revision of spss (2016) - discovering · pdf filerevision of ibm spss statistics ibm spss...
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Revision of IBM SPSS Statistics IBMSPSSStatistics™(SPSSfromnowon)isacomputerpackagedesigntocaterforyoureverystatisticalneed.Itcarriesoutanalysesthattakehoursbyhandinamatterofseconds.Here’salistofwhatSPSScanandcannotdo:
SPSSwill:• Docomplexstatisticalanalysisinafewseconds.• Producegraphs.• Saveyoutime.• Doanythingstatisticalthatyouaskittodo.SPSSwon’t:• Decide what test needs to be done in a givencircumstance.• Passyourexamforyou.• Maketea.So,althoughSPSSisaverypowerfultool,itworksonlywithinthelimits of your own statistical knowledge. In many respects it isratherstupidbecauseifyouprovideitwithdataandaskittorunacompletelymeaninglessanalysis,itwillhappilyoblige.Therefore,youstillneedtouseyourbraintoinitiatethecorrectanalysis.
Getting started with SPSS ThereareseveralexcellenttextsthatgiveintroductionstothegeneralenvironmentwithinwhichSPSSoperates.Inmyentirelyunbiasedopinion,thebestisField(2013)!SPSSmainlyusestwowindows:thedataeditor(thisiswhereyouinputyourdataandcarryoutstatisticalfunctions)andtheviewer (this iswhere the results of any analysisappear).
OnceSPSShasbeenactivated,astart-upwindowwillappear,whichallowsyoutoselectvariousoptions.Ifyoualreadyhaveadata fileondisk thatyouwouldliketoopenthenselectOpenanexistingdatasourcebyclickingonthe sothatitlookslike :thisisthedefault option. In the space underneath this optiontherewillbealistofrecentlyuseddatafilesthatyoucanselectwiththemouse.Ifyouwanttoopenadatafilethatisn’tinthelistthensimplyselectMoreFiles…withthemouseandclickon .
ThemainSPSSwindowincludesadataeditorforenteringdata.Thiswindowiswheremostoftheactionhappens.Thedataeditorhastwoviews:thedataviewandthevariableview.Thedataviewisforenteringdataintothedataeditor,andthevariableviewallowsustodefinevariouscharacteristicsofthevariableswithinthedataeditor.Atthebottom
ofthedataeditor,youshouldnoticethattherearetwotabslabelled‘DataView’and‘VariableView’()andallwedotoswitchbetweenthesetwoviewsisclickonthesetabs(thehighlightedtabtellsyouwhichviewyou’rein,althoughitwillbeobvious).
Please analysemydataforme.
SodOff!
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Figure1:TheSPSSDataEditor(‘DataView’)
Entering Data Using the Data Editor
Overview
WhenyoufirstloadSPSSitwillprovideablankdataeditor(showingthe‘dataview’)withthetitleNewData.Wheninputtinganewsetofdata,youmustinputyourdatainalogicalway.TheSPSSdataeditorisarrangedsuchthateachrowrepresentsdatafromoneparticipantwhileeachcolumnrepresentsavariable.Thereisnodiscriminationbetweenindependent and dependent variables, both types should be placed in a separate column. It follows from thisarrangementthatanyvariablemeasuredwiththesameparticipants(arepeatedmeasure)shouldberepresentedbyseveralcolumns(eachcolumnrepresentingoneleveloftherepeatedmeasuresvariable).However,whenabetween-groupdesignwasused(e.g.differentparticipantswereassignedtoeachleveloftheindependentvariable)thedatawillberepresentedbytwocolumns:onethathasthevaluesofthedependentvariableandonethatisacodingvariableindicatingtowhichgrouptheparticipantbelonged.Thisideawillbecomeclearerasyoulearnabouthowtocarryoutspecificprocedures.
http://www.youtube.com/watch?v=b163iBByycw
Thedataeditorismadeupoflotsofcells,whicharejustboxesinwhichdatavaluescanbeplaced.Whenacellisactiveitbecomeshighlightedwithablacksurroundingbox(asinFigures1and2).Youcanmovearoundthedataeditor,fromcelltocell,usingthearrowkeys¬ ¯ ®(foundontherightofthekeyboard)orbyclickingthemouseonthecellthatyouwishtoactivate.Toenteranumberintothedataeditorsimplymovetothecellinwhichyouwanttoplacethedatavalue,typeinthevalue,thenpressthearrowbuttonappropriatetothedirectioninwhichyouwishtomove.So,toenterarowofdata,movetothefarleftoftherow,typethefirstvalueandthenpress®(thisinputsthevalueandthenmovesyouintothenextcellontheright).
Thefirststepinenteringyourdataistocreatesomevariablesusingthe‘variableview’ofthedataeditor,andthentoinput your data using the ‘data view’ of the data editor.We’ll go through these two steps byworking through anexample.
The highlighted cell is the cell that is currently active
This shows that we are currently in the ‘Data View’
We can click here to switch to the ‘Variable View’
This area displays the value of the currently active cell
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Creating a Variable: The ‘Variable view’
Beforeweinputanydataintothedataeditor,weneedtocreatethevariables.Tocreatevariablesweusethe‘VariableView’ of the data editor. To access this view click on the ‘Variable View’ tab at the bottom of the data editor (
);thecontentsofthewindowwillchange(Figure2).
Figure2:The‘VariableView’oftheSPSSDataEditor
Everyrowof thevariableviewrepresentsavariable,andyousetcharacteristicsofaparticularvariablebyenteringinformationintothelabelledcolumns.Youcanchangevariouscharacteristicsofthevariablebyenteringinformationintothefollowingcolumns(playaroundandyou’llgetthehangofit):
Youcanenteranameinthiscolumnforeachvariable.Thisnamewillappearatthetopofthecorrespondingcolumninthedataview,andhelpsyoutoidentifyvariablesinthedataview.Therearecertainsymbolsyoucan’tuse(mainlysymbolsthathaveotherusesinSPSSsuchas+,−,$,&),andyoucan’tusespaces.(Manypeopleusea‘hard’spaceinvariablenames,whichreplacesthe space with an underscore; for example, Andy_Field instead of Andy Field.) If you use acharacter that SPSS doesn’t like you’ll get an errormessage saying that the variable name isinvalidwhenyouclickonadifferentcell,ortrytomoveoffthecellusingthearrowkeys.
Youcanhavedifferenttypesofdata.Mostlyyouwillusenumericvariables(whichjustmeansthatthevariablecontainsnumbers-SPSSassumesthisdatatype).Youwillcomeacrossstringvariables,whichconsistofstringsofletters.Ifyouwantedtotypeinpeople’snames,forexample,youwouldneedtochangethevariabletypetobestringratherthannumeric.Youcanalsohavecurrencyvariables(i.e.£s,$s,euro)anddatevariables.
Thenameofthevariable(seeabove)hassomerestrictionsoncharacters,andyoualsowouldn’twanttousehugelongnamesatthetopofyourcolumns(theybecomehardtoread).Therefore,youcanwritealongervariabledescriptioninthiscolumn.Thismayseempointless,butisactuallyoneofthebesthabitsyoucangetinto.
Thiscolumnisforassigningnumberstorepresentgroupsofpeople(seebelow).
Thisiswhereyoudefinethelevelatwhichavariablewasmeasured(Nominal,OrdinalorScale).Thisisimportant,especiallyifyouwanttodrawgraphs!
Data set 1: Between-group design
Tobeginwith,imaginethatafilmcompanydirectorwasinterestedinwhethertherewasreallysuchathingasa‘chickflick’(afilmthattypicallyappealstowomenmorethanmen).Hetook20menand20womenandshowedhalfofeachsampleafilmthatwassupposedtobea‘chickflick’(BridgetJones’Diary),andtheotherhalfofeachsampleafilmthatdidn’t fall into the category of ‘chick flick’ (Memento, a brilliant film by the way). In all cases he measured theirphysiologicalarousalasanindicatorofhowmuchtheyenjoyedthefilm.
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Table1:Filmdata
Male Female
BridgetJones’Diary Memento BridgetJones’Diary Memento
22 37 3 30
13 20 15 25
16 16 5 31
10 28 16 36
18 27 13 23
24 18 20 14
13 32 11 21
14 24 19 31
19 21 15 22
23 35 7 14
Weneed topay attention to the ruleof thedataeditor that scores fromdifferent peopleappear in different rowsbecausethisisnothowthedataarelaidoutinthetableabove(tosavespace).Rememberthateachscoreabovecomesfromadifferentperson,thereforewewillneed40rows(not10asabove).ToenterthedataaboveintotheSPSSdataeditorweneedtocreateseveralvariables.IfwebeginwiththevariableArousal,weshouldfollowthesesteps:
1. Movetheon-screenarrow(usingthemouse)tothefirstwhitecellinthecolumnlabelledName.
2. TypethewordArousal.
3. Moveoffofthiscellusingthearrowkeysonthekeyboard(youcanalsojustclickonadifferentcell,butthis isactuallyaveryslowwayofdoingit).
You’vejustcreatedyourfirstvariable!Youshouldnoticethatonceyou’vetypedaname,SPSScreatesdefaultsettingsforthevariable(suchasassumingit’snumericandassigning2decimalplaces).
Nowbecause Iwant you to get into good habits,move to the cell in the column labelled label and type ‘Averagephysiologicalarousalduringthefilm’.
Oncethevariablehasbeencreated,youcanreturntothedataviewbyclickingonthe‘dataview’tabatthebottomofthedataeditor( ).Thecontentsofthewindowwillchange,andyou’llnoticethatthefirstcolumnnowhasthelabelarousal.Toenterthedata,clickonthewhitecellatthetopofthecolumnlabelledarousalandtypethefirstvalue(22).Toregisterthisvalueinthiscell,wehavetomovetoadifferentcellandbecauseweareenteringdatadownacolumn,themostsensiblewaytodothisistopressthe¯keyonthekeyboard.Thisactionmovesyoudowntothenextcell,andthenumber22.00shouldappearinthecellabove.Enterthenextnumber(13)andthenpress¯tomovedowntothenextcell,andsoon.
Creating Coding Variables
Acodingvariables(alsoknownasagroupingvariable)isatypeofvariablethatyouwilluseonnumerousoccasions:itisavariableconsistingofaseriesofnumbersthatrepresentlevelsofanindependentvariable.Inexperiments,codingvariables are used to represent independent variables that have been measured between-groups (i.e. differentparticipantswereassignedtodifferentgroups).So,ifyouweretorunanexperimentwithonegroupofparticipantsinanexperimentalconditionandadifferentgroupofparticipantsinacontrolgroup,youmightassigntheexperimentalgroupacodeof1,andthecontrolgroupacodeof0.Whenyoucometoputthedataintothedataeditor,thenyouwouldcreateavariable(whichyoumightcallgroup)andtypeinthevalue1foranyparticipantsintheexperimentalgroup,anda0 foranyparticipant inthecontrolgroup.This tells thecomputerthatallof thecasesthathavebeen
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assignedthevalue1shouldbetreatedasbelongingtothesamegroup,andlikewisefortheparticipantsassignedthevalue0.
ThereisasimpleruleforhowvariablesshouldbeplacedintheSPSSdataeditor:levelsofthebetween-groupvariablesgodownthedataeditorwhereaslevelsofwithin-subject(repeatedmeasures)variablesgoacrossthedataeditor.Wewillpracticethisruleoverthecomingweeks.
Wehave two coding variable inourdata: theonedescribingwhether apersonwasmaleor female, anda seconddescribingwhetherthepersonwatched‘BridgetJones’diary’or‘Memento’.Let’screatethevariablegenderfirst.Tocreatethiscodingvariable,wefollowthestepsforcreatinganormalvariable,butwealsohavetotellthecomputerwhichnumericcodeshavebeenassignedtowhichgroups.So,firstofall,movetothecell inthesecondrowinthecolumnlabelledNametypeaname(let’scallitGender).I’mstilltryingtoinstilgoodhabits,somovealongthethirdrowtothecolumncalledLabelandgivethevariableafulldescriptionsuchas‘Participant’sgender’Thentodefinethegroupcodes,movealongtherowtothecolumnlabelled andintothiscell: .Clickon toaccesstheValueLabelsdialogbox(seeFigure3).
Figure3:DefiningcodingvaluesinSPSS
TheValueLabelsdialogboxisusedtospecifygroupcodes.Thiscanbedoneinthreeeasysteps.First,clickwiththemouseinthewhitespacenexttowhereitsaysValue(orpressAltanduatthesametime)andtypeinacode(e.g.1).Thesecodesarecompletelyarbitrary;forthesakeofconventionpeopletypicallyuse0,1,2,3,etc,butinpracticeyoucouldhaveacodeof495ifyouwerefeelingparticularlyarbitrary.Thesecondstepistoclickthemouseinthewhitespacebelow,nexttowhereitsaysValueLabel(orpressTab,orAltandeatthesametime)andtypeinanappropriatelabelforthatgroup.InFigure3Ihavealreadydefinedacodeof1forMales,andthenIhavetypedin2asmycodeandgiventhisalabelofFemale.Thethirdstepistoaddthiscodingtothelistbyclickingon .Whenyouhavedefinedallofyourcodingvaluesyoucanclickon andSPSSwillcheckyourvariablelabelsforspellingerrors(whichcanbeveryhandyifyouareasbadatspellingasIam).Tofinish,clickon ;ifyouclickon andhaveforgottentoaddyourfinalcodingtothelist,SPSSwilldisplayamessagewarningyouthatanypendingchangeswillbelost.InplainEnglishthissimplytellsyoutogobackandclickon beforecontinuing.Finally,codingvariablesalwaysrepresentcategoriesandsothelevelatwhichtheyaremeasuredisnominal(orordinalifthecategorieshaveameaningfulorder).Therefore,youshouldspecifythelevelatwhichthevariablewasmeasuredbygoingtothecolumnlabelledMeasureandselecting (or ifthegroupshaveameaningfulorder)fromthedrop-downlist.
Havingdefinedyourcodes,youcanthengotothedataview(byclickingthetabatthebottomofthewindow)andtypethesenumericalvaluesintotheappropriatecolumn(soifapersonwasamaletypeavalueof1,iftheywereafemale
Then,click onThis button activates
the ValueLabels dialog box
Click on the appropriate cell in the column labelled
Values
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typethevalue2).Youcangetthecomputertodisplaythenumericalcodes,orthevaluelabelsthatyougavethembyclickingon .
Havingcreatedthesevariablesarousalandgender,trytocreatethevariablefilmandentertherestofthedatayourself.SavethesedatainafilecalledChickFlick.sav–we’llneedthemlater.
Data set 2: repeated measures
Hiccupscanbeaseriousproblem:CharlesOsborneapparentlygotacaseofhiccupswhileslaughteringahog(well,whowouldn’t?)thatlasted67years.Peoplehavemanymethodsforstoppinghiccups(asurprise,holdingyourbreath),butactuallymedical sciencehasput itscollectivemind to the task too.Theofficial treatmentmethods include tongue-pullingmanoeuvres,massageofthecarotidartery,and,believeitornot,digitalrectalmassage(Fesmire,1988).Let’ssaywewantedtoputdigitalrectalmassagetothetest(asacureofhiccupsImean).Wetook15hiccupsufferers,andduringaboutofhiccupsadministeredeachofthethreeprocedures(inrandomorderandatintervalsof5minutes)aftertakingabaselineofhowmanyhiccupstheyhadperminute.Wecountedthenumberofhiccupsintheminuteaftereachprocedure.
Table2:Hiccupsdata
Baseline TonguePulling CarotidArteryMassage
DigitalRectalMassage
15 9 7 2
13 18 7 4
9 17 5 4
7 15 10 5
11 18 7 4
14 8 10 3
20 3 7 3
9 16 12 3
17 10 9 4
19 10 8 4
3 14 11 4
13 22 6 4
20 4 13 4
14 16 11 2
13 12 8 3
These data can be entered as above because each person took part in all four conditions – therefore, each rowrepresentsaparticularperson,andeachcolumnrepresentsthenumberofhiccupsthattheyhadduringaparticulartreatment.
® Youknowhowtocreatevariablesandenterdata,socreatefournewvariablesrepresentingthetreatmentsinthisexperiment.Labelthem,settheproperties,andthenenterthescoresforthe15participants.SavethecompleteddatafileasHiccups.sav.
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The SPSS Viewer AlongsidetheSPSSDataEditorwindow,thereisasecondwindowknownastheSPSSViewer.Figure4showsthebasiclayoutoftheSPSSviewer.Ontheright-handsidethereisalargespaceinwhichtheoutputisdisplayed.SPSSdisplaysbothgraphsandtheresultsofstatisticalanalysesinthispartoftheviewer.Itisalsopossibletoeditgraphsandtodothisyousimplydouble-clickonthegraphyouwishtoedit(thiscreatesanewwindowinwhichthegraphcanbeedited).Onthe left-handsideoftheoutputviewerthere isatreediagramillustratingthestructureof theoutput.This treediagramisusefulwhenyouhaveconductedseveralanalysesbecauseitprovidesaneasywayofaccessingspecificpartsoftheoutput.Thetreestructureisfairlyself-explanatoryinthateverytimeyoudosomethinginSPSS(suchasdrawingagraphorrunningastatisticalprocedure),itliststhisprocedureasamainheading.
Figure4:TheOutputViewer
http://www.youtube.com/watch?v=8dDIw_oJsBs
InFigure4Iconductedagraphingprocedurefollowedbyaunivariateanalysisofvariance(ANOVA)andsothesenamesappearasmainheadings.Foreachprocedurethereareaseriesofsub-headingsthatrepresentsdifferentpartsoftheanalysis.Forexample, intheANOVAprocedure,whichyou’ll learnmoreaboutlater inthecourse,thereareseveralsectionstotheoutputsuchasLevene’stestandatableofthebetween-groupeffects.Youcanskiptoanyoneofthesesub-componentsoftheANOVAoutputbyclickingontheappropriatebranchofthetreediagram.So,ifyouwantedtoskipstraighttothebetween-groupeffectsyoushouldmovetheon-screenarrowtotheleft-handportionofthewindowandclickwhereitsaysTestsofBetween-SubjectsEffects.Thisactionwillhighlightthispartoftheoutputinthemainpartoftheviewer
Graph of some data
Tree diagram of the current
output
Results of statistical analysis
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Presenting Data
Does and Don’ts of Presenting Data Theadventofcomputershasallowedpeopletospendvastamountsoftimeproducingverysnazzylookinggraphs,butIhopetoconvinceyouthatsnazzyisnotalwaysbest!Tuftehaswrittenextensivelyabouthowdatashouldbepresented(e.g.,Tufte,2001).SomeofbookslookatvarioustypesofvisualdisplaysanddiscussessomeofTufte’sadvice(Field,2013,2016);Briefly,Tuftepointsoutthatgraphsshould,amongstotherthings:
ü Showthedata
ü Inducethereadertothinkaboutthedatabeingpresented(ratherthansomeotheraspectofthegraph,likehownicethecolourschemeis).
ü Avoiddistortingthedata
ü Presentmanynumberswithminimumink!
ü Makelargedatasetscoherent
ü Encouragethereadertocomparedifferentpiecesofdata
ü Revealdata.
However,graphsoftendon’tdothesethings.Let’slookatanexampleofabadgraph.
Figure5:Acringinglybadexampleofagraphfromthefirsteditionofmybook‘DiscoveringStatistics…’(2000),andabetterexampleofhowitshouldbedone
Oneofthemosttrulyappallingexamplesofbadgraphicswasdonebymeinthefirsteditionofmybook‘DiscoveringStatistics…’(2000).OverexcitedbySPSS’sabilitytoputallsortsofuselesscrapongraphs(like3-Deffects,filleffectsandsoon)Iliterallywentintosomeweirdorgasmicstateandproducedanabsoluteabomination.It’sagraphshowingthemeannumberofobsessive thoughtsandbehavioursexperiencedbygroupsofpeople suffering fromobsessivecompulsivedisorderafterthreetypesoftherapy:CognitiveBehaviourTherapy(CBT),Behaviourtherapy(BT)andnotreatment.Figure5reproducesthisgraphandshowsanalternativedisplayofthesedata.What’swrongwiththeoriginal(left)?
û Thebarshavea3-Deffect:Neveruse3-Dona2-Dgraphbecauseallitdoesisobscurethedata.Inparticular,itmakesithardtoseethevaluesofthebarsbecauseofthe3-Deffect.Thisgraphisagreatexamplebecausethe3-Deffectmakestheerrorbarsalmostimpossibletoread.
TherapyCBT BT No Treatment
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Error Bars Show 95% CI
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û Patterns:Thebarsalsohavepatterns,whichalthoughverypretty,merelydistracttheeyefromwhatmatters(namelythedata).Thesearecompletelyunnecessary!
û Cylindricalbars:what’sthatallabout?
û Badlylabelledy-axis:‘number’ofwhat…delusions?Fish?Cabbageeatingsealizardsfromthe8thdimension?Idiotswhodon’tknowhowtodrawgraphs?
Now,takealookattheredoneversion(right).Whatimprovementshavebeenmade?
ü 2-D:Thecompletelyunnecessarythirddimensionisgonemakingitmucheasiertocomparethevaluesacrosstherapiesandthoughts/behaviours.
ü They-axishasamoreinformativelabel:wenowknowthatiswasthenumberofobsessivethoughtsoractionsperdaythatwasbeingmeasured.
ü Distractions:Therearefewerdistractionslikepatterns,cylindricalbarsandthelike!
Tufte(2001)goesastepfurtherandrecommendstryingtominimisetheamountofinkusedtopresentdata.
Governmentslovetoliewithstatistics,butscientistsshouldn’t.Howyoupresentyourdatacanmakeahugedifferencetothemessageconveyedtotheaudience.Figure6showstwographsthat,believeitornot,displayexactlythesamedata.Thefirstpanelshowshowthegraphshouldprobablybescaled.Theimportantthingisthatthey-axisbeginsat0,andthiscreatesthecorrectimpression:thattherepeoplehavemorenightmaresaftereatingcheese.However,imagineyouweretheministerforcheeseandyouwantedtocreatetheimpressionthatcheesedoesnotinducenightmares,allyouwouldneedtodoisre-scalethegraph(bynotstartingthey-axisatzero)andtheresuddenlyseemstobeonlyasmalldifferencedifference.Temptingasitis,don’tdothis(unlessyouplantobeapolitician,inwhichcaseyoumightaswellstartpracticingthiskindofdeceit).
TodrawacleargraphfollowafewofTufte’srecommendations:
ü Don’tcreatefalseimpressionsofwhatthedataactuallyshow(likewise,don’thideeffects!)byscalingthey-axisinsomeweirdway.
ü Abolishchartjunk:don’tusepatterns,3-Deffects,shadows,picturesofhippopotami,photosofyourgrandmaoranythingelse.
ü Avoidexcessink:thisisabitradical,butifyoudon’tneedtheaxis,thengetridofthem.
Figure6:twographsshowingthesamething
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The SPSS Chart Builder InSPSSwecanusetheall-singingandall-dancingChartBuildertoproducegraphs.Figure7showsthebasicChartBuilderdialogbox,whichisaccessedthroughthe menu.Therearesomeimportantpartsofthisdialogbox:
® Gallery: Foreach typeofgraph,agalleryofpossiblevariants is shown.Double-clickonan icon to selectaparticulartypeofgraph.
® Variablelist:Thevariablesinthedataeditorarelistedhere.Thesecanbedraggedintodropzonestospecifywhatisshowninagivengraph.
® Thecanvas:Thisisthemainareainthedialogboxandiswhereapreviewofthegraphisdisplayedasyoubuildit.
® Dropzones:Thesezonesaredesignatedwithbluedottedlines.Youcandragvariablesfromthevariablelistintothesezones.
Therearetwowaystobuildagraph:thefirstisbyusingthegalleryofpredefinedgraphsandthesecondisbybuildingagraphonanelement-by-elementbasis.Thegalleryisthedefaultoptionandthistab( )isautomaticallyselectedandisfineforourpurposes.
Figure7:TheSPSSChartBuilder
Variables list: variables
in the data editorare displayed here.
Gallery: Select astyle of graph by
clicking on an item on this list
Drop Zones: Variables can be dragged into
these zones.
The Canvas: An example graph will appear here
as you build it.
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Whenyoufirstusethechartbuildertodrawagraphyouwillseeadialogboxthatseemstosignalan impendingapocalypse. Infact,SPSSisjusthelpfully(?!)remindingyouthatfortheChartBuildertowork,youneedtohavesetthelevel of measurement correctly for eachvariable. That is, when you defined eachvariableyoumusthavesetthemcorrectlytobeScale,OrdinalorNominal.This isbecauseSPSS needs to knowwhether variables are categorical (nominal) or continuous (scale)when it
createsthegraphs.Ifyouhavebeendiligentandsetthesepropertieswhenyouenteredthedatathensimplyclickon tomakethedialogdisappear.Ifyouforgot to set the level of measurement for any variables then click on
togotoanewdialogboxinwhichyoucanchangethepropertiesofthevariablesinthedataeditor.
Graphing means: bar charts and error bars Howyoucreatebarcharts inSPSSdepends largelyonhowyoucollectedyourdata(whetherthemeanscomefromindependentcasesandare,therefore,independent,orcamefromthesamecasesandsoarerelated).Forthisreason,wewilllookatavarietyofsituations.Figure8showsthevariousoptionsinthechartbuilderundertheoption‘bar’.GivenwhatI’vesaidabove,avoidthe3-Doptions.Themainoptionsthatyou’llusewillbe:
® Simplebar:Usethisoptionwhenyoujustwanttoseethemeansofscoresacrossdifferentgroupsofcases.Forexample,youmightwanttoplotthemeanratingsoftwofilms.
® Clusteredbar:Ifyouhadasecondgroupingvariableyoucouldproduceasimplebarchart(asabove)butwithbarsproducedindifferentcoloursforlevelsofasecondgroupingvariable.Forexample,youcouldhaveratingsofthetwofilms,butforeachfilmhaveabarrepresentingratingsof‘excitement’andanotherbarshowingratingsof‘enjoyment’.
Figure8:Thebarchartgallery
Simple bar charts for independent means
Firstofall,let’sjustplotthemeanratingofthetwofilmsfromourfilmdatathatwetypedinearlier.Openthedatafilethatyousavedearlier(ChickFlick.sav).Wehavejustonegroupingvariable(thefilm)andoneoutcome(thearousal);therefore,wewantasimplebarchart.IntheChartBuilderdouble-clickontheiconforasimplebarchart(Figure8).Onthecanvasyouwillseeagraphandtwodropzones:oneforthey-axisandoneforthex-axis.They-axisneedstobethedependentvariable,orthethingyou’vemeasured,ormoresimplythethingforwhichyouwanttodisplaythemean.In
Simple Histogram
Stacked Histogram
Frequency Polygon
Population Pyramid
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thiscaseitwouldbearousal,soselectarousalfromthevariablelistanddragitintothey-axisdropzone().Thex-axisshouldbethevariablebywhichwewanttosplitthearousaldata.Toplotthemeansforthetwofilms,selectthevariablefilmfromthevariablelistanddragitintothedropzoneforthex-axis( ).
Figure9showsthecompletedChartBuilderforthebarchartandsomeotheroptions.The‘elementproperties’dialogboxshouldappearwhenyouselectthetypeofgraphyouwant,butifitdoesn’tclickon intheChartBuilder.Therearethreeimportantfeaturesofthisdialogbox.Thefirstisthat,bydefault,thebarswilldisplaythemeanvalue.Thisisfine,butjustnotethatyoucanplotothersummarystatisticssuchasthemedianormode.Second,justbecauseyou’veselectedasimplebarchartdoesn’tmeanthatyouhavetohaveabarchart.Also,youcanaskSPSStoadderrorbarstoyourbarcharttocreateanerrorbarchartbyselecting .Youhaveachoiceofwhatyourerrorbars represent. Normally, error bars show the 95% confidence interval, and I have selected this option (
).Note,though,thatyoucanchangethewidthoftheconfidenceintervaldisplayedbychangingthe‘95’toadifferentvalue.Youcanalsodisplaythestandarderror(thedefaultistoshow2standarderrors,butyoucanchangethisto1)orstandarddeviation(again,thedefault is2butthiscouldbechangedto1oranothervalue). It’simportantthatwhenyouchangethesepropertiesthatyouclickon : ifyoudon’tthenthechangeswillnotbeappliedtoChartBuilder.Clickon toproducethegraph.
Figure9:Dialogboxesforasimplebarchartwitherrorbar
Figure10showstheresultingbarchart.Thisgraphdisplaysthemean(andtheconfidenceintervalofthosemeans)andshowsusthatonaverage,peopleweremorearousedbyMementothantheywerebyBridgetJones’Diary.However,weoriginallywantedtolookforgendereffects,sothisgraphisn’treallytellinguswhatweneedtoknow.Thegraphweneedisaclusteredgraph.
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Figure10:Barchartofthemeanarousalforeachofthetwofilms.
Clustered bar charts for independent means
Todoaclusteredbarchartformeansthatareindependent(i.e.havecomefromdifferentgroups)weneedtodouble-clickontheclusteredbarcharticonintheChartBuilder(Figure8).Onthecanvasyouwillseeagraphaswiththesimple
barchartbutthereisnowanextradropzone: .Allweneedtodoistodragoursecondgroupingvariableintothisdropzone.Aswiththepreviousexample,selectarousalfromthevariablelistanddragitinto ,thenselectfilmfromthevariablelistanddragitinto .Inaddition,though,selecttheGendervariable
anddragit into .Thiswillmeanthatbarsrepresentingmalesandfemaleswillbedisplayedindifferentcolours.Asintheprevioussection,selecterrorbarsinthepropertiesdialogboxandclickon toapplythemtotheChartBuilder.Figure11showsthecompletedChartBuilder.Clickon toproducethegraph.
Figure11:Dialogboxesforaclusteredbarchartwitherrorbar
Figure12showstheresultingbarchart.LikethesimplebarchartthisgraphtellsusthatarousalwasoverallhigherforMementothanBridgetJones’Diary,butitalsosplitsthisinformationbygender.ThemeanarousalforBridgetJones’
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Diaryshowsthatmaleswereactuallymorearousedduringthisfilmthanfemales.Thisindicatestheyenjoyedthefilmmorethanthewomendid!ContrastthiswithMemento,forwhicharousallevelsarecomparableinmalesandfemales.Onthefaceofit,thiscontradictstheideaofa‘chickflick’:itactuallyseemsthatmenenjoychickflicksmorethanthechicks(probablybecauseit’stheonlyhelpwegettounderstandthecomplexworkingsofthefemalemind!).
Figure12:Barchartofthemeanarousalforeachofthetwofilms
Simple bar charts for related means
LoadthefileHiccups.savthatyousavedearlieron.Intheprevioustwoexampleswehaveusedgroupingvariablestospecifyaspectsofthegraph(e.g.weusedthegroupingvariablefilmtospecifythex-axis).Forrepeated-measuresdatawewillnothavethesegroupingvariablesandsotheprocessofbuildingagraphisalittlemorecomplicated(butnotalotmore).
Figure13:Specifyingasimplebarchartforrepeated-measuresdata
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ToplotthemeannumberofhiccupsgototheChartBuilderanddouble-clickontheiconforasimplebarchart(Figure8).Asbefore,youwillseeagraphonthecanvaswithdropzonesforthex-andy-axis.Previouslywespecifiedthecolumninourdatathatcontaineddatafromouroutcomemeasureonthey-axis,butforthesedatawehavefourcolumnscontainingdataonthenumberofhiccups(theoutcomevariable).Whatwehavetodothenistodragallfourofthesevariablesfromthevariablelistintothey-axisdropzone.Wehavetodothissimultaneously.First,weneedtoselectmultipleitemsinthevariablelist:todothis,selectthefirstvariablebyclickingonitwiththemouse.Thevariablewillbehighlightedinblue.Now,holddowntheCtrlkeyonthekeyboardandclickonasecondvariable.Bothvariablesarenowhighlightedinblue.Again,holddowntheCtrlkeyandclickonathirdvariableinthevariablelistandsoonforthefourth.Incasesinwhichyouwanttoselectalistofconsecutivevariables,youcandothisveryquicklybysimplyclickingonthefirstvariablethatyouwanttoselect(inthiscasebaseline),holddowntheShiftkeyonthekeyboardandthenclickonthelastvariablethatyouwanttoselect(inthiscasedigitalrectalmassage);noticethatallofthevariablesinbetweenhavebeenselectedtoo.Oncethefourvariablesareselectedyoucandragthembyclickingonanyoneofthevariablesandthendraggingtheminto asshowninFigure13.
Figure14:TheCreateSummaryGroupdialogbox
Figure15:SettingElementPropertiesforarepeated-measuresgraph
Onceyouhavedraggedthefourvariablesontothey-axisdropzonesanewdialogboxappears(Figure14).ThisboxtellsusthatSPSSiscreatingtwotemporaryvariables.OneiscalledSummary,whichisgoingtobetheoutcomevariable(i.e.whatwemeasured—inthiscasethenumberofhiccupsperminute).Theotheriscalledindexandthisvariablewill
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representourindependentvariable(i.e.whatwemanipulated—inthiscasethetypeofintervention).SPSSusesthesetemporarynamesbecause it doesn’t knowwhatourparticular variables represent, butwe should change them tosomethingmorehelpful!Justclickon togetridofthisdialogbox.
Weneedtoeditsomeofthepropertiesofthegraph.Figure15showstheoptionsthatneedtobeset:ifyoucan’tseethisdialogboxthenclickon intheChartBuilder.IntheleftpanelofFigure15justnotethatIhaveselectedtodisplayerrorbars(seetheprevioustwosectionsformoreinformation).ThemiddlepanelisaccessedbyclickingonX-Axis1(Bar1)inthelistlabelledEditPropertiesofwhichallowsustoeditpropertiesofthehorizontalaxis.ThefirstthingweneedtodoisgivetheaxisatitleandIhavetypedInterventioninthespacelabelledAxisLabel.Thislabelwillappearonthegraph.Also,wecanchangetheorderofourvariablesifwewanttobyselectingavariableinthelistlabelledOrderandmovingitupordownusing and .Ifwechangeourmindaboutdisplayingoneofourvariablesthenwecanalsoremoveitfromthelistbyselectingitandclickingon .Clickon forthesechangestotakeeffect.TherightpanelofFigure15isaccessedbyclickingonY-Axis1(Bar1)inthelistlabelledEditPropertiesofwhichallowsustoeditpropertiesoftheverticalaxis.ThemainchangethatIhavemadehereistogivetheaxisalabelsothatthefinalgraphhasausefuldescriptionontheaxis(bydefaultitwilljustsayMean,whichisn’tveryhelpful).Ihavetyped‘MeanNumberofHiccupsPerMinute’intheboxlabelledAxisLabel.Alsonotethatyoucanusethisdialogboxtosetthescaleoftheverticalaxis(theminimumvalue,maximumvalueandthemajorincrement,whichishowoftenamarkismadeontheaxis).MostlyyoucanletSPSSconstructthescaleautomaticallyanditwillbefairlysensible—andevenifit’snotyoucanedititlater.Clickon toapplythechanges.
Figure 16: Completed Chart Builder for a repeated-measuresgraph
Figure 17: Bar chart of the mean number of hiccups atbaselineandaftervariousinterventions
Figure16showsthecompletedChartBuilder.Clickon toproducethegraph.TheresultingbarchartinFigure17displays themean (and the confidence intervalof thosemeans)numberofhiccupsatbaselineandafter the threeinterventions.NotethattheaxislabelsthatItypedinhaveappearedonthegraph.Theerrorbarsongraphsofrepeated-measuresdesignsaren’tactuallycorrect(seeField,2013,Chapter9)butyou’llhavetoreadthebookifyouwanttosortthatproblemout!Wecanconcludethattheamountofhiccupsaftertonguepullingwasaboutthesameasatbaseline;however,carotidarterymassagereducedhiccups,butnotbyasmuchasagoodoldfashioneddigital-rectalmassage.Themoralhereis:ifyouhavehiccups,findsomethingdigitalandgoamuseyourselfforafewminutes.
Clustered bar charts for related means
Nowwehaveseenhowtoplotmeansthatarerelated(i.e.showdifferentconditionsappliedtothesamegroupofcases),youmightwellwonderwhatyoudoifyouhaveasecondindependentvariablethathadbeenmeasuredinthe
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samesample.You’ddoaclusteredbarchart,right?Wrong?Actually,theSPSSChartBuilderdoesn’tappeartobeabletocopewiththissituationatall—atleastnotthatIcanworkoutfromplayingaboutwithit.
Clustered bar charts for ‘mixed’ designs
TheChartBuildermightnotbeabletodochartsformultiplerepeated-measuresvariables,but itcangraphwhat isknownasamixeddesign.Thisisadesigninwhichyouhaveoneormoreindependentvariablesmeasuredusingdifferentgroups, and one ormore independent variablesmeasured using the same sample. Basically, the Chart Builder canproduceagraphprovidedyouhaveonlyonevariablethatwasarepeatedmeasure.
Weallliketotext-message.Whatwillhappentothechildren,though?Notonlywilltheydevelopsuper-sizedthumbs,theymightnotlearncorrectwrittenEnglish.Imagineweconductedanexperimentinwhichagroupof25childrenwasencouragedtosendtextmessagesontheirmobilephonesoverasix-monthperiod.Asecondgroupof25childrenwasforbidden fromsending textmessages for thesameperiod.Toensure thatkids in this lattergroupdidn’tuse theirphones, thisgroupwasgivenarmbandsthatadministeredpainfulshocks in thepresenceofmicrowaves (like thoseemittedfromphones).Theoutcomewasascoreonagrammaticaltest(asapercentage)thatwasmeasuredbothbeforeand after the intervention. The first independent variablewas, therefore, textmessageuse (textmessagers versuscontrols)andthesecondindependentvariablewasthetimeatwhichgrammaticalabilitywasassessed(baselineoraftersixmonths).ThedataareinthefileTextMessages.sav.
Figure18:Selectingtherepeated-measuresvariableintheChartBuilder
Tographthesedata,followtheprocedureforgraphingrelatedmeans.Ourrepeated-measuresvariableistime(whethergrammaticalabilitywasmeasuredatbaselineorsixmonths)andisrepresentedinthedatafilebytwocolumns,oneforthebaselinedataandtheotherforthefollow-updata.IntheChartBuilderselectthesetwovariablessimultaneouslybyclickingononeandthenholdingdowntheCtrlkeyonthekeyboardandclickingontheother.Whentheyarebothhighlighted click on either one and drag it into as shown in Figure 18. The second variable (whetherchildrentextmessagedornot)wasmeasuredusingdifferentchildrenandsoisrepresentedinthedatafilebyagrouping
variable(group).Thisvariablecanbeselectedinthevariablelistanddraggedinto .Thetwogroupswillbedisplayedasdifferent-colouredbars.
® Usewhatyoulearntearlierinthishandouttoadderrorbarstothisgraphandtolabelboththex-(Isuggest‘Time’)andy-axis(Isuggest‘MeanGrammarScore(%)’).
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ThefinishedChartBuilderisinFigure19.Clickon toproducethegraph.Figure20showstheresultingbarchart.Itshowsthatatbaseline(beforetheintervention)thegrammarscoreswerecomparableinourtwogroups;however,aftertheintervention,thegrammarscoreswerelowerinthetextmessagersthaninthecontrols.Also,ifyoucomparethetwobluebarsyoucanseethattextmessagers’grammarscoreshavefallenoverthesixmonths;comparethistothecontrols(greenbars)whosegrammarscoresarefairlysimilarovertime.Wecould,therefore,concludethattextmessaginghasadetrimentaleffectonchildren’sunderstandingofEnglishgrammarandcivilizationwillcrumble,withAbaddonrisingcacklingfromhisbottomlesspittoclaimourwretchedsouls.Maybe.
Figure 19: Completed dialog box for an error bargraphofamixeddesign
Figure20:Errorbargraphofthemeangrammarscoreoversixmonths in childrenwhowere allowed to text-messageversusthosewhowereforbidden
Editing Graphs Youcaneditgraphs,there’samoviethatshowsyouhow.
http://www.youtube.com/watch?v=en0t0QS9uo8
Tasks
Task 1 Aneurologistcarriedoutanexperimenttoinvestigatethedepressanteffectsofcertainrecreationaldrugs.Shetested20clubbersinall:10weregivenanecstasytablettotakeonaSaturdaynightand10wereallowedonlytodrinkalcohol.LevelsofdepressionweremeasuredusingtheBeckDepressionInventory(BDI)thedayafterandmidweek.
UsingwhatyouknowaboutenteringdatabelowintoSPSS,EnterthedataaboveintotheSPSSDataEditor.Drawtwobarcharts;onetoseewhetherecstasymakesyoumoredepressedthanalcoholthedayafter(Sunday)andtheothershowingwhetherecstasymakesyoumoredepressedthanalcoholmidweek(Wednesday).
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Table3:Drugdata
Participant Drug BDI(Sunday) BDI(Wednesday)
1 Ecstasy 15 28
2 Ecstasy 35 35
3 Ecstasy 16 35
4 Ecstasy 18 24
5 Ecstasy 19 39
6 Ecstasy 17 32
7 Ecstasy 27 27
8 Ecstasy 16 29
9 Ecstasy 13 36
10 Ecstasy 20 35
11 Alcohol 16 5
12 Alcohol 15 6
13 Alcohol 20 30
14 Alcohol 15 8
15 Alcohol 16 9
16 Alcohol 13 7
17 Alcohol 14 6
18 Alcohol 19 17
19 Alcohol 18 3
20 Alcohol 18 10
Task 2 Statisticsandmathsanxietyarecommonandaffectpeople’sperformanceonmathsandstatsassignments;womeninparticular can lack confidence in mathematics (Field, 2010). Zhang, Schmader, and Hall (2013) did a study did anintriguing study inwhich students completedamaths test inwhich someput theirownnameon the testbooklet,whereasothersweregivenabookletthatalreadyhadeitheramaleorfemalenameon.Participantsinthelattertwoconditionsweretoldthatthenamesonthebookletsweretoprotecttheiranonymityandthattheywouldbeusingthisotherperson’snameforthepurposeofthetest.Womenwhocompletedthetestusingadifferentnameperformedbetterthanthosewhocompletedthetestusingtheirownname.(Therewerenosucheffectsformen.)Zhangetal.concludedthatperformingunderadifferentnamefreedwomenfromfearsofself-evaluation,allowingthemtoperformbetter.
Table4containsasmallsubsetofthedatafromZhangetal.’sstudy.
UsingwhatyouknowaboutenteringdataintoSPSS,enterthedatabelowintotheSPSSDataEditor.Drawaclusteredbarcharttoshowthemeanmathstestscore(outof100)formalesandfemalescompletingthetestunderthethreedifferenttypesofnames.
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Table4:AsubsampleofZhangetal.’s(2013)data
Male Female
FemaleFakeName
MaleFakeName
OwnName
FemaleFakeName
MaleFakeName
OwnName
33 69 75 53 31 70
22 60 33 47 63 57
46 82 83 87 34 33
53 78 42 41 40 83
14 38 10 62 22 86
27 63 44 67 17 65
64 46 27 57 60 64
62 27 47 37
75 61 57 80
50 29
Task 3 According to some highly unscientific research done by a UK department store chain and reported inMarie Claremagazine(http://ow.ly/9Dxvy)shoppingisgoodforyou:theyfoundthattheaveragewomenspends150minutesandwalks 2.6 miles when she shops, burning off around 385 calories. In contrast, men spend only about 50 minutesshopping,covering1.5miles.Thiswasbasedonstrappingapedometeronamere10participants.AlthoughIdon’thavetheactualdata,somesimulateddatabasedonthesemeansarebelow.EnterthesedataintoSPSSandsavethemasShoppingExercise.sav.
Table5:Shoppingdata
Male FemaleDistance Time Distance Time
0.16 15 1.40 220.40 30 1.81 1401.36 37 1.96 1601.99 65 3.02 1833.61 103 4.82 245
Task 4 Iwastakenbytwonewstories.ThefirstwasaboutaSudanesemanwhowasforcedtoMarryGoatafterbeingcaughthavingsexwithit(http://ow.ly/9DyyP).I’mnotsurehetreatedthegoattoanicedinnerinaposhrestaurantbeforetakingadvantageofher,buteitherwayyouhavetofeelsorryforthegoat.I’dbarelyhadtimetorecoverfromthatstorywhenanotherappearedaboutanIndianmanforcedtomarryadogtoatoneforstoningtwodogsandstringingthemupinatree15yearsearlierhttp://ow.ly/9DyFn.Whyanyonewouldthink it’sagoodideatoenteradog intomatrimonywithamanwithahistoryofviolentbehaviourtowardsdogsisbeyondme.Still,Iwonderedwhetheragoatordogmadeabetterspouse.Ifoundsomeotherpeoplewhohadbeenforcedtomarrygoatsanddogsandmeasuredtheirlifesatisfactionand,also,howmuchtheylikeanimals.EnterthesedataintoSPSSandsaveasGoatorDog.sav.
Table6:Goatanddogdata
Goat Dog
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AnimalLiking LifeSatisfaction AnimalLiking LifeSatisfaction
69 47 16 5225 6 65 6631 47 39 6529 33 35 6112 13 19 6049 56 53 6825 42 27 3735 51 44 7251 42 40 46 23 27 37 48
References Fesmire,F.M.(1988).Terminationofintractablehiccupswithdigitalrectalmassage.AnnalsofEmergencyMedicine,
17(8),872.Field,A.P.(2010).TeachingStatistics.InD.Upton&A.Trapp(Eds.),TeachingPsychologyinHigherEducation(pp.134-
163).Chichester,UK:Wiley-Blackwell.Field,A.P.(2013).DiscoveringstatisticsusingIBMSPSSStatistics:Andsexanddrugsandrock'n'roll(4thed.).London:
Sage.Field,A.P.(2016).Anadventureinstatistics:therealityenigma.London:Sage.Tufte,E.R.(2001).Thevisualdisplayofquantitativeinformation(2nded.).Cheshire,CT:GraphicsPress.Zhang,S.,Schmader,T.,&Hall,W.M.(2013).L'eggoMyEgo:ReducingtheGenderGapinMathbyUnlinkingtheSelf
fromPerformance.SelfandIdentity,12(4),400-412.doi:10.1080/15298868.2012.687012
Terms of Use Thishandoutcontainsmaterialfrom:
Field,A.P.(2013).DiscoveringstatisticsusingSPSS:andsexanddrugsandrock‘n’roll(4thEdition).London:Sage.
ThismaterialiscopyrightAndyField(2000-2016).
This document is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 InternationalLicense,basicallyyoucanuseitforteachingandnon-profitactivitiesbutnotmeddlewithitwithoutpermissionfromtheauthor.