Short-Term or Working Memory 431account for this word length effect (e.g., Melton, 1963).
However, Baddeley et al. (1975) found that the performance
differences remained when all factors other than pronuncia-
tion time—or articulation rate—were controlled. For exam-
ple, the words bishopandFridayhave the same number of
letters and syllables but differ in pronunciation time (Friday
takes longer to say); lists of long words, defined solely in
terms of duration, still yielded poorer recall than lists of
matched short words.
As has been noted, the standard model assumes that decay
is the main culprit behind forgetting, and the word length ef-
fect is certainly consistent with this view. The longer it takes
to rehearse a set of items, the greater the chances that some of
the items will be lost prior to refreshing. Pronunciation time,
or articulation rate, is simply assumed to correlate with the
speed of internal rehearsal. Specifying capacity in terms of
items, then, is correct only in the sense that it usually takes
more time to rehearse a large number of items. Baddeley
et al. (1975) reported that the useful lifetime of a short-term
activity trace is around 2 s (see also Schweickert & Boruf,
1986); consequently, memory span should be roughly equal
to the amount of material that can be rehearsed in 2 s. On av-
erage, not surprisingly, we can rehearse somewhere around
seven plus or minus two items in 2 s.
This relationship between pronunciation time and imme-
diate memory span generally holds at the level of group data
as well as for individual subjects. In fact, it is possible to pre-
dict individual differences in memory span, for both children
and adults, by measuring overt articulation rate (see Hulme,
Thomson, Muir, & Lawrence, 1984; Tehan & Lalor, 2000).
Developmental changes in memory span are also associated
with rehearsal rate, to a certain extent, as are some differ-
ences that occur in span cross-culturally. For instance, digit
span tends to be higher in English and Chinese than in Arabic
or Welsh, presumably because it takes longer to say digit
names in the latter languages (see Ellis & Hennelly, 1980;
Naveh-Benjamin & Ayres, 1986).
Recent data, however, are qualifying these conclusions
somewhat. For example, it turns out that span differences
sometimes remain even when pronunciation times are held
constant. Memory span is typically lower for phonologically
similar lists of words, compared to phonologically dissimilar
lists, but similarity has little, if any, effect on pronunciation
rate (Hulme & Tordoff, 1989; Schweickert, Guentert, &
Hersberger, 1990). Hulme, Maughan, and Brown (1991)
found that words can produce higher memory spans than
nonwords, even when articulation rates are matched for the
item types; similar dissociations between articulation rate
and span have been found for concrete versus abstract words
(Walker & Hulme, 1999) and for high- and low-frequency
words (Hulme et al., 1997; Roodenrys, Hulme, Alban, &
Ellis, 1994). Any model that appeals simply to time-based
limits in storage capacity—for example, the standard re-
hearsal plus decay model—has no clear way of explaining
these data.
Even more troubling are recent reports suggesting that one
of the major conclusions of Baddeley et al. (1975)—namely,
that duration-based span differences exist for word sets
matched on all variables other than spoken duration—may
apply only to restricted sets of words. Caplan, Rochon, and
Waters (1992) found no memory advantage for short-
duration words in lists containing short- and long-duration
words matched for number of syllables and phonemes; in-
stead, they reported a reverse word length effect (long better
than short) when duration was implemented by using “lax”
vowels of short duration (e.g., carrot) and “tense” vowels of
long duration (e.g., spider). Caplan et al. (1992) suggested
that the phonological structure of a word, not its spoken du-
ration, determines the magnitude of the word length effect. A
similar conclusion was reached by Service (1998) using
Finnish stimuli, which allow one to vary duration by manip-
ulating combinations of the same articulatory and acoustic
features. Lastly, Lovatt, Avons, and Masterson (2000) varied
spoken duration in disyllabic words, holding constant a host
of potentially confounding factors (e.g., frequency, familiar-
ity, number of phonemes) and found no advantage for short-
duration words across several experiments; word duration
effects emerged only when the original word sets used by
Baddeley et al. (1975) were used as the to-be-remembered
stimuli.The Limits of AttentionIn retrospect, it is not surprising that factors other than time
contribute to limitations in immediate retention. Even within
the standard model, storage capacity is not fixed, but rather
arises from the trade-off between decay—which is purely
time-based—and a controlled process of rehearsal. Success-
ful retention depends on one’s ability to keep list items in an
active and recallable state, but also on the ability to counter-
act distraction and eliminate interference from nontarget
information in memory. Errors in immediate retention, for
example, often turn out to be intrusions from immediately
preceding trials (e.g., Estes, 1991; Wickelgren, 1967).
Some researchers believe that limits in immediate mem-
ory arise, at least in part, from the ability to use controlled at-
tention to ignore or filter out potentially interfering material
(see Dempster, 1992; Kane & Engle, 2000). There is some
evidence to suggest that individuals with low memory spans
are more susceptible to proactive interference than high-span