.com
Wilkinson L.
The Grammar of Graphics
New York: Springer Verlag, 1999
Download slides from: http://www.jolley.com.au
Summary
Howie’s Helpful Hints for bad graphs: Don’t show the data Show the data inaccurately Obfuscate the data
Steps for better graphs: Identify direction of cause & effect Exploit levels of measurement Accommodate visual perception principles Minimise non-data ink
Don’t use Excel unless you have to And if you have to, don’t use the default