Short-Term Memory


A system for temporarily storing and managing information required to carry out complex cognitive tasks such as learning, reasoning, and comprehension. Short-term memory is involved in the selection, initiation, and termination of information-processing functions such as encoding, storing, and retrieving data (, 2009[1]).

Theoretical frame

This term refers to information which is stored for a short time and then forgotten. The number of information we can process at one single time is limited and usually only for a few seconds. An example would be trying to recall a telephone number, forgetting it the next, and rereading it again before making the final dialing sequence. The “Magical Number Seven, Plus or Minus Two” is a famous quote or phrase which describes how much information our STM can hold on to at a particular moment. This phrase was coughed up by George A Miller, Harvard-University psychologists in 1956. His research and publication of his paper Psychological Review gave interesting insights into human memory which made a difference in our daily lives (Psychologists World, 2008 [3]).


Information is usually lost or forgotten unless repeated or rehearsed within that particular moment. Three factors contribute to memory forgetting: decay, lack of familiar pattern and overloading. Information tends to decay if not preserved through repetition, the best example being recalling numbers. Individuals tend to repeat the numbers in sequence a few times before making the final dialing sequence. With inadequate recognizing patterns, forgetting happens quicker as well. Lastly, overloading the capacity of STM also triggers forgetting, this is why it is not recommended to study at the last moment especially the night before an exam. However in long-term memory, forgetting happens differently, usually due to the inability to locate information, which completely varies from STM (Memory, 2001 [2]).

Supporting evidence

Decay Theory

The decay theory states that information is lost spontaneously over time, even when there is no interference from other materials (Reed, 2004, p. 72 [4]).

Peterson and Peterson (1959) at Indiana University raised the theory of what contributes to forgetting: Decay or interference? They developed a study to determine the duration of STM, using a hypothesis that STM lasts for about 20 seconds unless rehearsed thereby aiming to prove that information is lost through information decay (McLeod, 2007 [5]).

The study was tested on undergraduates on their skills to recall three consonants over a short interval. In order to prevent rehearsals, subjects were required to count backwards by 3s starting with a number that occurred after the consonants (Brown-Peterson technique). For example, subjects may hear the letters AXN followed the number 572. Subjects would then have to count backwards and paused until they see a light, which serves as a signal to recall the consonants. The independent variable (IV) was the time delay while the dependent variable (DV) was the number of trigrams recalled. Recalls have to be in order and 100% accurate to be credited as a recalled.

The results showed that recalls decreases steadily between 3 to 18 seconds and that the likelihood of a recall drops over the 18-second interval. This indicates to us that verbally rehearsing information keeps it fresh in STM. In addition, it can also be very likely that if we are distracted after searching for a telephone number, we will have to look it up again before dialing.

Results: The percentage recall was:
o After 3 seconds = 80%
o After 6 seconds = 50%
o After 18 seconds = less than 10%

(McLeod, 2007 [5])

Having said that, the rate of which we forget can be both good and bad in a way. It can be frustrating when trying to learn new information but there are many times when remembering briefly is all that we need. Using an example of telephone numbers again, imagined how many numbers we have ever dialed, most of which only once or twice and perhaps never again while some, constantly. If all the numbers are stored, it would be equally difficult as to retrieve information on numbers which we dial constantly (Reed, 2004 [4]).

The conclusion from the study tells us that any memory trace in STM is as good as gone after 18 seconds if not rehearsed quickly which clearly supports the hypothesis made by Peterson and Peterson. They refer to this loss of information in terms of trace decay. The memory fades over time until it disappears completely as oppose to reaching the maximum amount of interval time before forgetting (McLeod, 2007 [5]).

Refuting evidence

Interference Theory

The trace decay theory as an explanation for information loss however poses a wide opportunity for criticisms. One thing is for sure, it is almost impossible to simulate a real-situation simulation where the creation of a blank period of time (interval) between the presenting of material and the recall to test such a theory. Once material is presented, a subject’s first reaction would be to rehearse it. If however a technique is inserted to prevent rehearsal, this clearly indicates an interference to distract subjects from carrying out the any form of rehearsing.

Another possible explanation is the interference theory. The difference between the two is that while the decay theory says memory loss happens over time depending on the length of interval, the interference theory is determined by the number of interfering items.

Waugh and Norman (1965) conducted a test to determine whether information is really loss from decay or interference. The presented a list of 16 digits in which the last digit is referred to as the probe digit and will always occur exactly once in the 16-digit list. The task for subjects was to report the digit after the probe digit. For example, if the list were 5 1 9 6 3 5 1 4 2 8 6 2 7 3 9 4, the probe digit would be 4 and the correct answer would be 2 (test item). In the example above, there are 7 digits proceeding after the test item, therefore it can be assumed that the interfering item is 7. The independent variable would be the number of interference in each test. Waugh and Norman manipulated the interfering items as well as the location of the test digit in the list. From that, we can see that there were many interfering items if the test item occurred early in the list and vice versa.

The test also included the rate of presenting the digits in order to determine whether the probability of recalling the test item would be any different. They presented the 16 digits either in a 1 digit/sec or 4 digit/sec rate. Using the decay theory, a prediction can be made to assume that performance of the memory should be better for the faster rate of presentation because there would be less time for information to decay.

[Experiment taken from (Reed, 2004 [4]) and (Waugh & Norman, 1965 [6])].

However the results showed otherwise. The rate of presentation did not affect much of the probability of recalling as the decay theory would suggest. The difference between both the probabilities are not vast and only varies by fractions which suggests that memory is just slightly better in shorter interval situations. That is as far as an assumption can be made. While on the contrary, the number of interfering items had more effects in influencing retention. The probability of a recall decreases as the number of interfering items increases.

Waugh and Norman’s findings support the interference theory as opposed to the decay theory that contributes to the primary caused of forgetting. It is also a good thing because if information is lost through interference, the least we can do is prevent it by improving retention through the learning process whereas if information was loss from decay, it is inevitable to do anything about it. Also, the decay theory does not explain how many people can still have clear memories of distant events which happened years before and can successfully recall them.

Way forward (to do list)

1. (2009). Definition of short-term memory. Retrieved from on January 14, 2009.
2. Penn State University Learning Centres. (2001). Memory. Retrieved from on January 13, 2009.
3. Psychologists World. (2008). Millar's magic number: How much humans remembers. Retrieved from on January 14, 2009.
4. Reed, S.K. (2004) Cognition: Theory and applications. Thomson-Wadsworth: Belmont.
5. Simply Psychology Online. (2007). Forgetting: Trace decay theory of forgetting (STM). Retrieved from on January 14, 2009.
6. Waugh, N.C. & Norman, D.A. (1965). Primary Memory. Psychological Review, 72, 89-104.

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