MISRA C Conference 2002

had not yet been invented.In the 47 years since the publication of this paper, knowledge of the workings of the human mind has movedon and the invention of the integrated circuit has led to the wide spread use of computers, with a corre-sponding need for reliable software. However, perception of the 7±2 urban legend as being a scientiﬁcallyproved fact remains and it continues to be used to inform decisions on the maximum complexity of codingconstructs. The purpose of this paper is to dispell the urban legend that has grown up around 7±2. While noother rules, based on the workings of human memory, are yet available to replace it, people should at leaststop taking false comfort from this bogus model.What is the 7±2 urban legend? A common model of human memory divides it into two units; a shortterm memory and a long term memory. The short term memory is a limited capacity store for holdingtemporary information. The long term memory is usually treated as an inﬁnite capacity store capable ofholding information throughout a persons life. Information is held in short term memory before beingtransferred (or not) to long term memory. The 7±2 urban legend is that the capacity of short term memoryis seven plus or minus two items of information (some people being able to only hold ﬁve or six items, whileothers can hold eight or nine). The extent to which short term and long term memory really are differentmemory systems, and not simply two ends of a continuum of memory properties continues to be researchedand debated.Millers original paper is available on line at www.well. com/user/smalin/miller.html. Readerswill be surprised to ﬁnd that Miller never proposed 7 ±2 as the capacity of short term memory; he simplymade the observation that this range of values ﬁtted the results of several recent, 1950’s, experiments.Readers might like to try measuring their short term memory capacity, with one of the techniques used inearly memory studies. The lists of digits in the margin can be used. Slowly and steadily read the digits in asingle row, out loud. At the end of each row close your eyes and try to repeat the sequence of digits back,in the same order. If you make a mistake go onto the next row. The point at which you cannot correctlyremember the digits in any two rows, of a given length indicates your capacity limit; the number of digits in8704 the previous row.21933172573010294373619659420402586542173684917379316843617458276315088104296307239861578149306293486701721540683576208394140930672159261835740Measuring short term memory capacity using sequences of digits relies on several assumptions. It assumesthat short term memory, STM, treats all items the same way (what if letters of the alphabet had been usedinstead? ), and that individual concepts are the unit of storage. Subsequent studies have shown that both theseassumptions are incorrect.Any Chinese (or Welsh) speaking readers might like to do the exercise twice; once using English words andonce using Chinese (Welsh) words for the digits. Use of Chinese (Welsh) should enable readers to apparentlyincrease (decrease) the capacity of STM to an average of 9.9 digits [HS88] (5.8 in Welsh [EH80]). How can a persons native language affect their STM capacity? It turns out that the kind of STM people usefor remembering digits is based on the sound of those digits. A person has a two second, approximately,capacity limit [BTB75] on the amount of sound they can hold in STM. Approximately 7±2 English digitwords can be spoken in approximately 2 seconds. The difference is caused by people having differentspeech rates. The Chinese spoken words for the digits are shorter than the corresponding English spokendigit words, which in turn are shorter than the Welsh spoken digit words. This difference in sound length iswhat causes the difference in these speakers digit storage capacity.