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Statistics 120 Good and Bad Graphs - Department of Statistics

First Prev Next Last Go Back Full Screen Close QuitStatistics 120 good and Bad Graphs First Prev Next Last Go Back Full Screen Close QuitThe Plan In this lecture we will try to set down some basic rulesfor drawing good Graphs . We will do this by showing that violating the rulesproduces bad Graphs . Later in the course we see that there is a solidperceptual basis for some of these rules. First Prev Next Last Go Back Full Screen Close QuitData Content It does not make sense to use Graphs to display verysmall amounts of data. The human brain is quite capable of grasping one two,or even three values. First Prev Next Last Go Back Full Screen Close Quit First Prev Next Last Go Back Full Screen Close QuitMaleFemaleGenderPercentage0102030405 060 Class Gender Breakdown First Prev Next Last Go Back Full Screen Close QuitData Relevance Graphs are only as good as the data they display. No amount of creativity can produce a good graph fromdubious data. First Prev Next Last Go Back Full Screen Close Quit First Prev Next Last Go Back Full Screen Close QuitAuckland City Council:City Scene First Prev Next Last Go Back Full Screen Close QuitComplexity Graphs should be no more complex than the data whichthey portray.

Statistics 120 Good and Bad Graphs •First •Prev •Next •Last •Go Back •Full Screen •Close •Quit The Plan • In this lecture we will try to set down some basic rules for drawing good graphs. • We will do this by showing that violating the rules produces bad graphs.

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Transcription of Statistics 120 Good and Bad Graphs - Department of Statistics

1 First Prev Next Last Go Back Full Screen Close QuitStatistics 120 good and Bad Graphs First Prev Next Last Go Back Full Screen Close QuitThe Plan In this lecture we will try to set down some basic rulesfor drawing good Graphs . We will do this by showing that violating the rulesproduces bad Graphs . Later in the course we see that there is a solidperceptual basis for some of these rules. First Prev Next Last Go Back Full Screen Close QuitData Content It does not make sense to use Graphs to display verysmall amounts of data. The human brain is quite capable of grasping one two,or even three values. First Prev Next Last Go Back Full Screen Close Quit First Prev Next Last Go Back Full Screen Close QuitMaleFemaleGenderPercentage0102030405 060 Class Gender Breakdown First Prev Next Last Go Back Full Screen Close QuitData Relevance Graphs are only as good as the data they display. No amount of creativity can produce a good graph fromdubious data. First Prev Next Last Go Back Full Screen Close Quit First Prev Next Last Go Back Full Screen Close QuitAuckland City Council:City Scene First Prev Next Last Go Back Full Screen Close QuitComplexity Graphs should be no more complex than the data whichthey portray.

2 Unnecessary complexity can be introduced by irrelevant decoration colour 3d effects These are collectively known as chartjunk. First Prev Next Last Go Back Full Screen Close QuitAge Structure of CollegeEnrolment (1972-1976)This graph presents five values. Ituses six colours, unnecessaryperspective and a split axis to do Education Magazine. First Prev Next Last Go Back Full Screen Close Quit19721973197419751976282930313233 YearPercentlllllAge Structure of College EnrolmentPercent of Total Enrolment, Aged 25 and Over First Prev Next Last Go Back Full Screen Close Quit19721973197419751976 YearPercent05101520253035 Age Structure of College EnrolmentPercent of Total Enrolment, Aged 25 and Over First Prev Next Last Go Back Full Screen Close QuitShare ResultsThe extra dimension used in thisgraph has confused even the personwho created Washington Post, 1979. First Prev Next Last Go Back Full Screen Close Quit197219731974197519761977 Earnings Per Share and First Prev Next Last Go Back Full Screen Close QuitDistortion Graphs should not provide a distorted picture of thevalues they portray.

3 Distortion can be either deliberate or accidental. (Of course, it can be useful to know how to produce agraph which bends the truth.) First Prev Next Last Go Back Full Screen Close Quit First Prev Next Last Go Back Full Screen Close QuitTheologyFine ArtsMusicMedicineArchitectureEducationEn gineeringLawScienceCommerceArts020004000 60008000 Faculty Size First Prev Next Last Go Back Full Screen Close QuitEngineeringTheologyArchitectureComme rceScienceMedicineLawFine ArtsArtsMusicEducation020406080100 Percentage of Female Students First Prev Next Last Go Back Full Screen Close QuitThe Lie Factor Ed Tufte of Yale University has defined a lie factor asa measure of the amount of distortion in a graph . (Don ttake this too seriously don t learn it for the exam). The lie factor is defined to be:Lie Factor=size of effect shown in graphicsize of effect shown in data If the lie factor of a graph is greater than 1, the graph isexaggerating the size of the effect. First Prev Next Last Go Back Full Screen Close QuitData Effect= 1818= , graph Effect=.

4 ,Lie Factor= First Prev Next Last Go Back Full Screen Close Quit19781979198019811982198319841985 Required Fuel Economy Standards:New Cars Built from 1978 to 1985yearMPG051015202530 First Prev Next Last Go Back Full Screen Close QuitCommon Sources of Distortion The use of 3 dimensional effects is a common sourceof distortions in Graphs . Another common source is the inappropriate use oflinear scaling when using area or volume to representvalues. First Prev Next Last Go Back Full Screen Close QuitLie Factor = First Prev Next Last Go Back Full Screen Close QuitIs the bottom dollar note roughly half the size of the top one? First Prev Next Last Go Back Full Screen Close $ Power of the Diminishing Dollar First Prev Next Last Go Back Full Screen Close QuitDeliberate Distortion Sometimes Graphs contain deliberate distortions. Usually these are an attempt to hide some feature of thedata. First Prev Next Last Go Back Full Screen Close Quit First Prev Next Last Go Back Full Screen Close QuitYearlllllllllllllllllllllllll1939194 7195119551963196519671970197219731974197 51976010,00020,00030,00040,00050,00060,0 00 Median Net IncomesDoctorsOther Professionals First Prev Next Last Go Back Full Screen Close QuitYearlllllllllllllllllllllllll1940194 5195019551960196519701975010,00020,00030 ,00040,00050,00060,000 Median Net IncomesDoctorsOther Professionals First Prev Next Last Go Back Full Screen Close QuitErrors Sometimes distortions occur because of errors.

5 This can be because the artist tried to be clever andsometimes because they don t understand Statistics . First Prev Next Last Go Back Full Screen Close Quit First Prev Next Last Go Back Full Screen Close QuitDrawing good Graphs If the story is simple, keep it simple. If the story is complex, make it look simple. Tell the truth don t distort the data.


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