Digital versus conventional clocks a review

Hilary J. Sinclair

IPC Science and Technology Press Ltd

This article reviews two scientific papers by van Nes and Zeff, describing comparisons between digital and analogue (conventional) time displays. The first part deals with an investigation of speed and accuracy in reading small time differences. It was found that a small t ime difference can be determined more quickly and precisely from simultane- ously presented displays, if the displays are digital.

The second part describes an experiment devised to compare the two modes of t ime display wi th respect to the speed and accuracy of reading and logging. It was shown that it takes 3½ to 4 times longer to read (and 10 times more errors are incurred) from a conventional (analogue) time display than from a digital one.

Part 1. Reading time displays for mental calculation

Usually when a person consults his watch it is to deter- mine the difference between the perceived time and some point of time in the past or future. For example, when wishing to catch a train one needs to know the amount of time available between the actual time and the time of the train's departure. The information regarding the arrival and departure of trains, aeroplanes, ships and so on is usually given in digital form whereas most time pieces are analogue. This discrepancy prompted an investigation (van Nes) into the influence of the type of clock display on the speed and accuracy of determining small time differences.

Method

Three different series of time presentations were used: the first series (Fig 1) consisted of eight analogue time-display pairs; the second series (Fig 2) was made up of eight digital time-display pairs; and the third series (Fig 3) consisted of eight analogue-digital pairs. Each series constituted one test. The second and third series each contained four a.m. pairs and four p.m. pairs. The analogue displays contained one numeral only (at noon) as an orientation mark. Apparently the reason for omitting all the other numerals was that many

modern watches and clocks possess only a few numerals or none at all. Grether (1948), however, has shown that time pieces with twelve numerals are read faster and more accur- ately than those time pieces with numerals only at the 3, 6, 9 and 12 hourly positions.

Two groups of ten subjects were used in the experiment. The first group consisted of men from a laboratory popula- tion possessing college or university backgrounds. This group received no training in the experimental task, which involved reporting verbally for all pairs of time displays the time difference between the right hand display and the left hand one. The subjects' responses were timed and their errors recorded.

The second group of subjects contained five men and five women. These were allowed to accustom themselves to the experimental task, and were given two series of time displays similar to those in Figs 1 and 2. Any errors which were made were immediately conveyed to the subject who was allowed to correct his answer..

Results

Data obtained from the experiment, as shown in Table l , yield the following results for both groups of subjects:

1. Subtractions made whilst using the two digital displays, were carried out, on average, twice as fast as those made whilst using the two analogue displays.

178 Applied Ergonomics September 1971

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Fig 1 Analogue t ime-display pairs. Fig 2 Digital t ime-display pairs. Fig 3 Analogue-digital pairs.

Appl ied Ergonomics September 1971 179

Table 1 Results obtained from groups 1 and 2

Group 1

Time in seconds % errors

Type of time Ave- Ave- presentation Min rage Max Min rage Max

Analogue- an~ilogue 43 82 133 0 20 37.5

Digital- digital 15 46 75 0 10 37.5

Analogue- digital 29 79 126 0 12.5 50

Group 2

Time in seconds % errors

Type of time Ave- Ave- presentation Min rage M ax Min rage Max

Analogue- analogue 75 118 163 0 15 25

Digital- digital 43 51 62 0 5 25

Analogue- digital 77 115 164 0 22.5 37.5

2. There were 2 to 3 times as many errors for analogue- analogue subtractions, as compared with the digital-

- digital subtractions. 3. There was no significant difference between the

speeds for the compatible analogue-analogue subtrac- tion and the incompatible analogue-digital one.

4. No significant correlation was found, from either group, between speed and error rate of the individuals for any of the three series.

Error classification

Although the number of displays used was rather limited to provide a representative collection of errors, an attempt was made to analyse the subjects' errors. Apart from the errors recorded in the table, errors made by group 2 during the training session were also taken into account; and the errors made by five subjects whose results were not included in Table 1. Some subjects 'thought aloud' during the experiment a fact which helped to classify their errors.

The errors were tentatively classified as follows:- 1. Perceptual errors: 20 minutes for Fig 1, no 1. 2. Operational errors: those errors made in an opera-

tional stage of the subtraction, eg 12 minutes for Fig 1, no 5.

3. Mental arithmetic errors, (such errors are not included in the previous category). An example of this type of error is: 17 minutes for Fig 2, no 8.

4. Conversion errors, as: 27 minutes for Fig 3, no 5. 5. Concept errors, eg 96 minutes for Fig 2, no 1. This

particular error appears to indicate that under the

180 Applied Ergonomics September 1971

stress of the experiment- which may have its parallel in the hectic pace of everyday life- subjects can reckon an hour to comprise 100 minutes.

Discussion

These data show that if one has to determine a relatively small time difference quickly and accurately from simultan- eously presented displays, those displays should preferably be digital. It should be emphasised that this experiment does not test the situation of comparing the time 'now' on a watch with the remembered time for some event (eg train departure). However, since the remembered time, if a pre- cise one, may be verbalised in digital form (eg 'ten fifty four'),it is thought the results may perhaps be relevant to that situation also.

However, digital time displays have their disadvantages. For example, on many occasions, one does not observe a clock accurately but casually, so as to obtain a rough esti- mate of the time. For this usage, it may be that an analogue display is better. In order to facilitate both precise and rough estimates of time an ideal solution would seem to be to combine both analogue and digital display. For example, van Nes has suggested that a 'normal' watch might have a small digital time display as an insert, comparable to the date display on existing watches.

Part 2. Reading time displays for logging With the advent of automation, time recording is becom-

ing increasingly important, and logging is no longer reserved solely for navigators, air traffic controllers, and railway signalmen. It would seem that a digital timepiece might be preferable to an analogue one in this respect. However, it was not until 1965 that experimental research was carried out comparing the speed and accuracy of reading the time from conventional analogue docks, and digital clocks, for both 12-hour and 24-hour displays. Work had been done by Grether (1948) on comparing different formats of 24-hour time-pieces with particular reference to 12-hour and 24-hour division of the face.

Time-display design

In his study Zeff (1965) found that it was fairly easy to devise a suitable digital clock design so that the numerals, contrast, and proportions of the clock gave adequate legi- bility (Baker and Grether, 1954; Handbook, 1970).

The analogue clock design was assumed to be such that the numerals were changed completely from 1-12 to 13-24 at 30 seconds after 12 midday and back again at 30 seconds after midnight. It was hypothesised that such a clock would incur the minimum of error. The design details were in accordance with recommended instrument practice (British Standards Institution 1961, Maddock 1961 ; Handbook, 1970).

Method

The analogue and digital time-display designs were drawn, photographed and displayed to 20 subjects by projection on the screen of a modified ITM Grundytutor, which was connected to a response-time mechanism (see Fig 4).

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Fig 4 Examples of analogue and digital time-displays. The clocks in each pair show the same time for comparison.

Both types of time-display were shown on the teaching machine, each type appearing 40 times. Thus from 20 subjects there was a total of 1600 readings. The two differ- ent types of clock were not shown alternately, thus reducing the chance of the subjects becoming 'disoriented.'

When the time- display was presented, the subject was required to write down the time on a response form. As soon as the pen was used, it triggered a switch connected to an electronic timing device (Welford 1952). Thus the length of time between the appearance of the clock and the act of recording could be measured. As the display disappeared when the subject began to write, he was obliged to press a button to move onto the next display.

Table 2 Mean response times in seconds

Digital Conventional

0- 12 hours 0.93 3.37

13 - 24 hours 0.95 3.71

Mean 0.94 3.54

Table 3 Distribution of errors

Digital Conventional

Errors 0- 12 hours 2 29

Errors 13- 24 hours 2 21

Table 4 Percentage errors of a given magnitude

I

Digital Conventional

% errors V= hour or more 0.25 1.5

% errors 1 houror more 0.25 1.25

% errors 12 hours or more 0 0.25

Total error % 0.50 6.25

Results

The mean response time for all subjects is given in Table 2. It appears that the mean response time for the first 12 hours was not significantly different trom the second 12 hours for either type of time display.

Table 3 shows the distribution of errors. In the 800 trials with the analogue display there were 50 errors, and in the 800 trials with the digital display 4 errors. These 4 errors comprised 3 figure misreadings and 1 figure omission. Table 4 shows the percentages of the total readings, of the various degrees of error.

Discussion

From these results it can be said that it takes 3½ to 4 times longer to write down the time from a conventional analogue clock than t'rom a digital one. Also, the errors produced on reading and logging from a conventional clock are more than 10 times those produced from a digital clock.

It would seem from the results that where accuracy is critical (eg to air traffic controllers and the like), digital clocks are strongly to be recommended. The use of a digital clock would probably reduce errors and reading time in many practical situations to the extent that the relatively higher cost, at present, of such clocks would still be well justified.

References

Baker, C.A. and Grether, W.F. 1954 WADC Tech. Rept., 54-160. Visual presentation of

information. British Standards Institution

BS 3693. The graduation of industrial instruments for quantitative measurements.

Grether, W.F, 1948 J. appl. Psychol., 32, 159-169. Factors in the design

of clock dials which effect speed and accuracy of reading in the 2400 hour time system.

Handbook 1970 Applied Ergonomics, 1.2, 86-94. Chapter 3. Displays.

M a d d o c k , A.J. 1961 Brit. J. appl. Phys., 12, 33-43. Design of scales for

industrial instruments. van Nes F.L. 1971 Ergonomics, to be published. Welford, N. 1952 J. sci. lnstr., 29, 1-4. An electronic digital recording

machine - the 'SETAR'.

Zeff, C. 1965 Ergonomics, 8.3,339-345. Comparison of convention-

al and digital time displays.

Applied Ergonomics September 1971 181