revision of spss (2016) - discovering statistics · © prof. andy field, 2016 page 4 table 1: film...

©Prof.AndyField,2016 www.discoveringstatistics.com Page1

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!


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


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.


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


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


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.


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


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

Num

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ts/A

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ay

0

2

4

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18ThoughtsActions

Error Bars Show 95% CI


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


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.


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


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.


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’


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


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


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


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(%)’).


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).


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.


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


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.

http://creativecommons.org/licenses/by-nc-nd/4.0/

revision of spss (2016) - discovering statistics · © prof. andy field, 2016 page 4 table 1: film...

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