Memory is the ability to store and recall information which has either been experienced or learnt. There are two components, short-term and long-term memory. The nature of memory consists of duration (length of time), encoding (means of remembering) and capacity (amount of information stored). The nature of short-term memory is different from the nature of long-term memory. Short-term memory is the information which is immediately accessible or active. However, it has a limited duration time. Peterson and Peterson (1959) did a study using trigrams and found that duration lasted 18 seconds when recall was prohibited.
Encoding in short-term memory is predominantly done acoustically, by sound, rather than visually, sight or semantically, meaning. Conrad (1964) conducted a study with acoustically similar and acoustically different letters; he concluded that similar letters such as ‘B’ and ‘P’ caused confusion in recall due to acoustic similarity. The capacity of information in short-term memory is also limited. The first systematic experiment on the short-term memory span was done by Jacobs (1887). The aim of the experiment was to see how much information could be stored in the short-term memory or active memory.
The study consisted of using the serial digit span technique, using both letters and numbers; however, excluding ‘w’ and ‘7’ because they contain two syllables. Jacobs found that on average the digit span (9.3) was greater than the letter span (7.3). His theory for this difference being that there are only 9 digits whereas there are 25 letters, consequently there would be less confusion between the digits. Another factor observed was that digit span increased with age, due to either, the increased brain capacity or to formed strategies to remember more material, such as chunking, reorganising information into smaller groups or clusters.
Wickelgren (1964), observed that when trying to recall a number, for instance a telephone number, recall improved if the numbers where grouped by pausing briefly after every three digits. Typically, the first and the last number in a group were better remembered than the middle digit. Therefore, 791 862 534 would be better remembered than 79 18 62 53 2. However, is it better remembered because of the number of digits in a group or the total number of groups? By grouping in three’s there are three groups (chunks) but grouping in pairs there are five chunks.
Simons (1974) established that the size of the chunk did have an effect on memory. He found that fewer longer chunks, such as eight word phrases, were recalled compared to the shorter chunks, one syllable words. Slak (1970) found that subjects who found a number sequence such as 265070193 very difficult, by using a mnemonic, a word or group of words that can be associated with the information being remembered, like BAFDILTUN could learn the letter-code equivalent much quicker. This is because instead of learning nine digits there are only three chunks, BAF, DIL and TUN.
Miller (1956) cited a study done by a Scottish philosopher, Hamilton (1788-1856) who wrote ‘If you throw a handful of marbles on the floor, you will find it difficult to view at once more than six, or seven at most, without confusion.’ Jevons (1834-1882) followed up this theory and found that he never made a mistake with three or four, was sometimes right about five, and usually wrong with fifteen. In the classic paper “The Magic number seven, plus or minus two” Miller established that more items were remembered (immediate memory span increased) when information was ‘chunked’ rather than the number of items, when the amount of information averages seven chunks.
This is why a familiar date such as 1066 is likely to be a chunk rather than a less meaningful date as 1325. The memory span for letters is about six when in random order and nine when a constant-vowel-constant syllable, increasing the span to approximately 50 when the words made a meaningful sentence, however, the number of chunks remains at about six.