Answer. Thequestionsaddressedbyscientificinquiryarebasedonobservationsor

APBIOLOGYNAME______

DESIGNINGACONTROLLEDEXPERIMENT

STEP1:DEFININGTHEPROBLEM

Everyscientificinvestigationbeginswiththequestionthatthescientistwantsto

answer. Thequestionsaddressedbyscientificinquiryarebasedonobservationsor

oninformationgainedthroughpreviousresearch,oronacombinationofboth. Just becauseaquestioncanbeanswereddoesn’tmeanthatitcanbeanswered scientifically.

1.Discussthefollowingquestionswithyourlabgroupanddecidewhichofthem youthinkcanbeansweredbyscientificinquiry.

WhatisthecauseofAIDS?

Areserialkillersevilbynature?

Whyisthegrassgreen?

Whatisthebestrecipeforchocolatechipcookies?

WhenwilltheBigEarthquakehitSanFrancisco?

Howcanthemaximumyieldbeobtainedfromapeanutfield?

Doeswatchingtelevisioncausechildrentohaveshorterattentionspans?

2.Howdidyoudecidewhatquestionscanbeansweredscientifically?

STEP2: IDENTIFYINGTHEDEPENDENTVARIABLE(S)

Thedependentvariableiswhattheinvestigatormeasures(orcountsorrecords).

Itiswhattheinvestigatorthinkswillvaryduringtheexperiment. Forexample,he

maywanttostudypeanutgrowth. Onepossibledependentvariableistheheightof thepeanutplants.

3.Namesomeotheraspectsofpeanutgrowththatcanbemeasured.

Alloftheseaspectsofpeanutgrowthcanbemeasuredandcanbeusedas dependentvariablesinanexperiment. Therearedifferentdependentvariables possibleinanexperiment. Theinvestigatorcanchoosetheonehethinksismost important,orhecanchoosetomeasuremorethanonedependentvariable.

STEP3: IDENTIFYINGTHEINDEPENDENTVARIABLE

Theindependentvariableiswhattheinvestigatordeliberatelyvariesduringthe

experiment. Itischosenbecausetheinvestigatorthinksitwillaffectthedependent variable.

4.Namesomefactorsthatmightaffectthenumberofpeanutsproducedby peanutplants.

Inmanycases,theinvestigatordoesnotmanipulatetheindependentvariable

directly. Hecollectsdataandusesthedatatoevaluatethehypothesis,ratherthan doingadirectexperiment. Forexample,thehypothesisthatmorecrimesare committedduringafullmooncanbetestedscientifically. Thenumberofcrimes committedisthedependentvariableandcanbemeasuredfrompolicereports. The phaseofthemoonistheindependentvariable. Theinvestigatorcannot

deliberatelychangethephaseofthemoon,butcancollectdataduringanyphase hechooses.

Althoughmanyhypothesesaboutbiologicalphenomenacannotbetestedbydirect manipulationoftheindependentvariable,theycanbeevaluatedscientificallyby collectingdatathatcouldprovethehypothesisfalse. Thisisanimportantmethod

inthestudyofevolution,wheretheinvestigatorisattemptingtotesthypotheses abouteventsofthepast.

Theinvestigatorcanmeasureasmanydependentvariablesashethinksare importantindicatorsofpeanutgrowth. Bycontrasthemustchooseonlyone independentvariabletoinvestigateinanexperiment. Forexample,ifthescientist wantstoinvestigatetheeffectthattheamountoffertilizerhasonpeanutgrowth,

hewillusedifferentamountsoffertilizerondifferentplants;theindependent variableisamountoffertilizer.

5.Whyisthescientistlimitedtooneindependentvariableperexperiment?

Timeisfrequentlyusedastheindependentvariable. Theinvestigatorhypothesizes thatthedependentvariablewillchangeoverthecourseoftime. Forexample,he

maywanttostudythediversityofsoilbacteriafoundduringdifferentmonthsofthe year. However,theunitsoftimeusedmaybeanywherefromsecondstoyears, dependinguponthesystembeingstudied.

6.Identifythedependentandindependentvariablesinthefollowingexamples.

a.Heightofbeanplantsisrecordeddailyfor2weeks.

Dependentvariable:______Independentvariable:______

b.Guineapigsarekeptatdifferenttemperaturesfor6weeks. Percent weightgainisrecorded.

Dependentvariable:______Independentvariable:______

c.Thediversityofalgalspeciesiscalculatedforacoastalareabeforeand

afteranoilspill.

Dependentvariable:______Independentvariable:______

d.Lightabsorptionbyapigmentismeasuredforred,blue,green,and yellowlight.

Dependentvariable:______Independentvariable:______

e.Batchesofseedsaresoakedinsaltsolutionsofdifferent concentrations,andgerminationiscountedforeachbatch.

Dependentvariable:______Independentvariable:______

f.Aninvestigatorhypothesizesthattheadultweightofadogishigher whenithasfewerlittermates.

Dependentvariable:______Independentvariable:______

STEP4: IDENTIFYINGTHESTANDARDIZEDVARIABLES

Athirdtypeofvariableisthestandardizedvariable. Standardizedvariablesare

factorsthatarekeptequalinalltreatments,sothatanychangesinthedependent

variablecanbeattributedtothechangestheinvestigatormadeintheindependent variable.

Sincetheinvestigator'spurposeistostudytheeffectofoneparticularindependent variable,hemusttrytoeliminatethepossibilitythatothervariablesareinfluencing theoutcome. Thisisaccomplishedbykeepingtheothervariablesatconstant

levels,inotherwords,bystandardizingthesevariables. Forexample,ifthe

scientisthaschosentheamountoffertilizerastheindependentvariable,hewants

tobesurethattherearenodifferencesinthetypeoffertilizerused. Hewoulduse thesameformulationandsamebrandoffertilizerthroughouttheexperiment.

7.Whatothervariableswouldhavetobestandardizedinthisexperiment?

STEP5: WRITINGTHEHYPOTHESIS

Ascientificquestionisusuallyphrasedmoreformallyasahypothesis,whichis

simplyastatementofthescientist’seducatedguessattheanswertothequestion.

Ahypothesisisusableonlyifthequestioncanbeanswered“no”.Ifitcanbe answered“no”,thenthehypothesiscanbeprovenfalse. Thenatureofscienceis suchthatwecanproveahypothesisfalsebypresentingevidencefroman investigationthatdoesnotsupportthehypothesis. Butwecannotprovea hypothesistrue. Wecanonlysupportthehypothesiswithevidencefromthis particularinvestigation.

Scientificknowledgeisthusanaccumulationofevidenceinsupportofhypotheses:

itisnottoberegardedasabsolutetruth. Hypothesesareacceptedonlyonatrial basis. Futureinvestigationsmaybeabletoproveanyhypothesisfalse. Current scientificstudiesyoureadaboutinthenewspaper(forexample,investigationsof theeffectsofcaffeine)aresometimesquitepreliminaryandthereforetentativein

nature. Often,studiesarepublishedwhoseresultscontradicteachother.However, thisdoesnotmeanthatscientificknowledgeisflimsyandunreliable. Muchofthe informationinyourtextbook,forexample,isbaseduponmanyexperimentscarried outbynumerousscientistsoveraperiodoftime.

Thescientificmethod,thenappliesonlytohypothesesthatcanbeprovenfalse throughexperimentation. (Thereareothertypesofscientificinvestigation,suchas observationandcomparisonthatdonotinvolvehypothesistesting.) Itisessential

tounderstandthisinordertounderstandwhatisandisnotpossibletolearn throughscience. Consider,forexample,thishypothesis: Morepeoplebehave immorallywhenthereisafullmoonthanatanyothertimeofthemonth. The

phaseofthemooniscertainlyawell-definedandmeasurablefactor,butmoralityis notscientificallymeasurable. Thusthereisnoexperimentthatcanbeperformedtotestthehypothesis.

8.Proposeatestablehypothesisforhumanbehaviorduringafullmoon.

9.Whichofthefollowingwouldbeusefulasscientifichypotheses? Givethe

reasonforyourdecisions.

a.Plantsabsorbwaterthroughtheirleavesaswellasthroughtheirroots.

b.Micerequirecalciumfordevelopingstrongbones.

c.Dogsarehappywhenyoufeedthemsteak.

d.Anactivevolcanocanbepreventedfromeruptingbythrowingavirgin

intoitduringeachfullmoon.

e.Thehighertheintelligenceofananimal,themoreeasilyitcanbe

domesticated.

f.HIV(humanimmunodeficiencyvirus)canbetransmittedbycatfleas.

_

Theinvestigatordevisesanexperimentorcollectsdatathatcouldprovethe

hypothesisfalse. Heshouldalsothinkthroughthepossibleoutcomesofthe experiment(whetherthehypothesisissupportedorprovenfalse)andmake predictionsabouttheeffectoftheindependentvariableonthedependentvariable

ineachsituation. Forexample,ascientisthasmadethefollowinghypothesis: Increasingtheamountoffertilizerappliedwillincreasethenumberofpeanuts produced. Hehasdesignedanexperimentinwhichdifferentamountsoffertilizer

areaddedtoplotsoflandandthenumberofpeanutsyieldedperplotismeasured. Thepredictionsshouldstatespecificallyhowthedependentvariablewillchangein relationtotheindependentvariableandmustbestatedasanIf…Thenstatement. ThegeneralformatforanIf…Thenstatementis“iftheindependentvariableis changedinthisway,thenthedependentvariablewillchangethisway.” For

example,iftheamountoffertilizerappliedtoafieldisdoubled,thenthenumberof peanutsproducedwilldouble. Or,ifthetemperatureofthereactantsinachemical reactionincreases,thentherateofthereactionwillincrease.

10.Writeahypothesisforeachofthefollowing:

a.Guineapigsarekeptatdifferenttemperaturesfor6weeks. Percent weightgainisrecorded.

b.Batchesofseedsaresoakedinsaltsolutionsofdifferent

concentrationsandthenumberofseedsthatgerminateiscountedfor eachbatch.

STEP6:SETTINGTHELEVELSOFTREATMENT

Oncetheinvestigatorhasdecidedwhattheindependentvariableforanexperimentshouldbe,hemustalsodeterminehowtochangeorvarytheindependentvariable.Thevaluessetfortheindependentvariablearecalledthelevelsoftreatment. Forexample,anexperimentmeasuringtheeffectoffertilizeronpeanutyieldhasfivetreatments. Ineachtreatment,peanutsaregrownona100-m2 plotofground, andadifferentamountoffertilizerisappliedtoeachplot. Thelevelsoftreatmentinthisexperimentaresetas200g,400g,600g,800g,and1000gfertilizer/100m2.

Theinvestigator'sjudgmentinsettinglevelsoftreatmentisusuallybasedonprior knowledgeofthesystem. Forexample,ifthepurposeoftheexperimentisto investigatetheeffectoftemperatureonweightgaininguineapigs,thescientist shouldhaveenoughknowledgeofguineapigstouseappropriatetemperatures. Subjectingtheanimalstoextremelyhighorlowtemperaturescankillthemandno usefuldatawouldbeobtained. Likewise,thescientistattemptingtodeterminehow muchfertilizertoapplytopeanutfieldsneedstoknowsomethingabouttheamountstypicallyusedsohecouldvarythetreatmentsaroundthoselevels.

STEP7: IDENTIFYINGTHECONTROLTREATMENT

Itisalsonecessarytoincludecontroltreatmentsinanexperiment. Acontrol

treatmentisatreatmentinwhichtheindependentvariableiseithereliminatedoris setatastandardvalue. Theresultsofthecontroltreatmentarecomparedtothe resultsoftheexperimentaltreatments. Inthefertilizerexample,theinvestigator mustbesurethatthepeanutsdon'tgrowjustaswellwithnofertilizeratall. The controlwouldbeatreatmentinwhichnofertilizerisapplied. Anexperimentonthe effectoftemperatureonguineapigs,however,cannothavea"notemperature" treatment. Instead,thescientistwilluseastandardtemperatureasthecontrol

andwillcompareweightgainatothertemperaturestoweightgainatthestandard temperature.

11.Tellwhatanappropriatecontroltreatmentwouldbeforeachofthefollowing examples.

a.Aninvestigatorstudiestheamountofalcoholproducedbyyeastwhen

itisincubatedwithdifferenttypesofsugar.

ControlTreatment:______

b.Theeffectoflightintensityonphotosynthesisismeasuredby collectingoxygenproducedbyaplant.

ControlTreatment:______

c.TheeffectofNutraSweetsweetenerontumordevelopmentin laboratoryratsisinvestigated.

ControlTreatment:______

d.Subjectsaregivensquaresofpaperthathavebeensoakedinabitter-

tastingchemical. Theinvestigatorrecordswhethereachpersoncan tastethechemical.

ControlTreatment:______

e.AsolutionismadeuptosimulatestomachacidatpH2. Maalox antacidisaddedtothesolutioninsmallamounts,andthepHis measuredaftereachaddition.

ControlTreatment:______

STEP8: DETERMININGREPLICATION

Anotheressentialaspectofexperimentaldesignisreplication. Replicatingthe

experimentmeansthatthescientistrepeatstheexperimentnumeroustimesusing

exactlythesameconditionstoseeiftheresultsareconsistent. Beingableto replicatearesultincreasesourconfidenceinit. However,weshouldn'texpectto getexactlythesameanswereachtime,becauseacertainamountofvariationis normalinbiologicalsystems. Replicatingtheexperimentletsusseehowmuch variationthereisandobtainanaverageresultfromdifferenttrials.

Aconceptrelatedtoreplicationissamplesize. Itisriskytodrawconclusions

basedupontoofewsamples. Forinstance,supposeascientististestingtheeffects

offertilizeronpeanutproduction. Heplantsfourpeanutplantsandappliesa differentamountoffertilizertoeachplant. Twooftheplantsdie.

12.Canheconcludethattheamountsoffertilizerusedonthoseplantswere lethal? Whatotherfactorsmighthaveaffectedtheresults?

STEP9: WRITINGTHEMETHOD

Afterformulatingahypothesisandselectingtheindependentanddependent

variables,theinvestigatormustfindamethodtomeasurethedependentvariable;

otherwise,thereisnoexperiment. Methodsarelearnedbyreadingarticles

publishedbyotherscientistsandbytalkingtootherscientistswhoare knowledgeableinthefield. Forexample,ascientistwhoistestingtheeffectof

fertilizeronpeanutswouldreadaboutpeanutgrowthandvariousfactorsthataffect

it. Hewouldlearntheacceptedmethodsforevaluatingpeanutyield. Hewould

alsoreadaboutdifferenttypesoffertilizersandtheircomposition,theiruseson differentsoiltypes,andmethodsofapplication. Thescientistmightalsogetin touchwithotherscientistswhostudypeanutsandfertilizersandlearnabouttheir work. Scientistsoftendothisbyattendingconferenceswhereotherscientists presentresultsofinvestigationstheyhavecompleted.

Inthiscourse,methodsaredescribedinthelabs.

QUESTIONS

13.Agroupofstudentshypothesizesthattheamountofalcoholproducedin

fermentationdependsontheamountofglucosesuppliedtotheyeast. They wanttouse5%,10%,15%,20%,25%,and30%glucosesolutions.

a.Whatistheindependentvariable?

b.Whatisthedependentvariable?

c.Whatcontroltreatmentshouldbeused?

d.Whatvariablesshouldbestandardized?

14.Havinglearnedtheoptimumsugarconcentration,thestudentsnextdecideto

investigatewhetherdifferentstrainsofyeastfermentglucosetoproduce differentamountsofalcohol. Brieflyexplainhowthisexperimentwouldbe setup.

15.Agroupofstudentswantstostudytheeffectoftemperatureonbacterial growth. Togetbacteria,theyleavePetridishesofnutrientagaropenona shelf. Theythenputthedishesindifferentplaces:anincubator(37oC),a refrigerator(10oC),andafreezer(0oC). Bacterialgrowthismeasuredby estimatingthepercentageofeachdishcoveredbybacteriaattheendofa3- daygrowthperiod.

a.Whatistheindependentvariable?

b.Whatisthedependentvariable?

c.Whatvariablesshouldbestandardized?

16.Ateamofscientistsistestinganewdrug,XYZ,onAIDSpatients. They

expectpatientstodevelopfewerAIDS-relatedillnesseswhengiventhedrug, buttheydon'texpectXYZtocureAIDS.

a.Whathypothesisarethescientiststesting?

b.Whatistheindependentvariable?

c.Whatisthedependentvariable?

d.Whatcontroltreatmentwouldbeused?

e.Whatvariablesshouldtheresearchersstandardize?

17.Agroupofstudentsdecidestoinvestigatethelossofchlorophyllinautumn leaves. Theycollectgreenleavesandleavesthathaveturnedcolorfrom sugarmaple,sweetgum,beech,andaspentrees. Eachleafissubjectedto ananalysistodeterminehowmuchchlorophyllispresent.

a.Whatisareasonablehypothesisforthesestudents?

b.Whatistheindependentvariable?

c.Whatisthedependentvariable?

d.Whatwouldyouadvisethestudentsaboutreplicationforthis

experiment?

18.Ascientistwantstostudymatingbehaviorincrickets. Shehypothesizesthat

malesthatwinthemostmale-vs.-malecontestsmatewiththemostfemales. Sheobservesthecricketstoobtaindata. Foreachmale,shecountsthe numberofmale-malefightshewinsandthenumberoffemaleshemates

with.

a.Whatistheindependentvariable?

b.Whatisthedependentvariable?

c.Whatconstitutesreplicationinthisexperiment?

19.Designacontrolledexperimenttodeterminetheeffectofexerciseonheart rate.

PROBLEM
(intheformofa question)
Dependent variable
Independent variable
Standardized variables
(listatleast4 factorsthatmust bekeptequalinall treatments)
HYPOTHESIS
Levelsof treatment
(valuessetforthe independent variable)
Control treatment
(independent variableeither eliminatedorsetto
standardvalue)
REPLICATION
METHOD
(whatstepswill
youfollow)
Expectedresults
Explanationof expectedresults