detailed in Ward and Lockhead (1970), is only summarized here.

Transcription

1 Respnse system prcesses in abslute judgment* LAWRENCE M. WARDt and G. R. LOCKHEAD Duke University, Durham, Nrth Carlina 2778 Cnsistent relatinships are fund between Ss' abslute judgments f the value f a stimulus and the previus sequence f bth stimuli and respnses. The frm and magnitude f these sequential effects are shwn t depend n the presence r absence f feedback and n task difficulty. The pattern f the sequential effects fund allws the cnclusin that they are due t purely respnse-system prcesses. A tw-stage mdel f the judgment prcess is prpsed, and it is argued that bserved assimilative effects accunt fr the central tendency effects bserved in categry judgments. The name r respnse given t a stimulus in an abslute-judgment experiment depends in part n the previus sequence f stimuli. Respnses tend t be assimilated tward the value f the immediately previus stimulus and are either cntrasted frm r assimilated tward earlier stimuli in the sequence, depending n the presence r absence f feedback (knwledge f the "crrect" respnse) (Hlland & Lckhead, 1968; Ward & Lckhead, 197). The effects f previus respnses in the series f judgments were nt reprted in the abve papers and, in general, have nt been the subject f extensive investigatin. Garner (1953) and McGill (1957) did shw a cntingency between the immediately previus respnse and the respnse t the current stimulus. Suggested explanatins f the abve effects have been based n aberratins in the use f the shrt-term memry f the previus stimulus as a standard fr cmparative judgment. The purpse f the present paper is t develp a pattern f sequential effects in different experimental cnditins that allws the cnclusin that they are due t the use f purely respnse-system prcesses in mediating respnse uncertainty. These respnse-system prcesses are shwn t take the frm f "guessing" strategies. STUDY 1: SEQUENTIAL EFFECTS WITHAND WITHOUT FEEDBACK The purpse f this study was t determine if the presence r absence f feedback affected the frm f the effects f previus respnses n the current respnse. These are previusly reprted data analyzed nw fr the effects f the previus respnses, as well as the already reprted stimulus effects. The methd, detailed in Ward and Lckhead (197), is nly summarized here. Methd S made abslute judgments f 1 randmly presented 1,-Hz sinusids that varied in intensity. Adjacent tnes were 1 db apart and the ttal range was apprximalely frm 55 t 64 db re.2 dynes/em". Respnses cnsisted f the numbers 1-1, recrded n an electric typewriter. The stimulus duratin was 1 msec, and there was a fixed 3.5-sec interval between trials. Three Ss each gave 5 respnses per day fr 4 cnsecutive days, fllwing a practice day nt reprted, with feedback given as an illuminated numeral n every trial. An additinal three Ss perfrmed in the same way but with n feedback given. Results Figures I and 2 shw the effects f the.5 a. Feedback: Stimulus Analysis z.4 ' a. ltj -. I a::: _ g W e GJ -:6 StimulusI Respnse Vlue n Trial N 1,2 3,4,t.----A 5,6.. 7, :1. 9,1 --- previus stimuli and thse f the previus respnses n the respnse t the current stimulus fr the feedback and n-feedback experiments, respectively. The sequential effects are essentially the same fr all Ss in this and in the fllwing studies, and the data in these and subsequent figures are averaged ver Ss fr ease f presentatin. The data are als cllapsed acrss adjacent pairs f stimuli fr ease f analysis and presentatin. The figures take the same frm when nt averaged in this way. Each pint in the figures represents the average errr f the respnses t all stimuli that ccurred n Trial N + K when ne r the ther f a particular pair f stimuli r respnses ccurred n Trial N. K ranges frm ne t six, which is as far as the analyses were cnducted. Fr example, the tp leftmst pint in Fig. 1a represents an average errr f +.41 categry units n Trial N + 1 when the stimulus n Trial N was a 9 r a 1. Crrespndingly, the same pint in Fig. 1b represents an averageerrr f +.44 categry units n Trial N + 1 when the respnse n Trial N was a 9 r a 1. Fr stimuli and respnses further back in the sequence than ne trial, the average errr f respnse was calculated withut respect t what stimuli r respnses ccurred between the relevant previus trial and the current trial. Since the stimuli were randm, it wuld be expected that each stimulus ccurred abut equally ften in the intervening series f trials. Thus, the data reprted here d nt refer t the effects f particular sequences f stimuli r respnses. These culd be calculated frm b. Feedback: Respnse Analysis *This research was supprted in part by Grant 5-FOI-MH483-2 frm the Natinal Institute f Mental Health. t Requests fr reprints shuld be sent t Lawrence M. Ward, Department f Psychlgy. Duke University. Durham, Nrth Carlina Trials (K) since Stimulus/ Respnse n Trial N Fig. 1. Average errr f the respnse at Trial N +K attributable t the stimulus/respnse at Trial N fr the ludness-feedback study. (There are apprximately 1,2 bservatins per pint.i Perceptin & Psychphysics, 1971, Vl. 9 (18) Cpyright I <)71. Psvchnmic Jurnals, Inc.. A ustin, Texas 73

2 :x: + z c I Q. III e I. GI CI e 1.5 a. NFeedback: Stimulus Analysis b. N Feedback: Respnse A.rdtSs -.:. StinUus/Respn.. Value n 1)'11 N 1, ,4.----A 5,6.. 7,8 b , Trials (K) since Stimulus I Respnse n Trial N the average results nly if there were n interactin between elements and trials. In the feedback study (Fig. 1), a large valued stimulus r respnse n Trial N results in a psitive average errr f the respnse n Trial N + 1, and small valued stimuli and respnses are fllwed by negative average errrs. Thus, respnses are "assimilated" tward the values f the previus stimulus and respnse. But the reverse effect ccurs n Trials N + 2 thrugh N + 6; respnses are "cntrasted" with stimuli and respnses tw and mre trials back in the sequence. It shuld be cautined that the rdering f pints in Fig. 1 with respect t the value f the 'previus stimuli and respnses des nt imply that, fr example, Stimulus 1,2 "pulls" respnses tward it mre than des Stimulus 5,6. In fact, an apprximately equal amunt f assimilatin is assciated with each stimulus (see Fig. 1 in Ward & Lckhead, 197). The average effects are rdered as in Fig. 1 because mst stimuli that fllw 1,2 are larger than 1,2; thus, mst f the errrs f assimilatin are negative, and the average f these is negative. Hwever, half f the stimuli fllwing 5,6 are larger than 5,6, resulting in a negative errr, and half are smaller, resulting in a psitive average errr f the fllwing respnse. The average f these effects is zer average errr. In the n-feedback study (Fig. 2), respnses are assimilated tward stimuli at least tw trials back and tward respnses at least five trials back in the sequence. Cntrast seems t appear fr trials further back than mentined abve. The magnitude, defined as the difference in average errr between 1,2 and 9,1, fthe assimilative effect f previus respnses is cnsiderably greater than that f previus stimuli in this cnditin. Infrmatin transmissin averaged 1.27 and.95 bit in the feedback and n-feedback experiments, respectively, and percent crrect averaged 36.1 and Since stimuli and respnses are crrelated, there is n way 't determine frm these data whether the sequential effects are due t stimuli r respnses r bth. STUDY 2: SEQUENTIAL EFFECTS IN EASY AND DIFFICULT TASKS Perfrmance level affects the crrelatin between stimuli and respnses; as perfrmance appraches chance, the crrelatin appraches zer. The purpse f this study was t investigate the relatin between sequential stimulus effects and sequential respnse effects when the S's task was the same in tw cnditins but perfrmance was made different by manipulating the discriminability f the stimuli. A secnd purpse f the study was t establish the generality f enduring sequential effects beynd ludness judgments. The experiment was cnducted. fr a different purpse and reprted earlier, but the data had nt been analyzed fr the sequential effects reprted here. The experimental prcedure is summarized here and detailed in Lckhead (1966). Methd The stimuli were IO hrizntal black lines.3 in. wide and centered n an 8 x 8 in. viewing field. The lines varied in length frm 2. t 2.9 em in O.l-em steps and were assigned the respnse labels 1-1 fr the shrtest t the lngest line. In the easy cnditin, three Ss each gave 15 respnses per day fr 8 days and three gave Fig. 2. Average errr f the respnse at Trial N + K attributable t the stimulus/respnse at Trial N fr the ludness-n-feedback study. (There are apprximately 1,2 bservatins per pint.) 15 respnses per day fr 4 days. The stimuli were presented tachistscpically fr 2 msec, and the bluish-white stimulus and adapting fields were easily seen. In the difficult cnditin, five f the six Ss in the easy cnditin (the ther S had difficulty seeing the stimuli in this cnditin because f pr acuity and was nt used) each gave 15 respnses per day fr 8 days. In this cnditin the stimulus duratin was reduced t 8 msec, and the verall luminance was reduced by means f a 1.D-lg unit neutral-density filter. The fields were thus quite dim, and S reprted smetimes nt seeing the stimulus. Ss partially dark adapted befre participating in this cnditin and were required t respnd n all trials. Feedback was given after each respnse in bth cnditins. Bth cnditins were self-paced, and Ss wrked at abut the same rate, arund 5 sec per stimulus. There was a brief warm-up perid befre each sessin. Ss recrded their respnses n a keybard that perated a remte electric typewriter. Thus, Ss culd nt see the previus sequence f stimuli and respnses as they culd in Study 1, where they typed their respnses and the feedback, when prvided, directly n an electric typewriter. Results Fr the easy cnditin infrmatin transmissin averaged ver Ss and days was 1.7 bit and percent crrect perfrmance was 35.. In the difficult cnditin infrmatin transmissin averaged.19 bit and percent crrect was 15. (chance perfrmance is 1%). Thus, the crrelatin between stimuli and respnses is different in the tw cnditins, while the stimuli were still visible enugh t be judged in the difficult cnditin. Figures 3 and 4 shw the analyses f the data frm the easy and difficult cnditins, respectively, fr the effects f previus stimuli and previus respnses n the current respnse. The frm f the sequential effects in the easy cnditin (Fig. 3) is cmparable t that frm the ludness-feedback study (Fig. 1), althugh the magnitude f the assimilatin effects is less and the variability f the curves is greater fr the line-length study. These differences may be due t the smaller number f trials per day run in this study (1SO vs 5) r t the lnger intertrial time (5 sec vs 3.5 sec). The majr effect, 74 Perceptin & Psychphysics, 1971, Vl. 9 (IB)

3 +.5 Z.4 ' : [ r: -.1 a:: E -:4 & -.5 e -.6 <! a. Easy: Stimulus Analysis Stll1llUlRnpnse value n Tril N 1,2 3, , ,8 A--l> 9, z QI II) b. Easy: Respnse Analysis Trials (K) since Stimulus/Respnse n Trial N assimilatin t the previus stimulus and respnse, fllwed by cntrast t stimuli and respnses further back in the sequence, in an abslute judgment with feedback study, replicates in a mdality ther than ludness. The sequential effects in the difficult cnditin (Fig. 4) are different frm thse btained when the stimuli are relatively easy t judge (Figs. I and 3), althugh the Ss' task was the same in all three cnditins. The first-rder assimilative effect is smaller in the respnse analysis than in the stimulus analysis, a result that had nt been bserved earlier. Apparently the perfrmance manipulatin (Fig. 3 vs Fig.4) affects the first-rder sequential effects withut markedly changing the higher-rder effects, while the feedback manipulatin (Fig. 1 vs Fig. 2) affects bth the first-rder and higher-rder sequential effects. It is particularly nted that there is a large amunt f assimilatin t the previus respnse and much less assimilatin t the previus stimulus when feedback is nt prvided, while just the reverse is true when the task is difficult but feedback is prvided. STUDY 3: SEQUENTIAL EFFECTS WITH NOSTIMULUS PRESENTED In this study we asked Ss t perfrm in an "abslute judgment withut stimulus" experiment with feedback prvided. The purpse f the study was t determine the Fig. 4. Average errr f the respnse at Trial N + K attributable t the stimulus/respnse at Trial N fr the difficult line-length study. (There are apprximately 1,2 bservatins per pint.) presence r absence f sequential effects when there culd be n errrs f stimulus judgment invlved. Methd Three Ss each guessed which f the numerals 1 1 wuld ccur next in a randm sequence with "feedback" given after every trial. The study was identical t Study I with feedback, except that the ludness generatr was turned ff and Ss were instructed t respnd immediately after a light in frnt f them flashed; the light flashed every 3.5 sec. Each S made 5 predictins per day n each f 2 days. Results The average percent crrect was a. II) & W QI -. e -.7 <! a.difficult: Stimulus Analysis Trials Stimulus/Respnse Value n Tril N ,4.----, 5, ,8 e,-----,e. 9, (K) Fig. 3. Average errr f the respnse at Trial N + K attributable t the stimulus/respnse at Trial N fr the easy line-length study. (There are apprxima tely 1,1 bservatins per pint.] Figure 5 shws the results f the analyses f the sequential effects. Average errr means the difference between the mean guess fllwing a particular previus "stimulus" (r "feedback" number) and the average "stimulus" that actually ccurred in the trials fllwing that previus "stimulus" r respnse. The difference between this prcedure and the use f 5.5 as the ideal value f the average stimulus fllwing any previus stimulus r respnse value shuld be negligible, since the sequence was randm and the number f trials cnsidered at each pint was large. There is marked assimilatin f the respnses (guesses) t the value f the previus stimulus (numeral presented) and n cnsistent relatin between respnses n the current and previus trial. The higher-rder effects, as in the abve feedback cnditins, generally shwed cntrast fr bth analyses. The remarkable similarity between Fig. 4 (difficult task, feedback given) and Fig. 5 (guessing, feedback given) is strng supprt fr the argument that the tw situatins are quite similar with respect t the causes f sequential effects. In the limiting case f chance respnses, the tw cnditins may be equivalent. DISCUSSION The particular respnse S selects fr naming a given stimulus in an abslute judgment situatin is determined b. Difficult: Respnse Analysis 6 I 2 3 Stimulus / Respnse n Trial N 6 Perceptin & Psychphysics, 1971, Vl. 9 ( I B) 7S

4 a. Guessing:"Stimulus"Analysis z 1.2 b. Guessing: Respse Analysis Fig. S. Average errr f the respnse at Trial N + K attributable t the stimulus/respnse at Trial N fr the guessing study. (There are apprximately 6 bservatinsper pint.) are prpsed t reflect the S's decisin r respnse prcesses in determining which f these severalpssible respnses t select. -.2 e -4. II II -I.O "Stinulus"/Respnse value n Trial N ir---...ti I 2 3 4!5 Trials (K) since "Stimulus"/Respnse n Trial N by at least tw classes f factrs: memry In their classic paper n abslute and respnse r number usage variables. judgment, Wever and Zener (1928) With few stimuli and a relatively large reprted that it takes nly a few trials fr physical spacing between stimuli, Ss t learn rughly the scale upn which identificatin perfrmance is perfect and, they are t respnd in a givenexperiment. thus, there is n evidence f respnse Alng with many ther investigatrs, we prcesses affecting the judgments. This have als nticed this and'mdify the perfrmance is attributed t memry. As bservatin nly by smallerrrs due t the the psychlgical separatin between lng-term memry fr previusly learned stimuli is decreased and/r the number f scales. It appears that Ss learn the range stimuli is increased, Ss begin t make ver which the stimuli vary in a given errrs, and sequential effects have been situatin in a few trials if the range is fixed bserved in all such situatins investigated. r if it varies systematically. Thus, after a The reasns fr errrs are attributed t number f trials there culd exist a difficulties f discriminability and t a lng-term memry f the part f the ttal variable memry r variable criteria. The pssible range f stimuli being used in the sequential effects bserved when errrs are particular study. Ss are thus prpsed t made are attributed t the actin f have a lng-term memry f what the respnse-systemprcesses which Ss emply varius parts f this range "sund," "feel," when there is uncertainty abut the crrect "lk," etc., like, and f which numbers respnse. These tw factrs are discussed are t be attached t the varius parts. separately belw. The judgmental prcess, then, is viewed as fllws. The S cmpares his "perceptin" r "shrt-term memry" f the currently presented stimulus with his lng-term memry f the range f stimuli in use. He determines an area f that scale that matches the stimulus. The number assigned t the matching area culd then be the respnse t a particular stimulus. With many stimuli, r a small spacing between stimuli, r a lw SjN rati, the prcess prvides nly a rugh judgment f The Rle f Memry in Judgment Lng-term memry has been shwn t be a factr in abslute judgments. Ward and Lckhead (197) shwed that if the entire stimulus scale is shifted up r dwn 5 db frm the level f the previus day, whether r nt filedback is prvided, the average errr f respnse shifts in the directin f the scale shift. Fr this t ccur, a tendency must exist t respnd t the new set f stimuli n the basis f the previus day's scale. The magnitude and frm f the sequential effects in that study did nt depend n the stimulus scale but were superimpsed n the cnstant errrs due t the lng-term memry. Thus, it seems that the causes f sequential effects and f lng-term memry effects are nt related. the area f the subrange in which the presented stimulus lies. Because f this imprecisin f nnverbal lng-term memry, the presented stimulus may "match" a large part f the ttal pssible range f stimuli, and S culd assignany f several different numbers t the stimulus and still be cnsistent with all f his stimulus infrmatin. Sequential effects Respnse System Prcessesin Judgment We first summarize the pattern f the sequential effects bserved. The basic finding is that in mst abslute judgment situatins the present respnse is clser t the value f the immediately previus stimulus r respnse, and is further frm the values f stimuli r respnses further back in the sequence, than wuld be expected by chance. Fr brevity we say there is assimilatin fllwed by cntrast but d nt imply any relatin between these effects and cntrast and assimilatin as investigated in such areas as simultaneus brightness cntrast. There are tw exceptins t the generality. First, when n infrmatin feedback is given, assimilatin extends t stimuli tw trials back and t respnses as far as five trials back in the sequence. The magnitude f the assimilatin t the previus respnses in this case is very much larger than that t previus stimuli. Secnd, with feedback given, as perfrmance deterirates the magnitude f assimilatin t the previus stimulus grws larger, while that t the previus respnse becmes much smaller. When there are n stimuli at all, i.e., when the state f uncertainty as t which is the "crrect" respnse extends t all f the pssible respnses in the guessing situatin, assimilatin t the value f the previus "stimulus" is largest f all, and there is n cnsistent effect f the previus respnse n the present respnse. The effects f stimuli and respnses further than ne trial back in the sequence generally remain stable and alike as perfrmance decreases t chance in the feedback cnditins. But in the n-feedback situatin these effects change and it appears that the cntrast effects are being canceled by the larger assimilatin effects. Garner (1953) reprted data shwing that the immediately previus respnse and stimulus affect the current respnse in an abslute judgment withut feedback situatin and that the magnitude f the effect increases as the number f categries (stimuli) increases. McGill (1957), als in a n-feedback study, shwed that the cntingency between the previus and current respnses increases with decreasing 76 Perceptin & Psychphysics, 1971, Vl. 9 (IB)

5 '15 15 d: 1..."\ Expeetfin if s maximize.--. :'S;I WI -=n -\\ '\_ "' : D;tference in categry Steps between SlN-1)and RlN) Fig. 6. The frequency distributin f abslute differences between the stimulus at Trial N - I and the respnse at Trial N fr the data f the guessing study and the expected frequency distributins if SS maximized r prbability matched. (There were apprximately 3, respnses available fr analysis.) stimulus discriminability. He reprted the cntingency in terms f an uncertainty analysis that des nt allw determinatin f its frm, and we assume that his result was ne f increasing assimilatin t the previus respnse as the task became mre difficult. Thus, withut feedback assimilatin between the previus and present respnses increases with task difficulty, while with. feedback th is increasing assimilatin is assciated with the previus stimulus (ur Studies 2 and 3). Tw guessing strategies are prpsed here as pssible explanatins f these varius sequential effects. One f these wuld accunt fr assimilatin and the ther fr cntrast. The strategy assciated with assimilatin may be as fllws. Cnsider the prbability distributin f pssible differences, in categry steps, between successive stimuli. When stimuli are presented randmly, this distributin is skewed with a maximum at a difference f ne categry step. With 1 stimuli presented randmly and equally ften, the prbability that successive stimuli d nt differ (stimulus repetitin) is.1; that they differ by ne step is.18; by tw steps,.16; and by three thrugh nine steps,.14,.12,.1,.8,.6,.4, and.2, respectively. Figure 6 shws the frequency distributin f the differences between the respnse n Trial N [R(N)] and the stimulus n Trial N - 1 [SeN - 1») fr the three Ss in the guessing study. Als shwn are the expected distributins f these differences if Ss fllwed a strategy f maximizatin with respect t the prbability distributin f differences between successive stimuli and if they prbability matched with respect t that distributin. As shwn in Fig. 6, Ss guess shrt steps (, I, 2) mre ften, and larger steps less ften, than they wuld if they were prbability matching. They d nt maximize (minimize abslute errr). The strategy they appear t adpt is between these extremes. A strategy f guessing mstly small distances necessarily results in "assimilatin." The data frm Studies I and 2 take the same frm as thse frm the guessing study in Fig. 6. In general, the smaller the magnitude f assimilatin, the smaller the deviatins frm the prbability matching curve. We prpse that Ss use strategies similar t this guessing behavir when there is a stimulus t be judged but perfrmance is nt perfect. After any given stimulus presentatin, Ss are likely t be in a state f uncertainty as t which f several acceptable respnses is mst likely t be "crrect." Ss may mediate this uncertainty by chsing, frm this subset f acceptable respnses, respnses that tend t minimize the difference between the value f the previus stimulus and that f the current respnse. If the "crrect" respnse was n the average in the middle f the grup f acceptable respnses, the result f this strategy wuld be assimilatin f the current respnses tward the value f the previus stimulus (and thrugh the crrelatin between them t the previus respnse) regardless f the value f the previus r present stimulus. Fr example, cnsider all thse ccasins n which an S is presented with Stimulus 4. Assume that his rugh match t his "stimulus scale" always turns up the numbers 3, 4, and 5 as acceptable respnses t this stimulus. We wuld expect that, in the instances where the previus stimulus value was lwer than 4, the S wuld respnd with 3 mst ften, 4 next mst, and 5 least ften. Just the ppsite wuld happen in thse instances where the previus stimulus was higher than 4. This means that the average errr f respnse t Stimulus 4 wuld be negative when 4 was preceded by a lwer stimulus and psitive when preceded by a higher stimulus; i.e.. respnses t 4 wuld be "assimilated" tward the value f the previus stimulus. In the n-feedback cnditin S is nt given veridical infrmatin cncerning the value f the previus stimulus. In this case, we assume that S uses the best infrmatin available, his wn previus respnse. He tends tward maximizatin f respnse-respnse differences in the same way as utlined fr stimulus-respnse differences. This wuld accunt fr the assimilatin effect being larger t the previus respnse than t the previus stimulus in this study, stimuli and. respnses nt being perfectly crrelated. Further, if S assimilates his respnse t the value f the previus respnse, a cmpunding f the assimilatin effect wuld be expected, since the previus respnse itself was assimilated t the respnse previus t it, and s frth. This wuld explain why the assimilatin effects last s many trials back in the sequence. Accrding t this explanatin, the assimilatin effect wuld f)1ecessity grw smaller with increases in the number f trials frm Trial N. The mre intervening trials, the mre different directins f respnse are interplated between them, and thus, the mre these separate assimilatin effects wuld tend t cancel each ther. The mdel prpsed here is cnsistent with the fact that the magnitude f assimilatin grws larger as perfrmance deterirates. In a difficult task the presented stimulus carries less infrmatin and thus the number f acceptable respnses n any particular trial increases. The magnitude f the assimilatin effect shuld increase with any increase in the number f acceptable respnses, since tending t minimize the difference between the respnse and the previus stimulus wuld result in Ss giving a respnse that is further away frm the "crrect" ne. This is cnsistent with a very large assimilatin effect in a pure guessing situatin. The cn trast f respnses t stimuli further back in the sequence might be accunted fr in the fllwing way_ We knw that S tends t use his available respnses equally ften ver a large number f trials (Parducci, 1965). This empirical result may be the average f attempts t use respnse areas equally ften ver smaller numbers f trials, perhaps as few as 5 t 1. We prpse that, when presented with a shrt sequence f stimuli in which the use f ne area f the respnse scale predminates, S adjusts his respnse scale s as t increase the prbability f use f anther part f it. This is equivalent t packing categry bundaries mre clsely int areas f previusly higher stimulus prbability. Thus, fr example, fllwing an intermixed series f lw and intermediate respnses, Perceptin & Psychphysics, 1971, Vl. 9 ( I B) 77

6 an intermediate stimulus wuld tend t be verestimated. This wuld result in the cntrasting f the respnse t a stimulus ccurring after such adjustment with the stimuli ccurring befre it. The invariability f these effects ver perfrmance indicates that this tendency perates regardless f the infrmatin carried by the stimulus and, thus, is a fairly stable respnse-system prcess. That cntrast des nt appear until mre than tw trials back in the n-feedback study is attributed t the cmpunding f the assimilative errr canceling ut the smaller cntrast effects. Central Tendency r Assimilatin There are many studies that demnstrate that, in abslute judgment-like tasks, small stimuli are verestimated, large stimuli are underestimated, and intermediate stimuli generally have n systematic errr assciated with them. This bservatin, first made by Hllingwrth, has been called the "central tendency effect" and has been attributed t a statistical effect due t imperfect discriminatin (Guilfrd, 1954). Jhnsn and Mullally (1969, p. 26) state that "when the categry judgment is treated as a matching task, crrelatin analysis leads t a simple explanatin f central tendency." Hwever, if assimilatin t a previus stimulus is a fact, and if stimuli are distributed with equiprbability ver the range, then: P(SN-l > SN) > P(SN-1 SN) if SN is lw, P(SN-1 >SN) ::::: P(SN-1 SN) if SN is medium, P(SN-l > SN) < P(SN-1 SN) if SN is high, where P(SN-1> SN) is the prbability that the stimulus value n Trial N - 1 is greater than the stimulus value n Trial N. If assimilative effects are cnstant ver stimuli, then the effect is prprtinal t the prbabilities abve, i.e., average errr is psitive if SN is lw, average errr is zer if SN is medium, average errr is negative if SN is high. That is, if SN is lw, the prbability that the previus stimulus was larger than SN is great, and, thus, if RN is assimilated t the value f the previus stimulus, there will result a psitive average errr. On the abve level f analysis, central tendency and assimilatin are equivalent. The tw pints f view lead t divergent predictins, hwever, with a mre mlecular analysis f the data. Cnsider, fr example, the situatin when a lw- r a high-valued stimulus is presented in an abslute judgment situatin and is fllwed by an intermediate stimulus. The central tendency r crrelatin pint f view predicts that errrs in the average respnse t the intermediate stimulus wuld be in the directin f the mean f the stimulus distributin. The assimilatin pint f view predicts that the in t ermediate stimulus wuld be underestimated when preceded by a lw-valued stimulus and verestimated when preceded by a high-valued stimulus. The data presented in bth Hlland and Lckhead (1968) and Ward and Lckhead (197) supprt the assimilatin interpretatin. Fr example, in Fig. 1 in Ward and Lckhead (197), the average errr f respnses when Stimulus 3,4 is preceded by 1,2 is -.25, but when the same stimulus is preceded by 9,1, the average errr is +.6. There is sme variability in the data, but this result btains in general. Respnses are assimilated tward the previus stimulus rather than tward the mean f the stimulus distributin. This analysis breaks dwn at the extremes (Stimuli 1,2 and 9,1) because errrs can be in nly ne directin; i.e., since Stimulus 1 cannt be underestimated and 1 cannt be verestimated because f the limits n the respnse scale, any ccasinal large errrs in judging these stimuli must prduce mean errrs that are tward the mean f the distributin. This leads t an apparent central tendency effect fr these cases nly. Hwever, it still appears mre reasnable t attribute these average respnse effects t an assimilatin prcess than t a "central tendency effect" such as statistical regressin. REFERENCES GARNER, W. R. An infrmatinal analysis f abslute judgments f ludness. Jurnal f Experimental Psychlgy, 1953, 46, GUILFORD, J. P. Psychmetric methds. New Yrk: McGraw-Hili, HOLLAND, M. K., & LOCKHEAD, G. R. Sequential effects in abslute judgments f ludness. Perceptin & Psychphysics, 1968, 3, JOHNSON, D. M., & MULLALLY, C. R. Crrelatin-and-regressin mdel fr categry judgments. Psychlgical Review, 1969, 76, LOCKHEAD, G. R. Effects f dimensinal redundancy n visual discriminatin. Jurnal f Experimental Psychlgy, 1966,72, McGILL, W. J. Serial effects in auditry threshld judgments. Jurnal f Experimental Psychlgy, 1957,53, PARDUCCI, A. Categry judgment: A range-frequency mdel. Psychlgical Review, 1965,72, WARD, 1. M., & LOCKHEAD, G. R. Sequential effects and memry in categry judgments. Jurnal f Experimental Psychlgy, 197, 84, WEVER, E. G., & ZENER, K. E. The methd f abslute judgment in psychphysics. Psychlgical Review, 1928, 35, (Accepted fr publicatin May 15,197.) 78 Perceptin & Psychphysics, 1971, Vl. 9 (IB)