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ENGINEERING
HUMAN FACTORS
Henry Petroski
W
hen something is designed, decisions
are necessarily made. Deliberate
progress cannot proceed without choices—as to whether a part goes to the right or the
left of another part, whether a component is larger or smaller. In some cases a clear historical
record documents for us the evolutionary design
process that brought us the thing that we contemplate. Looking into that record in detail can help
us understand why certain things are the way
they are and, perhaps more importantly, help us
understand how things in general come to take
the forms that they do. One case study that has
these attributes encompasses the keypads of two
familiar artifacts that most of us use daily: those
of the telephone and the hand-held calculator.
In Invention by Design: How Engineers Get from
Thought to Thing, I posed the question of why
these two common devices have different numberpads, the telephone’s having the numbers 1, 2,
3 on its top row and the calculator’s having 7, 8, 9
in the same place. The purpose of the question
was to encourage readers, especially students, to
look closely at two things they encounter every
day and to think about and speculate on what
design reasons there might be for the development of two different solutions to essentially the
same problem—inputting numbers. I did not expect readers to delve into the history of the keypads, but rather to reflect on a curious example of
a lack of standardization in the world of invention and design and on whether that lack has serious consequences for users. Here, however, I
would like to delve into the historical record.
Competition for Automation
The most commonly given answer as to why the
two keypads have different layouts is that modern calculators and telephones derive from different technological roots. The calculator’s ancestors are cash registers and calculating machines,
on which the keys were arranged in an ascending
order, from the bottom to the top. Push-button
Henry Petroski is A. S. Vesic Professor of Civil Engineering and
a professor of history at Duke University. He is the author of
nine books, the most recent of which is titled The Book on the
Bookshelf. Address: Box 90287, Durham, NC 27708-0287.
304
American Scientist, Volume 88
telephones, on the other hand, derive from rotary-dial telephones, which had to incorporate,
in addition to digits, letters of the alphabet,
which naturally are arranged in the order in
which we would read a Western text. There is a
nugget of truth in this explanation, but the full
story is a bit more complicated.
The rotary-dial telephone dates from at least
1891, when a patent was issued to Almon Strowger, a Kansas City, Missouri undertaker who is
said to have become suspicious that telephone operators were accepting bribes to direct calls intended for his mortuary to his competitors. He invented an “automatic telephone exchange” that
eliminated the need for an operator, at least for
local calls. Initially employing buttons, by 1905
Strowger’s scheme had evolved into one using a
dial wheel. The American Telephone & Telegraph
Company licensed Strowger’s invention in 1916,
but even so rotary-dial telephones were not widely used in the Bell System until the 1950s.
The AT&T rotary telephone dial was, of
course, a wheel with 10 finger holes in it. Most of
the holes were labeled with a triplet of letters in
addition to a number, and the process of dialing
each letter or numeral was carried out by inserting the finger (women with long or delicate fingernails used the eraser end of a pencil or a special plastic dialing aid, similar in design to the
head of the popular Bic four-color pen) in the appropriate hole, turning the dial wheel to the stop,
and releasing the dial to allow it to return to its
rest position, ready for the next digit to be dialed.
The spring-loaded wheel clicked when turned
and returned with a characteristic ratcheting
sound to its neutral position after each number
was dialed, a loud and slow process compared to
“dialing” today’s touch-tone telephones.
The finger holes were arranged around the
dial wheel in ascending order counterclockwise,
so that the number 1 and the letters ABC (paired
with the number 2) took the least time to dial,
since they required the least travel time to rotate
them to the stop. This may all be obvious to those
of us who once used rotary-dial telephones, but
an engineer who collects the devices has reported
that “one ten-year-old boy who was trying to dial
a three put his finger in the zero dial hole,
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Ergonomics Becomes Human
Human factors is the shorthand name for humanfactors engineering, which is also known as human engineering and engineering psychology.
This field of specialization deals with the humanmachine interface and considers the human user
to be part of a person-machine system. Among
the principal goals of human-factors engineering
is to contribute to the design of instrument displays and controls and to the efficiency, safety and
reliability of the use of machines. The field is a
part of the larger one of industrial engineering and
traces its roots back to the scientific-management
movement spearheaded by Frederick Winslow
Taylor in the early 20th century. Research in the
area greatly intensified during World War II, especially in Britain, where it was known as ergonomics, although that term has largely given way to
the more direct one of human factors.
The objects of human-factors studies can range
from the size, shape and arrangement of the dials,
buttons and levers in the control room of a nuclear-power plant to the effect of pencil-lead diameter on the response time for filling out multiple-choice examination answer sheets. When in
the 1950s Bell Laboratories began looking seriously at the use of push buttons on telephone sets,
the spatial arrangement, physical action, and ease
and reliability of use of the buttons became natural subjects for study by human-factors engineers.
Push-button devices were nothing new. Indeed,
early light switches were operated by push buttons, and these can still be found in use in some
1
2
3 4 5 6 7
8
9
0
7 8 9
4 5 6
1 2 3
0
5 6
8
9
3
2
1
5
4
1 2 3 4 5
6 7 8 9 0
4
0
8
2
0
5 6
7
3
2
1
9
3
7
4
8
9
0
7
1
6
1 2 3
4 5 6 7
8 9 0
1 2 3
4 5 6
7 8 9
0
4
8
0
8
0
7
6
5
8
9
0
9
7
1
2
3
8
7
6
5
4
0 1
1
9
1 2 3
5 6 7
9 0
4
5
6
1 6
2 7
3 8
4 9
5 0
1 2 3
4 5 6
7 8 9
0
3
2
1
1 2 3 4
5 6 7
8 9
0
2
3
4
2
9
8
3
4
6 5
7
4
1 2 3 4 5
6 7 8 9 0
3
2
8
9
1
5
6
7
0
brought it to three, and then released it.” (We
tend to forget that technology that is second nature to operate for those of us who are familiar
with it can be mysterious to those who have never experienced it.)
The combination of letters and numbers on a
rotary telephone dial was necessary because older
telephone numbers comprised named and numbered exchanges, which served as prefixes for
four-digit numbers. The mystery movie Butterfield
8 derived its title from the exchange BUtterfield 8,
which was connected to by dialing B-U-8 and, incidentally, played a central role in providing a crucial clue in solving a murder. In New York in the
1950s, a telephone call was made by dialing a
complete seven-digit phone number, such as MU
5-1234, the MU indicating that the telephone being
called was located in the Murray Hill section of
the city. (The hyphenated combination of alphanumeric exchanges and four-digit numbers
has come down to us in the way most of us group,
pronounce and remember the purely numerical
numbers of today, for example, 685-1234.) The key
arrangement on today’s push-button telephones—the top row comprising 1, 2, 3 (along
with ABC and DEF, the 1 key not having been
paired with letters of the alphabet)—has more to
do with human-factors research than tradition,
however, as the historical record demonstrates.
Figure 1. Telephone keypad arrangements were tested in groups of
three to determine the optimal array for this now-familiar device.
Two that showed “significantly shorter keying time” were retested
in the final test (bottom row, left and middle). The arrangement at
bottom left was eventually chosen, despite the fact that the bottom
middle arrangement was “significantly more preferred.”
older homes. Automobile radios with push buttons were also common by the 1940s, and I recall
our family’s first car, a 1948 Dodge, having them.
Such push-button devices were, however, principally on-off switches, whose function was indicated by location, and they were not used for extensive data entry, such as of a series of numbers.
Keysets, on the other hand, consisted of a matrix
arrangement of keys, each corresponding to a
unique digit. These also were not new in principle,
for cash registers and mechanical calculators had
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2000
July–August
305
long used multiple columns of numbered keys,
one column for each decimal place. By the early
1950s, keysets of the kind we might recognize today were in use on coding devices, computers and
communications equipment. However, as a 1955
article in the Journal of Applied Psychology noted,
there appeared to have been “few systematic studies concerned with the design factors that make
keysets easy or hard to use.”
The study that prompted this remark was motivated by the fact that long-distance telephone
operators were making errors in entering numbers like 815 RE 4-0267 using a ten-button keyset
arranged as follows:
4 5
3 6
2 7
1 8
0 9
(The letters of the alphabet were located on the
usual keys.) According to the investigators, the
patterns of errors associated with using this keyset suggested that a different arrangement of the
keys might help reduce the error rate. As a first
step in seeking the best arrangement of the alphanumeric keys, the engineers decided to “find
out where people say they would expect to find
letters and numbers on six different keyset configurations,” that is, ones arranged as follows:
O O
O O
O O
O O O
O O O O O
O O O
O O O O O
O O O
O O
O
O O
O
O O O
O O O
O O O
O O O
O O O O
O O O O
O O O
O O O
O O O
The first four arrangements were also the top
four choices, by frequency, of the subjects of the
study, who arranged the numbers (and corresponding letters) as follows:
1 2
3 4
5 6
1 2 3
0
1 2 3 4 5
4 5 6
1 2 3
6 7 8 9 0
7 8 9
4 5 6
0
7 8 9
7 8
9 0
The leftmost of these arrangements, incidentally,
is the one that in 1973 would appear on the first
hand-held cellular telephone, which weighed
about two pounds and was the size of a box of tis306
American Scientist, Volume 88
sues or a thick paperback book. More than a
decade before that, however, the “most obvious
finding,” and perhaps not surprising result, of the
study was “that people arrange numbers and letters in [the] order in which they normally read.”
The researchers commented on the fact that, of
several contemporary “calculating devices” they
were able to look at, only the IBM punch-card
machine used an arrangement of keys that the
study found highly preferred—the one on the far
right in the last set. The arrangement third from
the left was found on the multiplier keys of the
Frieden calculator and on the keyset of the Remington Rand adding machine. As for “most other
calculators,” they had “keys reading upward in
vertical rows of ten,” as on cash registers, with
the 0 located below the 1, a placement seldom
found preferable in the tests. (The electronic calculator had not yet arrived, of course.) The researchers admitted to having “no assurance that
these differences between calculator keysets are of
any practical importance” and considered their
study “only a first step toward finding the keyset
which would give the fewest errors and shortest
keying times.” Thus was the quest begun.
Those conducting human-factors engineering
studies on the design and use of pushbutton keysets admitted that
[t]he number of possible key arrangements,
force-displacement characteristics and button tops is very large—too large to be tested.
A selection of characteristics was, therefore,
made on the basis of prior knowledge, user
expectation and broad engineering requirements, so that we could examine only the
region around an expected maximum.
This is a not uncommon situation in engineering
design, whether it be a problem of choosing the
material for a nuclear reactor core, the type and
style of bridge for a particular crossing, or the
arrangement of the keys on a telephone set. Engineering judgment, insight and simple common
sense contribute to narrowing down the seemingly limitless field of choices to a manageable
few that can be compared in a rational way in a
reasonable amount of time.
By the late 1950s, field trials of push-button
telephones were under way, but research continued at Bell Labs to determine the optimal
arrangement of the keyset, as well as the size,
spacing and tactile characteristics of the action of
the buttons themselves.
Among the arrangements of buttons that had
been tried, in addition to the variety of rectangular arrays, were circles, triangles and a cross pattern. It had also been established that a sevendigit telephone number could be keyed in as
much as five times faster than it could be dialed.
(Until I tried my first office push-button telephone, I thought dialing time to be unimportant.
However, after getting used to operating a new
push-button telephone installed in my office in
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with permission only. Contact [email protected].
the early 1970s, I did find our old rotary phone at
home frustratingly slow to dial.) But dialing time
was found to be approximately equal among the
rectangular array of two horizontal rows, that of
the three-by-three array with a 0 key added below the 8, and two circular arrays.
Circular arrays did, of course, most closely
mimic the rotary dial arrangement. Indeed, one
circular configuration of the number keys effectively placed the buttons in the respective hole
positions on the conventional dial—that is, in the
shape of a letter C with the digits increasing from
1 to 0 in a counterclockwise direction (see Figure
1). This arrangement was, understandably,
termed the “telephone” arrangement. Alternatively, the number keys were arranged in a circle
open at the bottom, with the digits progressing
from 1 to 0 in a clockwise fashion. This was
termed the “speedometer” arrangement.
A study found the difference in the top five
arrangements, in terms of keying time, to be insignificant. Interestingly, the now-familiar telephone keypad arrangement, then termed the
“right-reading 3 by 3 plus 1,” neither produced
the fastest keying time nor was it the most preferred. The most preferred arrangement was the
“two horizontal rows” of buttons, and the fastest
in keying time was the arrangement that mimicked the conventional rotary dial, the “telephone.”
The two horizontal rows of buttons were used in
early trials, in which a standard telephone had its
rotary dial replaced with the keypad arrangement and was mounted on a special table so that
the seven-inch-long prototype pushbutton mechanism could project out the bottom, allowing the
telephone to have familiar-looking proportions.
In the final analysis, however, it was the now-familiar telephone keypad arrangement that was
chosen, “since it uses the available space efficiently and permits a simplified design in the initial application.” In other words, the final choice
was a judgment call and a compromise, in that
the keypad arrangement, after the mechanism
was reduced in size, fit neatly and relatively attractively into existing telephone bodies. It is difficult, however, to identify one single consideration that led willy-nilly to the final choice.
Numbers vs. Letters vs. Patterns
Regardless, the arrangement of the push-button
telephone keypad was firmly established by the
early to mid-1960s. Some of those who resist technological change or who have little motivation to
go along with it still use rotary dials four decades
later. The rotary dial, with its roots in the alphanumeric telephone numbers of an earlier era,
does perhaps have an advantage in dialing those
numbers that we are supposed to remember by
mnemonic devices, such as FOR-FOOD, which
could designate the telephone number 367-3663
of the local pizza-delivery service. Such devices
might have been a natural when we were used to
telephone exchanges, but in the all-numbers
Figure 2. Keypad arrangement for the first electronic calculator (built
at Texas Instruments in 1967) departed from the familiar touch-tone
phone array. (Photograph courtesy of Texas Instruments.)
world of today, they can be a nuisance. I, for one,
find “dialing” my portable phone frustratingly
slow when I have to input a series of words rather
than the numbers whose keys I have come to
know like the back of my hand.
There are times when the letter keys are convenient, however, such as when a company or
organization has an electronic directory that allows the telephone keys to be used to spell out
the name of an employee whose extension we do
not know. (The letters Q and Z were not on old
rotary dials, of course, and neither are they on
most alphanumeric keysets today. It has been
proposed, however, that they be added, either in
alphabetical order on the 7 and 9 keys, respectively, or as an odd pair on the 1 key.)
Most of all I like the telephone keypad for its invaluable assistance to me in remembering my various “PIN numbers,” as they are redundantly called.
I remember these access codes not by the numbers
but by the patterns my finger traces out in keying
them in. Indeed, I have become so accustomed to
remembering the PIN by its pattern that I have to
punch out the pattern in my mind if I am asked to
recite the numbers to a service representative.
The telephone keypad arrangement was about
a decade old by the early 1970s, when the first
hand-held electronic calculators began appearing
on the market. The Texas Instruments prototype
dates from 1967, and it had not an LED display
but a thermal printer for displaying results. Most
important for this discussion, however, is the fact
that it had a keypad not with the telephone
arrangement of buttons but with the now-familiar
7, 8, 9 buttons in the top row. This was the natural
way to arrange the calculator keys, because it was
the way they were arranged on desktop adding
machines. Adding machines were increasingly following a British standard dating from 1963, which
had gotten a fingerhold just as the telephone key-
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with permission only. Contact [email protected].
2000
July–August
307
pad was being introduced. It was soon recognized
by human-factors engineers that:
[i]t seems highly likely that in the near future
many people will be frequently involved in
two numeral data-entry operations, using two
keysets with identical configuration but very
different numeral arrangement. Before even
considering the question of confusion in concurrent use of two different layouts, it is worth
asking whether one or the other is more efficient for its purpose.
All of the concerns and studies of the ergonomicists were soon moot, however, for the future did come, as it always does—and with two
keysets. The companies that came to compete for
the early calculator business did not have the luxury of time that comes with a monopoly for conducting extensive human-factors tests, and the
pocket scientific calculators, which date from
1972 and were immediately embraced by engineers and scientists, all came with the 7, 8, 9 keys
in the top row. The placement of the 0 was, however, another story, as was what accompanied it
in the bottom row of buttons. The 0 could be in
any of the three positions and still is, and it could
share the bottom row with the decimal point,
clear key, +, –, =, π or a host of other keys.
Few if any calculator/telephone users appear
to have become finger-tied, however, and,
whether adding or dialing, we seem to adapt to
the machine before us. This calls to mind the familiar adage that people conform to technology,
but the observation need not be interpreted in a
pejorative sense. The fact that we users of technology can switch freely between two keypads
with similar geometric arrangements but with
grossly different key designations illustrates not
conformity to so much as mastery of technology.
There appears to be something in our makeup
that allows us to adjust immediately to the task
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American Scientist, Volume 88
at hand. It is not an indictment of but an accolade for the human machine that it can do this.
And we do it well beyond the telephone and
calculator. My son’s coffee table has four or five
remote-control devices sitting on it, each with a
different keyset arrangement and different still
from the cellular telephone and electronic calculator nearby. He uses them as would a percussionist in an orchestra, with nary a missed beat
in switching from triangle to cymbal to tympanum to glockenspiel. Technology, like music, is
enriched by variety.
Acknowledgment
I am grateful to Jeon Nam Kil, a Korean inventor who
after reading Invention by Design in translation sent
me copies of an article and a standard about push-button telephones that led me to the literature on human
factors cited here. He also sent me a prototype of a telephone with a modified keypad, with the 0 key on the
top, where he believes it is more conveniently located to
input international access codes and Korean cellular
and beeper codes that frequently begin with 01. He has
applied for a patent for his new keyset arrangement.
Bibliography
Conrad, R., and A. J. Hull. 1968. The preferred layout for
numerical data-entry keysets. Ergonomics 11:165–173.
Dallaire, Gene. 1975. Pocket electronic calculators zoom.
Civil Engineering, February, pp. 39–43.
Deininger, R. L. 1960. Desirable push-button characteristics. IRE Transactions on Human Factors in Electronics,
May, pp. 24–30.
Deininger, R. L. 1960. Human factors engineering studies
of the design and use of pushbutton telephone sets.
Bell System Technical Journal 39:995–1012.
Lutz, Mary Champion, and Alphonse Chapanis. 1955.
Expected locations of digits and letters on ten-button
keysets. Journal of Applied Psychology 39:314–317.
Petroski, Henry. 1996. Invention by Design: How Engineers
Get from Thought to Thing. Cambridge, Mass.: Harvard
University Press.
Wikell, Goran. 1982. The layout of digits on pushbutton
telephones. Tele no. 1:34–45.
© 2000 Sigma Xi, The Scientific Research Society. Reproduction
with permission only. Contact [email protected].