How to Produce Statistical Graphics General Clinical Research Center August 15, 2005 Rachel Enriquez
What are we going to talk about? • Why should we care about statistical graphics? • What is the theoretical framework for statistical graphics? • When do we make statistical graphics? • How can we produce good quality graphics?
Communication of results is the last step in the scientific process • Many people can comprehend the results better by seeing them in a figure than they can by reading them in a table. • Do you have an opinion? • Graphics can help persuade. • Objectively correct graphics can call attention to the result you WANT the viewer to see.
Get Attention • Can you produce exceptional statistical graphics? (me neither) • Do you want people to know that you are committed to the scientific process? • If people understand your research, they’ll listen to you and do what you tell them to.
Data Visualization • Visually encode the data. • Viewers decode the picture • Easy to figure out • Learn something new • See the right comparisons
Hierarchy of Visual Perception Position along common scale Position along nonaligned scale Length Angle / Slope Area Volume Color
Aesthetics • A personal matter • Unless you ask Tufte • Data / Ink ratio • Avoid 3-D • Fill patterns are bad • Obtain good resolution • Text can be small • (in print)
For preliminary analysis • Speed
For Publication in Journals • Data density is good. • Excellent resolution is required. • Color is difficult. • Column width is a consideration. • MS office is frequently not an option. • Too many tables!
A plot is better Confounding variable
Oral Presentations • Data density should be moderate. • Color is available. • LABEL! • Hope you have interesting data
Posters • Smaller audience • Experimentation is good. • Graphics will bring you customers!
How much time do you have? • It is not easy. • There is no perfect, easy to use, cheap software that is going to solve your problems.
Books are not very helpful • Software changes quickly. • People use different software. • You want to do it NOW, not after reading for 5 hours. • Surfing the net is frequently useful.
Vector Graphics vs Bitmaps • Vector graphics. • A set of instructions that tells the device how to display the document. • Adobe software is the most common way to edit vector graphics. • Bitmaps • Resolution depends on the size of the computer file. • Easy to open and publish on-line. • Generally not accepted for publication. • Vector graphics can be made into bitmaps. • Bitmaps cannot be made in vector graphics.
Bitmaps, compression, and enlarging • Compression can be ‘lossy’ • We are familiar with the grainy effect of enlargement.
Software • SPSS • Many chart options • Graphics can be edited • Can export vector graphics. • SAS • Known for poor graphics. • However, some people produce very good graphs with SAS. • Hope SAS improves and use something else for now?
Stata • Any comments? • R • It is free. • Produces good graphics that can be exported in various formats. • Infinitely customizable • Difficult for the novice statistician / programmer • R clinic • SyStat • EpiInfo • S+ • Spotfire • Prism – also available in GCRC computer lab. • Others…..
Sigma Plot • Can be used with Excel and SPSS • Opens other data formats • Menu driven • Multiple graphics options • Easily produces compound graphics • Exports graphics in multiple formats.
MS Office • Windows Metafile is a vector graphic format. • Excel • More control over graphics • Limited selection of graph types • User typically provides the S.E.s and effect estimates. • PowerPoint • Surprisingly good at managing bitmaps. • If you already use it, then improve your graphics by applying aesthetic rules.
Scanners • Scanned figures are an option. • Good way to clean up figures from journals if you’re proficient in Photoshop • The bitmap resolution problem remains • Which file format and program will avoid lossy compression?
Art Software • As a novice graphic preparator, I appreciate the ability to draw on graphs. • Can also ‘cover’ unwanted parts with white shapes. • Group the resulting collection of shapes and save as a picture.
Adobe Illustrator • Adobe Photoshop • These programs may seem counter-intuitive at first use. • Paint, MS office, etc. • Easy to use • Bitmap products.
Call the experts • The Medical Illustrators at VUMC will improve your graphs. • $50/hr • Average graph is 20 minutes. • Grow your own group ‘expert’.