Isolation and Serial Position Effect
Running head: ISOLATION AND THE SERIAL POSITION EFFECT
The Impact of an Isolated Item on the Serial Position Effect
Lisa A. Way
University of Baltimore
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Isolation and Serial Position Effect
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Abstract
In an experiment investigating the affect of an isolated item on the serial position
effect, the investigators studied the recall of 30 subjects for the first, middle, and last
words in presented lists. Fifteen subjects were shown 15 lists of words in which the
middle word was highlighted. The other 15 subjects were shown 15 lists of words that
did not contain a highlighted word. The data was collected in order to find the number of
times each subject recalled the first, middle, and last words in each list for each group. A
proportion of the number of times the middle word was recalled to the sum of the number
of times the first and last words were recalled was computed for each subject. The results
found significantly supported the hypothesis that in the presence of an isolated item the
serial position effect would be diminished.
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The Impact of an Isolated Item on the Serial Position Effect
This article deals with how the presence of a distinct item among other
nondistinct items affects the recall of individuals. This is known as the von Restorff
effect, also called the isolation effect. Specifically, this study will focus on the effect that
isolation of an item has on serial position recall. It was hypothesized that the isolation
effect would override the serial position effect.
The serial position effect has two parts, according to Atkinson and Shiffrin
(1968). These two parts are known as the primacy effect and the recency effect. The
probabilities of an item being recalled are usually plotted as a function of its serial
position, this is known as the serial position curve and is usually a U-shaped curve
(Rouder & Gomez, 2001).
Atkinson and Shiffrin define the primacy effect as the
advantage in recalling the first items in a given list, these items then being stored in longterm memory. This is due to the beginning items being rehearsed more than the ending
items (Rundus, 1971), allowing time for these items to be placed into long-term memory.
The rehearsal allows for the words to be pulled from long-term memory much easier than
words that have not been rehearsed. The recency effect, on the other hand, involves
recalling the items at the end of the list at a higher rate than those in the middle of the list.
Atkinson and Shiffrin (1968) propose that this effect is due to short-term memory. The
items in short-term memory are easily accessible and temporary. Therefore the items at
the end of the list are more easily recalled because they are still in the short-term memory
store.
The serial position effect has been studied in many past research investigations.
Andrade, Werniers, May, and Szmalec (2002) used a visual noise technique to investigate
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whether it disrupted visual short-term memory involved in serial position recall. With
their conclusion that this visual noise did not have an affect on visual short-term memory,
they proposed that visual working memory is not analogous to verbal working memory.
Similarly, Beaman (2002) investigated the modality effect, which is, “the recall
advantage over the last one or two serial positions for verbal items presented auditorily
over verbal items presented visually” (Beaman, 2002, p.371). Beaman found there to be
many errors in theories that assume this modality effect regardless of the circumstances.
A sufficient way to invert the modalities was also found.
There have also been many investigations regarding the nature of the coding of
the positions of items for serial recall. Ng and Maybery (2002) studied whether position
information is time dependent. Through three different experiments, they concluded that
recall for serial order does not depend on temporal information. Their conclusion has
been found in many other similar investigations as well.
The isolation effect proposes that an isolated item, in a list of otherwise similar
items, would be remembered better than an item in the same relative position in a list in
which all items are similar (Main, Leland, & Bartlett, 1998). This effect has been shown
in many research investigations.
Cimbalo and Siska (1986) investigated their hypothesis that the isolation effect
would be greater when there are more background items. They concluded that there was
an “overall list facilitation effect (OLFE)” (Cimbalo & Siska, p.70). In other words this
means that when there was an isolated item, in their study, subjects remembered both the
isolated item and the surrounding items better.
Cimbalo, McQuestion, and Witting
(1980) found that overall memory for an entire list containing an isolated item is neither
Isolation and Serial Position Effect
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improved nor decreased. They stated that when the list was presented simultaneously the
memory for the list was improved, however, when the items in the list were presented
successively, the isolated item was recalled much more than the other items in the list.
Main, Leland, and Barlett (1998) specifically investigated the isolation effect to
facial feature recognition. They hypothesized that simpler, distinctive stimuli would be
recalled better than more complex stimuli. Their findings supported their hypothesis.
They found evidence for three properties of one isolated item.
A single, simple, distinctive stimulus in an otherwise cohesive set is remembered
better than two or more such stimuli; in a simultaneous-presentation paradigm,
with a wide variety of contexts, it leads to better memory for the whole set than
do zero, two, or more such stimuli; and it produces better behavioral control than
zero, or two, or more such stimuli. (p.204).
Although there have been thorough investigations conducted on both the isolation
effect and the serial position effect, few researchers have investigated the effects of one
against the other. The present study was designed to incorporate all of the previously
discussed research. Given all of the previous knowledge and the theoretical background
of the two effects, it was hypothesized that when lists of items are presented to subjects,
some containing an isolated item, the distinct item will override the serial position effect
of that particular list of items. Specifically, that the isolated item will be recalled better
than the other items, thus decreasing the primacy and recency effects.
Method
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Participants
The participants in this study were all undergraduate students at the University of
Baltimore. There were 15 males and 15 females making the total number of subjects in
the study 30. The participants ranged in age from 17 to 35 years (M = 20.25, SD = 2.25).
Materials
The experiment was done in a standard classroom, which contained chairs and
desks for the subjects to sit in. Each subject was given a sheet of paper and a pen or
pencil.
There were thirty lists of words typed out onto transparencies.
contained 15 randomly generated words.
Each list
In half of the lists, the eighth word was
highlighted with a yellow highlighter. A projector was used to present the lists of words
onto a screen in the front of the classroom. A sheet of paper with a hole cut out of it,
large enough for the subjects to see only one word at a time, was used to present the lists.
The person displaying the lists of words to the subjects was a randomly selected professor
at the University of Baltimore, who was unaware of the research question.
This
controlled for any experimenter bias effects.
Procedure
All participants were asked to spend a few minutes on a memory task. They were
told that they could stop at any time. All subjects willingly agreed to complete the task
(Webster, Leland, & Bartlett, 1997). The participants were then randomly assigned to
group 1 and group 2, with 15 subjects in each group.
Group 1 was asked to leave the room and wait in the hallway. Once all of group1
were out of the room, the door was shut and group 2 was given a sheet of paper and a pen
or a pencil. The professor explained to the participants that they were going to be shown
Isolation and Serial Position Effect
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15 lists of words, one list and one word at a time, and that after each list, they would be
asked to write down, on the sheet of paper distributed, as many of the words that they
could recall. The projector was turned on. The 15 lists not containing the highlighted
word were shown to this group. The piece of paper with the whole cut out of it was used
to only allow one word to be seen at a time. Starting at the top of the list, the words were
shown and read aloud. Each word was displayed for 5 seconds. When all of the words
were displayed, the participants were given 30 seconds to write down, in any order, all of
the words that they recalled. The subjects were then shown the remaining lists of words.
The same procedure was followed for each list.
Group 2 was then asked to go out into the hallway and group 1 was asked to come
into the room. The same procedure was followed for this group. The only difference was
that group 1 was shown the 15 lists with the highlighted word. Fifteen lists were shown to
each group in order to obtain a more accurate result. When group 1 was finished, group 2
was asked back into the room. The participants were then given a debriefing by the
experimenter. The purpose of the study and the studies conditions were discussed.
The data was then recorded. The experimenter recorded the number of times that each
subject recalled the first, middle, and last words out of the 15 lists that they were shown.
This was done for each subject in both groups. In this study the independent variable was
the presence or absence of the highlighted word in the list. The dependent variable was
the proportion of the number of times the middle words, also the highlighted words, were
recalled to the sum of the number of times the first and last words were recalled. An
independent-samples t-test was used to determine if the difference between the
proportions in group 1 and group 2 were significant.
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Results
A proportion of the number of times the middle words were recalled to the sum of
the number of times the first and last words were recalled was found for each subject in
each group. As predicted, the proportion of recall was found to be higher for the
participants that were shown the lists containing the highlighted word (M = 3.144, SD =
3.8645) than the proportion of recall for the participants that were shown the lists without
the highlighted word (M = .371, SD = .2914). These results are shown in Table 1. The
results were found to be statistically significant using an alpha level of .05, t(28) = 2.772,
p = .05. These results are shown in Table 2.
Figure 1 shows the differences in recall between group 1 and group 2. This graph
displays the mean number of times the first, middle, and last words were recalled for each
group. It shows the presence of the serial position curve for the recall of group 2. This is
compared with the absence of the serial position curve for the recall of group 1. For
group 1, shown the lists with the highlighted word, the mean recall for the middle word,
or the highlighted word, is much higher than the mean recall for the first and last words.
Group 2, shown the lists without the highlighted word, had a higher mean recall for the
first and last words than for the middle word. The exact values are shown in Table 3.
Figure 1 is used to display the relationship graphically.
Discussion
The hypothesis that the presence of an isolated item would block the serial
position effect was supported through the results. In this study, the first and last words
were used to define the serial position effect. This idea incorporated Atkinson and
Shiffron’s (1968) model of the serial position effect, which stated that the serial position
Isolation and Serial Position Effect
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effect was composed of the primacy and recency effects. Thus, the use of the first word
in the lists represented the primacy effect and the use of the last word in the lists
represented the recency effect. The isolated item was placed in the middle of the list in
order to emphasize the results.
Figure 1 clearly shows the difference in recall between group 1 and group 2. The
results suggest that when the middle word was highlighted, the first and last words were
not recalled as often. When the middle word was not highlighted, the first and last words
were recalled much more often than the middle word. The U-shaped serial position
curve, which was suggested by Atkinson and Shiffron (1968) and supported by many
following researchers including Rouder and Gomez (2001), was seen in the recall of
group 2 and it was not seen in the recall of group 1.
One possible explanation for this could pertain to memory strategies. Rundus’
(1971) assumption that the primacy and recency effects are due to the long-term and
short-term memory stores can explain the results found for group 2, which portray the
serial position effect. In contrast, the results found for group 1 cannot be explained by
Rundus’ (1971) assumption. Cimbalo and Siska (1986) suggest that the isolation effect
could be due to the encoding processes of memory. They concluded that when the
isolation of an item is embedded in an otherwise nondistinct array of items, the isolation
provides extra clarity of the item and thus stronger encoding for memory. This may help
to explain the results found for group 1, in that the encoding for the highlighted word
may have been stronger than that for the first and last word due to its clarity. The
stronger encoding for the highlighted word may have blocked the serial position effect.
Isolation and Serial Position Effect 10
This may be related to the reason that students tend to highlight important readings in
their studies. This may help them encode the information better and thus recall it better.
This study did not investigate the affects of the modality used, as in the studies
conducted by Beaman (2002) and Andrade, Werniers, May, and Szmalec (2002). These
studies investigated the differences and affects of verbal memory and visual memory.
The present study controlled for those differences by having the experimenter show as
well as read each word aloud one at a time. It is possible that this method strengthened
the results by allowing both modalities for the encoding process.
This study also did not investigate the overall list facilitation effect (OLFE) that
has been studied by many researchers, including Cimbalo and Siska (1986), and Cimbalo,
McQuestion, and Witting (1980). The present study did not take this effect into account
when analyzing the results. It did not investigate the idea and therefore concentrated on
the highlighted word and not the words in close proximity to it. This may have limited
the studies external validity. It cannot support and therefore cannot be generalized to
those investigations involving the overall list facilitation effect.
Relating to time for encoding, was an assumption taken from a study conducted
by Webster, Leland, and Bartlett (1997). This study states that an isolation, “must be a
very simple stimulus, because a complex stimulus would itself require processing time
that would detract from the time available for encoding the surrounding material”
(Webster, Leland, & Bartlett, 1997, p.404).
The present study therefore simply
highlighted the isolated word in order to keep the isolation simple and so as not disturb
the results of the study. This may have strengthened the internal validity of the study, as
well as the external validity by making the results more generalizable. The present study
Isolation and Serial Position Effect 11
also controlled for the affects of time on recall between the two groups by having the
experimenter display each word for 5 seconds.
This also strengthened the internal
validity of the study.
The internal validity, however, may have been threatened by certain undetermined
variables. The experimenter, for example, may not have followed the directions precisely
and therefore not allowed exactly 5 seconds between each word. If this time varied, even
slightly, it may have had an affect on the results of this study in comparing the two
groups. Also it is possible that the experimenter may have paused slightly longer at the
highlighted word, thus allowing more time for encoding. If this were to happen the
results would have been greatly threatened.
The external validity of the study could also be threatened.
It is risky to
generalize the results of the present study to situations beyond auditory and visual lists of
words. The participants were placed in a classroom setting and not in their natural
environments. They knew that they were being tested on their memory ability and may
have wanted to do well. Although this also may affect the internal validity of the study,
this needs to be taken into account when generalizing the results of the present study to
other situations.
This study is very important because few researchers have conducted research
regarding the affects of the isolation effect on the serial position effect. The results of
this study may be generalized to suggest that the isolation effect may override the serial
position effect. This would lead to the assumption that when an isolated item is placed
among other nondistinct items, the serial position effect would be less prominent or
nonexistent.
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References
Andrade, J., Kemps, E., Werniers, Y., May, J., & Szmalec, A. (2002). Insensitivity of
visual short-term memory to irrelevant visual information. The Quarterly Journal
of Experimental Psychology, 55, 753-774.
Atkinson, R.C., & Shiffrin, R.M. (1968). Human memory: A proposed system and its
control processes. In K.W. & J.T. Spence (Eds.), The psychology of learning and
motivation: Advances in research and theory, 2, 89-195.
Beaman, C.P. (2002). Inverting the modality effect in serial recall. The Quarterly Journal
of Experimental Psychology, 55, 371-389.
Cimbalo, R.S., McQuestion, C., & Witting, D.M. (1980). Short-term memory isolation:
Null, facilitative, and debilitative effects and their preconditions. The Journal of
General Psychology, 103, 171-178.
Cimbalo, R.S., & Siska, B.L. (1986). Isolation effect in simultaneously presented lists of
different lengths. The Journal of General Psychology, 114, 69-73.
Main, K.M., Leland, Jr, L.S., & Bartlett, G.C. (1998). The properties of one: Facial
memory and the isolation effect. The Journal of General Psychology, 125, 192206.
Ng, H.L.H., & Maybery, M.T. (2002). Grouping in short-term verbal memory: Is position
coded temporally? The Quarterly Journal of Experimental Psychology, 55, 391424.
Rouder, J.N., & Gomez, P. (2001). Modelling serial position curves with temporal
distinctiveness. Memory, 9, 301-311.
Isolation and Serial Position Effect 13
Rundus, D. (1971). Analysis of rehearsal processes in free recall. Journal of
Experimental Psychology, 89, 63-77.
Webster, R.A., Leland, Jr, L.S., & Bartlett, G.C. (1997). The properties of one: Single
distinctiveness and their effects. The Journal of General Psychology, 124, 391409.
Isolation and Serial Position Effect 14
Table 1.
Summary Statistics for Each Set of Word Lists
Proportion
Isolation
Highlighted
Word
Not Highlighted
N
Mean
Std. Deviation
Std. Error
Mean
15
3.144
3.8645
.9978
15
.371
.2914
.0752
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Table 2.
Independent-Samples t-Test
t-test for Equality of Means
t
Porportion of middle word
to first and last combined
df
Sig. (2-tailed)
Mean
Difference
Std. Error
Difference
Equal variances
as sumed
2.772
28
.010
2.773
1.0006
Equal variances
not ass umed
2.772
14.159
.015
2.773
1.0006
Isolation and Serial Position Effect 16
Table 3.
Comparison of Recall for First, Middle, and Last Words
Isolation
FIRST
Highlighted Word
Mean
N
Std. Deviation
Not Highlighted
Mean
N
Std. Deviation
Total
Mean
N
Std. Deviation
MIDDLE
LAST
3.73
12.47
3.13
15
15
15
2.712
2.295
1.727
9.13
5.53
9.00
15
15
15
3.563
2.850
3.723
6.43
9.00
6.07
30
30
30
4.150
4.347
4.127
Isolation and Serial Position Effect 17
Figure Caption
Figure 1. The number of times the first, middle, and last words were recalled for group 1
and group 2.
Isolation and Serial Position Effect 18
Number of Times Recalled
14
Group 1
Group 2
12
10
8
6
4
2
0
First, Middle, and Last Words