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ChoosingAppropriateGraphs or
Whattodowithallthatdata…
BarGraphsareappropriatefordatathatarenon‐numericalanddiscreteforatleastonevariable,i.e.theyaregroupedintoseparatecategories.Therearenodependentorindependentvariables.Importantfeaturesofthistypeofgraphinclude:
• Dataarecollectedfordiscontinuous,non‐numericalcategories(e.g.place,colour,andspecies)sothebarsdonottouch.
• Datavaluesmaybeenteredonorabovethebarsifyouwish.• Multiplesetsofdatacanbedisplayedside‐by‐sidefordirectcomparison(e.g.males
andfemalesofthesameagegroup).• Axesmaybereversedsothatthecategoriesareonthex‐axis,i.e.thebarscanbe
verticalorhorizontal.Whentheyarevertical,thesegraphsaresometimescalledcolumngraphs(MSExcelusesthisnameforverticalbargraphs).
Histogramsareplotsofcontinuousdataandareoftenusedtorepresentfrequencydistributions,wherethey‐axisshowsthenumberoftimesaparticularmeasurementorvaluewasobtained.Forthisreason,theyareoftencalledfrequencyhistograms.Importantfeaturesofthistypeofgraphinclude:
• Thedataarenumericaland
continuous(e.g.heightorweight)sothebarstouch.(AcolumngraphfromExcelcanbemadeintoahistogram).
• Thex‐axisusuallyrecordstheclassinterval.They‐axisusuallyrecordsthenumberofindividualsineachclassinterval(frequency).
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PieGraphscanbeusedinsteadofbargraphs,generallyincaseswheretherearesixoffewercategoriesinvolved.Apiegraphprovidesstrongvisualimpactoftherelativeproportionsineachcategory,particularlywhereoneofthecategoriesisverydominant.Featuresofpiegraphsinclude:
• Thedataforonevariablearediscontinuous(non‐numericalorcategories).• Thedataforthedependentvariableareusuallyintheformofcounts,proportionsor
percentages.• Piegraphsaregoodforvisualimpactandshowingrelativeproportions.• Theyarenotsuitablefordatasetswithalargenumberofcategories.
LineGraphsareusedwhenonevariable(theindependentvariable)affectsanother,thedependentvariable.Linegraphscanbedrawnwithorwithoutameasureofspread(errorbars).Importantfeaturesoflinegraphsinclude:
• Thedatamustbecontinuousforbothvariables.• Thedependentvariableisusuallythebiologicalresponse.• Theindependentvariableisoftentimeortheexperimentaltreatment.• Incaseswherethereisanimpliedtrend(e.g.onevariableincreaseswiththeother)
alineofbestfitisusuallyplottedthroughthedatapointstoshowtherelationship.• Iffluctuationsinthedataarelikelytobeimportant(e.g.withclimateandother
environmentaldata)thedatapointsareusuallyconnecteddirectly(point‐to‐point).• Linegraphsmaybedrawnwithameasureoferror.Thedataarepresentedas
points(thecalculatedmeans),withbarsaboveandbelow,indicatingameasureofvariabilityorspreadinthedata(e.g.standarderror,standarddeviationor95%confidenceintervals).
• Wherenoerrorvaluehasbeencalculated,thescattercanbeshownbyplottingtheindividualdatapointsverticallyaboveandbelowthemean.Byconvention,barsarenotusedtoindicatetherangeofrawvaluesinadataset.
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Whenyouaretryingtocomparemultipledatasets,itismostusefultodosoonthesameaxes:
• Ifthetwodatasetsusethesamemeasurementunitsandasimilarrangeofvaluesfortheindependentvariables,onescaleonthey‐axisisused.
• Ifthetwodatasetsusedifferentunitsand/orhaveaverydifferentrangeofvaluesfortheindependentvariable,twoscalesforthey‐axisareused(seeexampleprovided).Thescalescanbeadjustedifnecessarytoavoidoverlappingplots.
• Thetwocurvesmustbedistinguished(usealegend!)
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Kitegraphsareidealforrepresentingdistributionaldata(e.g.abundancealonganenvironmentalgradient).Theyareelongatedfiguresdrawnalongabaseline.Importantfeaturesofkitegraphsinclude:
• Eachkiterepresentschangesinspeciesabundanceacrossalandscape.Theabundancecanbecalculatedfromthekitewidth.
• Theyofteninvolveplotsformorethanonespecies;thismakesthemgoodforhighlightingprobabledifferencesinhabitatpreferencesbetweenspecies.
• Athinlineonakitegraphrepresentsspeciesabsence(zerovalue).
• Theaxescanbereverseddependinguponpreference.
• Kitegraphsmayalsobeusedtoshowchangesindistributionwithtime,forexamplewithdailyorseasonalcyclesofmovement.
Note–MSExcelwillonlydoakitegraphifyouhavetheMerlinAdd‐In.(PConly)ScatterGraphsareacommonwaytodisplaycontinuousdatawherethereisarelationshipbetweentwointerdependentvariables.Importantfeaturesofscattergraphsinclude:
• Thedataforthegraphmustbecontinuousforbothvariables.• Thereisnoindependent(manipulated)variable,butthevariablesareoften
correlated,i.e.theyvarytogetherinsomepredictableway.• Scattergraphsareusefulfordeterminingtherelationshipbetweentwovariables.• Thepointsonthegraphneednotbeconnected,butalineofbestfitisoftendrawn
throughthepointstoshowtherelationshipbetweenthevariables.o Thismaybedrawnbyeyeorcomputer‐generated.
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