Investigating more powerful discrimination tests with consumers - effects of memory and response bias.pdf - Artykuły - janocz

Food Qualityand Preference 13 (2002)39–45

www.elsevier.com/locate/foodqual

Investigatingmorepowerfuldiscriminationtestswithconsumers:

eﬀectsofmemoryandresponsebias

Benoı ˆ tRousseau*,StefanieStroh,MichaelO’Mahony

Department of Food Science and Technology, University of California, Davis CA 95616, USA

Received4April 2000;receivedin revisedform19 June 2001;accepted20 July 2001

Abstract

Two experiments were conducted to investigate the sensitivity of four discrimination methods when they were performed by

consumers. InExperiment I, the inﬂuence of memory in the duo-trio method was studied. Three versions of theduo-trio method

with diﬀerent memory requirements were considered. Calculated d 0 values indicated a higher sensitivity for the duo-trio with the

referencetasted between thetwotestsamples (DTM), illustrating theimportance ofmemoryinsensory discrimination testing.In

Experiment2,fourdiscriminationtestswerecompared:thetriangle,theDTM,thesame–diﬀerentandthedual-pairtests.Thedual-

pairtestwaspredictedtoincreasethed 0 valueofthesame–diﬀerenttestbyeliminatingthelargeresponsebiasvariationsbetween

consumers. Results indicated no signiﬁcant diﬀerences in d 0 among the protocols. Thus the dual-pair method was not able to

improvethesamplediscriminationabilityofthesame–diﬀerenttest. # 2001ElsevierScienceLtd.Allrightsreserved.

Keywords: Discriminationtests;Power;Consumers;Memory;Responsebias

1. Introduction

& Sauvageot, 1988; Gridgeman, 1955; Helm & Trolle,

1946; Hopkins, 1954; Hopkins & Gridgeman, 1955;

MacRae & Geelhoed, 1992; Pfaﬀman, 1954; Pokorny,

Marcı ´ n, & Davı ´ dek, 1981; Raﬀensperger & Pilgrim,

1956; Vessereau, 1965). The work of Thurstone (1927),

laterusedbyFrijters(1979)tosolvethe‘paradoxofdis-

criminatorynon-discriminators’(Byer&Abrams,1953;

Gridgeman, 1970), allowed an explanation of these

inconsistencies and provided a more consistent analysis

of results obtained from discrimination tests and other

sensory evaluation procedures. This approach involves

thecalculationofad 0 valuewhichisanindexdescribing

theperceiveddegreeofdiﬀerencebetweentwoproducts.

Thelargerthevalue,themorediﬀerenttheproducts.

This approach has been frequently applied in recent

studies in order to investigate which sensory protocol

would be the most appropriate to study small sensory

diﬀerences between products (Delwiche & O’Mahony,

1996; Dessirier & O’Mahony, 1998; Frijters, 1980;

Geelhoed, Macrae, & Ennis, 1994; Hautus & Irwin,

1995; Huang & Lawless, 1998; Irwin, Hautus, & Still-

man, 1992; Irwin, Stillman, Hautus, & Huddleston,

1993; Masuoka, Hatjopoulos, & O’Mahony, 1995;

Rousseau & O’Mahony, 1997, 2000, 2001; Rousseau,

Meyer, & O’Mahony, 1998; Rousseau, Rogeaux, &

The area of discrimination testing has recently been

an active area of research in sensory evaluation. Dis-

crimination methods are broadly used both in the

industry and academia, their applications ranging from

daily quality control measurements, to the study of the

impact of ingredient or process changes, to the investi-

gation of the ability of consumers to discriminate

amongproducts.Becauseofthecostlyimplicationsthat

can be induced by imprecise results or conclusions,

numerous studies have been conducted to attempt to

deﬁnethemostappropriatediscriminationmethods.By

‘most appropriate’ was meant the most powerful

methodofinvestigation.Themorepowerfulthetest,the

morelikelythedetectionofasensorydiﬀerencewhenit

exists. The analysis of results in early studies was often

based on the guessing model (illustrated by the use of

binomial tables) and yielded conﬂicting results (Bucha-

nan, Givon, & Goldman, 1987; Byer & Abrams, 1953;

Dawson&Dochterman,1951;Filipello,1956;Franc¸ois

* Corresponding author. Tel.: +1-530-752-6389; fax: +1-530-752-

4759.

E-mail address: bdrousseau@ucdavis.edu(B.Rousseau).

0950-3293/01/$ -see frontmatter # 2001ElsevierScienceLtd. All rightsreserved.

PII: S0950-3293(01)00055-6

B. Rousseau et al./Food Quality and Preference 13 (2002) 39–45

O’Mahony, 1999; Stillman, 1993; Stillman & Irwin,

1995;Tedja,Nonaka,Ennis,&O’Mahony,1994).

While the majority of these studies have been con-

ducted under very controlled conditions in the labora-

tory, few have been conducted under less controlled

conditions of consumer testing. Stillman (1993) con-

ﬁrmed the paradox of discriminatory non-dis-

criminators using consumers; the subjects could

discriminatesigniﬁcantlybetterbetweentwoversionsof

anonionpartydipusinga3-AlternativeForcedChoice

(3-AFC) than using a triangle test. This illustrated the

higherpowerofthe3-AFCoverthetriangletest(Ennis,

1993) because of its more eﬀective cognitive strategy.

One of the drawbacks of the 3-AFC test, or its two-

sample counterpart, the paired comparison test (2-

AFC), is that it requires the speciﬁcation of the nature

of the diﬀerence in the instructions. Yet, this informa-

tionisnotalwaysreadilyavailabletotheexperimenter.

Another study by Rousseau et al. (1998) compared the

performance of the duo-trio, triangle and same–diﬀer-

enttestswithconsumers.Thesethreeprotocolshavethe

advantageofnotrequiringanydescriptionofthedimen-

sion of the diﬀerence; however, they lack the statistical

power of the 2-AFC and 3-AFC tests (Ennis, 1993). In

that study, Rousseau et al. found that the same–diﬀer-

enttestwasthemostpowerfulandmostsensitiveofthe

three protocols thanks to its more eﬀective cognitive

strategyandlowermemoryrequirements,respectively.

While some studies have shown the importance of

memory when using scaling procedures (Kim &

O’Mahony, 1998), more studies have investigated its

eﬀects in discrimination testing. For example, memory

decay over time has been shown to aﬀect performance

in the same–diﬀerent test (Cubero, de Almeida, &

O’Mahony, 1995; de Almeida, Cubero, & O’Mahony,

1999). Furthermore, memory is thought to be the main

factor giving an advantage to the same–diﬀerent over

the triangle test (Rousseau et al., 1998, 1999; Rousseau

&O’Mahony,2000;2001).Memoryisalsosuspectedof

beingpartlyresponsibleforthehighersensitivity ofthe

2-AFC over the 3-AFC (Dessirier & O’Mahony, 1998;

Rousseau&O’Mahony,1997).

A potentially useful way of further conﬁrming the

signiﬁcant eﬀects of memory would be to change only

slightly the design of a particular protocol, while alter-

ing its memory properties, and observe its eﬀect on the

protocol’s performance. A candidate for such an inves-

tigation is the duo-trio method. The traditional pre-

sentationinvolvesthetastingofthereferenceﬁrst,then

the two unknown or ‘test’ samples. It is possible that

oncethethirdsampleisevaluated,thememorytraceor

‘engram’ofthereferencestoredintheshort-termmem-

ory would have degenerated; it would be ‘blurred 0 , and

might cause the judge to provide an incorrect answer.

One could slightly modify the mode of presentation by

introducing the reference between the two alternatives,

so that both alternatives are evaluated immediately

before or after the reference, reducing memory require-

ments for the comparisons. This issue was investigated

inExperiment1.

While the same–diﬀerent protocol is more sensitive

and powerful than the traditional triangle and duo-trio

tests, it might be possible to further improve its perfor-

mance. The factor that needs to be taken into con-

sideration is response bias, in the form of their t

criterion,apsychological‘yardstick’ofdegreeofdiﬀer-

ence. If the perceived diﬀerence between the two sam-

ples is smaller than his t criterion, the subject will

consider the stimuli as ‘same’, while a larger diﬀerence

will induce him to consider them as ‘diﬀerent’. Signal

Detection Theory and Thurstonian models successfully

deal with this response bias to yield an unbiased

measure of discrimination, the d 0 value. However, one

ofthelimitationsofcombiningdatafromdiﬀerentsub-

jects in the same–diﬀerent test is that subjects exhibit t

criteria of diﬀerent sizes. Such combination will cause

anunderestimationofthed 0 value(Hautus,1997;Mac-

millan&Kaplan,1985).

Thedual-pairtestisthusinvestigatedinanattemptto

control this eﬀect. In the dual-pair test, also called 4-

IntervalAXor4IAXinpsychology(e.g.Kaplan,Mac-

millan,&Creelman,1978),thesubjectispresentedwith

two pairs of samples simultaneously. One is a pair of

identical samples, the other of diﬀerent samples. In the

pair consisting of two diﬀerent samples, one of the

samples is the same as the sample used in the pair of

identicalstimuli.Thetaskofthesubjectistodetermine

which pair has the same and which pair has diﬀerent

samples. This protocol has been studied previously

under controlled conditions (Rousseau & O’Mahony,

2000,2001),whileaThurstonianmodelhasbeendevel-

oped allowing the investigation of its statistical power

(Rousseau & Ennis, 2001). Information obtained from

these experiments and new models did not indicate an

advantage for the dual-pair test. Its statistical power

wasfoundtobemoresimilartothatoftheduo-triotest

and thus inferior to that of the same–diﬀerent test.

Furthermore, it did not yield larger d 0 values. Not

yielding larger d 0 values than the same–diﬀerent test

might have been due to the fact that in controlled con-

ditions subjects were familiar with the products under

study,limitingthelargevariationsinthesubjects’ t cri-

teria which would have hindered performance on the

same–diﬀerent test. It is possible that such variations

could have a larger inﬂuence on the same–diﬀerent test

when testing highly ‘uncalibrated 0 subjects such as con-

sumers. Here, an advantage of the dual-pair test over

thesame–diﬀerenttestmightpossiblybeobserved.This

particularissuewasstudiedinExperiment2.

Thus, in summary, the present experiments further

investigatedtheeﬀectofmemoryindiscriminationtest-

ing and the possible experimental advantages of the

B. Rousseau et al./Food Quality and Preference 13 (2002) 39–45

dual-pair paradigm. In Experiment 1 concerning mem-

ory,thesensitivitiesofthreeversionsoftheduo-triotest

were compared using 96 consumers. The objective was

to investigate the eﬀect of the position of the reference

versus that of the two alternative samples. In Experi-

ment 2, the relative performances of the dual-pair and

same–diﬀerent tests were compared using 144 con-

sumers. The duo-trio and triangle tests were also inclu-

ded in the study so as to provide more complete

informationabouttheadvantagesanddisadvantagesof

thetraditionaldiscriminationprotocols.

twotestsamples(DTM)andathirdversiondesignated

as the 2-Distance-AFC or 2-D-AFC. For the ﬁrst two

protocols,thesubjectwasaskedtoindicatewhichofthe

two alternatives was the most similar to the reference.

The 2-D-AFC had the same presentation order as the

DTM, but the subject was asked to indicate which

changeofsensationwasthelarger.Thus,theyreported

whether the sensation change was greater between the

ﬁrst and the second sample, or between the second and

thethirdsample,ratherthanindicatewhichsamplewas

themoresimilartothemiddlestimulus.Here,theywere

nottoldthatamongthethreesamples,twowereidentical.

Onedeionizedwaterrinsewastakenbeforeeachtest.

Before tasting the ﬁrst sample of the triad, a primer

sample was taken in order to prevent the distorting

eﬀect of the rinsing water on the taste of the ﬁrst sam-

ple. The primer was always sample A. Subjects were

requiredtotakethewhole10mlofeachsampleatonce

and swallow it. Water rinses were also swallowed. No

retasting was allowed so as to allow memory to have a

larger inﬂuence. The order of presentation of the tests

and the order in which samples appeared in a test were

counterbalancedoverjudges.Inordertopreventdiﬀer-

ential sequence eﬀects among the various protocols,

only two presentation orders were used, namely ‘AAB’

and‘BBA’.Eachjudgeencounteredonlyoneofthetwo

possibletriadsoverthethreeprotocolsperformed.Jud-

ges responded verbally, no feedback was given. Session

lengths from interception to termination lasted

approximately10–15mins.

2. Experiment 1

2.1. Materials and methods

2.1.1. Judges

Ninety-six(96)subjects(51M,45F;agerange13–56

yearsaverageage22.7years)participatedinExperiment

I. They were students, faculty, staﬀ and their friends

intercepted on the campus of the University of Cali-

fornia,Davis.

2.1.2. Products

The stimuli for the experiment consisted of two ver-

sions of a non-carbonated orange ﬂavored beverage

(The Gatorade company, Chicago, IL, USA), available

asaninstantmix.Onestimulus(A)waspreparedusing

80 g of product per liter, mixed with deionized water.

Thesecondstimulus(B)waspreparedthesamewaybut

10 g of sucrose (Fischer Grade, Fisher Scientiﬁc, Fair

Lawn,NJ,USA)werealsoaddedtocreateasmallsen-

sorydiﬀerence.Sampleswereservedin10mlaliquotsin

approximately thirty-milliliter clear plastic cups (Serco

1-oz Plastic Cups, S. E. Rykoﬀ & CO., Los Angeles,

CA,USA).

2.2. Results

Thed 0 valuesforthethreeversionsoftheduo-triotest

are presented in Table 1. The d 0 values for the duo-trio

and DTMwere obtained from the proportions of cor-

rect responses using tables (Ennis, 1993). The d 0 value

for the 2-D-AFC was also obtained using the tables,

sinceitcanbearguedthatthecognitivestrategyusedby

the subjects to perform this particular protocol is the

sameasthatassumedfortheduo-trio.

Variances for testing signiﬁcant diﬀerences were

obtainedusingtheapproachproposedbyBi,Ennis,and

O’Mahony (1997). It was found that the d 0 value

obtained with the DTMwas signiﬁcantly higher than

those obtained with the traditional duo-trio and the

2.1.3. Procedure

Having intercepted the judge, an experimenter ﬁrst

established rapport and then took personal details. A

second experimenter then explained the methods and

performedthetest.

Each judge performed three versions of the duo-trio

test: the traditional duo-trio with the reference tasted

ﬁrst, the duo-trio with the reference tasted between the

Table1

Resultsforthethreepro t ocolscomparedinExperiment1—proportionofcorrectanswers,d 0 ,varianceofd 0 and95%conﬁdenceintervals a

Protocol

Numberofcorrectanswers(outof96)

d 0

Variance

95%Conﬁdenceintervals

Duo-Trio

1.4a

0.08

(0.8;2.0)

DTM

2.2b

0.08

(1.6;2.7)

2-D-AFC

1.3a

0.09

(0.8;1.9)

a DTMd 0 valuesigniﬁcantlydiﬀerentfromduo-trio(P=0.05)and2-D-AFC(P=0.03).DTM,duo-triowithreferenceinthemiddle;2-D-AFC,

2-distance-alternativeforcedchoice.

B. Rousseau et al./Food Quality and Preference 13 (2002) 39–45

2-D-AFC(speciﬁcallyat P=0.05and0.03,respectively,

after an overall screening at P=0.06), while the latter

two procedures did not show any signiﬁcant diﬀerence

(P=0.8).

For each of the three duo-trio protocols, no diﬀer-

enceswerefoundbetweenthetwodiﬀerentsequencesof

tasting (AAB, BBA). d 0 values are given in Table 2.

Diﬀerenceswerenotsigniﬁcant(P 50.33).

ItwasdecidedtousetheDTMinExperiment2sinceit

appearedtobethemostsensitiveversionoftheduo-trio.

sisted of two successive pairs of samples, one being of

identical samples, the other of diﬀerent samples. How-

ever, subjects were not aware of this arrangement. The

version of the same–diﬀerent test used here, sometimes

labeledasthe‘longer’version(Rousseauetal.,1999),is

statistically more powerful than the traditional triangle

and duo-trio tests (Ennis, 1996). For each pair of sti-

muli,subjectswereaskedtostatewhethertheythought

thesampleswerethesameordiﬀerentandwhetherthey

weresureornotsosureoftheiranswer.

Each consumer was ﬁrst introduced to the two sam-

ples A and B to be tested and was allowed to sample

both.This‘familiarizationprocedure’wasperformedto

allow the judge to have some preconceived idea of the

extent of the diﬀerence to be detected. This allowed

themtoadjusttheircriterionofdiﬀerence(Macmillan&

Creelman, 1991) such that if they detected a diﬀerence,

theywouldreportit.

Theorderofpresentationofthetestsandtheorderin

which samples appeared in a test were counterbalanced

overjudges.Ablockof48subjectspermittedacomplete

counterbalancing. Thus three of these blocks were used

here.Judgesrespondedverbally,nofeedbackwasgiven.

Session lengths from interception to termination lasted

approximately10–15min.

3. Experiment 2

3.1. Materials and methods

3.1.1. Judges

One hundred and forty-four (144) subjects (78 M, 66

F;agerange18–71;meanage24.3years)participatedin

Experiment2.Theywereselectedfromthesamepoolof

subjectsasinExperiment1.

3.1.2. Products

Thestimulifortheexperimentwerethesameasthose

used in Experiment 1. The only diﬀerence was a reduc-

tionofthesugarconcentrationinsampleBto8g/lfrom

10g/l,toassurebetterconfusabilityofthestimuli.

4. Results

3.1.3. Procedure

The testing procedures were similar to those used in

Experiment 1. This included the rinsing protocol and

the use of a primer. Consumers swallowed the stimuli

and the rinses. Each consumer performed one test of

each protocol: triangle, DTM, dual-pair and same–dif-

ferent. It should be noted that the last protocol con-

ResultsarepresentedinTable3.FortheDTM,dual-

pairandtriangletests,d 0 valuesandtheirvarianceswere

obtainedfromtheproportionofcorrectresponsesusing

tables (Ennis, 1993; Rousseau & Ennis, 2001). For the

same–diﬀerent test,ananalysis usedinprevious studies

(Rousseau et al., 1998, 1999; Rousseau & O’Mahony,

2000; 2001) was applied. This analysis was based on

Thurstone’s law of categorical judgment (Ennis, 1998;

Thurstone,1927).Theanalysiswasappliedtodiﬀerence

ratingsandusedthemethod ofmaximum likelihoodto

estimate d 0 , t and the variance–covariance matrix for

theseestimates.

Using the approach proposed by Bi et al. (1997), no

signiﬁcant diﬀerences were observed among the four d 0

values(P=0.7).However,atrendwasobservedforthe

triangle test to be slightly less sensitive than the other

threeprocedures.

Table2

d 0 valuesforeachduo-triosequence(n=48)

Protocol

AAB

BBA

Duo-Trio

1.6 a

1.1 a

DTM

1.9 b

2.5 b

2-D-AFC

1.5 c

1.1 c

a,b,c For each protocol, d 0 values for sequences AAB and BBA are

notsigniﬁcantlydiﬀerent(P 50.33).DTM,duo-triowithreferencein

themiddle;2-D-AFC,2-distance-alternativeforcedchoice.

Table3

ResultsforthefourprotocolscomparedinExperiment2—proportionofcorrectanswers,d 0 ,varianceofd 0 and95%conﬁdenceintervals

Protocol

Numberofcorrectanswers(outof144) d 0

Variance 95%Conﬁdenceintervals

Triangle

1.1 a

0.06

(0.6;1.6)

DTM

1.5 a

0.05

(1.1;2.0)

Same–diﬀerent N/A

1.4 a

0.04

(1.0;1.8)

Dual-pair

1.4 a

0.06

(0.9;1.9)

a d 0 valuesarenotsigniﬁcantlydiﬀerent(P=0.7).

B. Rousseau et al./Food Quality and Preference 13 (2002) 39–45

Table4

Distribution of responses for the four pairs presented in the same–

diﬀerenttest(inbold,responseswhichcanbescoredas‘correct’) a

5. Discussion

The results from Experiment 1 conﬁrmed the sig-

niﬁcanteﬀectplayedbymemoryindiscriminationtests.

Merely changing the position of the reference in the

duo-triotestsigniﬁcantlyimprovedthesensitivityofthe

protocol. This result substantiates the higher sensitivity

oftestswithonlytwosamples,suchasthesame–diﬀer-

ent and 2-AFC tests over those with three samples,

observed in previous studies. This ﬁrst result seems to

indicate that tasting the reference ﬁrst in the duo-trio

test is not the most appropriate design for this parti-

cular protocol. In order to obtain the best discrimina-

tion between products, the reference should be tasted

betweenthetwoalternativesamples.

However, it needs to be noted that subjects were not

allowed to retaste the samples, therefore permitting

memoryeﬀectstohaveanon-negligibleinﬂuenceonthe

outcome of the test. It is possible that retasting might

limit the advantage of the DTM; further experimenta-

tion is needed to investigate this issue. Nevertheless,

whenthenatureoftheproductdoesnotallowretasting

becauseofsensoryfatigue(e.g.redpepper,hardliquors,

...),theDTMappearslikethemostappropriateversion

oftheduo-triotest.

The2-D-AFCdidnotshowanyparticularadvantage.

Itmayhavebeenexpectedtogiveahigherproportionof

testscorrectliketheDTM.Yet,subjectsreportedthatthe

task was diXcult and they might have reverted to an

alternativestrategy.Perhapsmorepracticed and trained

subjectsinlaboratoryconditionsmayhavemoresuccess.

Experiment2didnotshowanysigniﬁcantdiﬀerences

among the sensitivities of the four protocols. These

results would conﬁrm Thurstonian predictions. How-

ever, trends seen in previous research (Rousseau et al.,

1998, 1999; Rousseau & O’Mahony, 2000; 2001; Still-

man & Irwin, 1995) were observed. Of the four proto-

cols, the same–diﬀerent test was the most powerful as

indicatedbyitslowervariance.

Thedual-pairdidnotyieldalargerd 0 valuethanthat

ofthesame–diﬀerenttestaspredicted.Thereasoncould

be that larger memory requirements counterbalanced

any stabilizing eﬀect on the judges’ t criteria that was

providedbyusingaforced-choicemethod.Asecondpos-

sibilityisthatitmightbeduetothefactthatthefamiliar-

ization procedure performed by the subjects prior to

starting the experiment was suXcient to prevent large

variations in the size of the subjects’ t criteria in the

same–diﬀerenttestandsonotreduceitsperformance.

Nevertheless, this result indicates that the dual-pair

methoddoesnotappeartobeagoodalternativetothe

same–diﬀerentmethod.Itexhibitsatbestthesamesen-

sitivity as the same–diﬀerent, while its statistical power

is more limited. Thus, the same–diﬀerent test with a

familiarization procedure appears more appropriate as

asensorydiscriminationmethodforconsumertesting.

Pair Numberofanswers(/72)

‘‘Correct’’ b

‘‘Incorrect’’ b

Same

AA 39

BB 41

Diﬀerent AB 42

BA 44

a The pattern of responses is not signiﬁcant ( 2 , P=0.7 for com-

parison of ‘same’ pairs, P=0.7 for comparison of ‘diﬀerent’ pairs).

Thecategories‘‘Diﬀerentsure’’and‘‘Diﬀerentnotsure’’ononehand,

and ‘‘Same sure’’ and ‘‘Same not sure’’ on the other have been com-

binedtopermiteasiercomparison.

b For the pairs AB and BA, a response ‘Diﬀerent’ will be con-

sideredcorrect,whileforthepairsAAandBB,aresponse‘Same’will

beconsideredcorrect.

Table5

d 0 valuesforeachtypeofsequenceforthreetestingprotocols(n=72)

Protocol

‘B’odd

‘A’odd

Triangle

0.9 a

1.2 a

DTM

1.9 b

1.2 b

Dual-pair

1.7 c

1.0 c

a,b,c Foreachprotocol,d 0 valuesofeachhalfofthetastingsequen-

ces (‘B’ odd vs. ‘A’ odd) are not signiﬁcantly diﬀerent (P 5 0.17).

DTM,duo-triowithreferenceinthemiddle.

As for Experiment 1, no clear trend was apparent

when looking at possible sequence eﬀects. This can be

observed separately for diﬀerent test protocols. First,

for the same–diﬀerent test, the pattern of sequences

obtainedwiththefourpossiblepairsofstimulicouldbe

examined. This is an approach similar to that used in

thesequentialsensitivityanalysis(O’Mahony&Odbert,

1985). The appropriate way of performing the analysis

would be to carry out a 2 test including the four

sequences. However, thisanalysis iscomplicated bythe

existence of response bias. For instance, if the subjects

exhibited a very large t criterion, the results of the pair

AA and BB would tend to appear more ‘correct’ than

those of the pairs AB and BA, giving a signiﬁcant 2

test. This would obscure the eﬀect of adaptation and

sequencingsincetheresultswouldbecontaminatedbya

psychological bias. However, it is still relevant to com-

pare the responses of the pairs AA with BB on one

hand,andABwithBAontheotherhand(Table4).No

signiﬁcantdiﬀerencesareobserved( 2 ,P=0.7foreither

comparison).

For the remaining tests, the eﬀects can be examined

by looking at the two halves of each of the triangle,

DTMand dual-pair tests. Results are presented in

Table 5. Using the same analysis as in Experiment I

(Table 2), no signiﬁcant diﬀerence between the two

halves(‘A-odd 0 and‘B-odd 0 )wereobserved(P 5 0.17).

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