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Speed-accuracy trade-off and time of day
Acta Psychologica North-Holland
SPEED-ACCURACY
TRADE-OFF
Angus CRAIG and Ruth CONDON UWWS//J($ Su.ssrs.UK Accepted
115
58 (1985) 115-122
AND TIME OF DAY *
April 1984
Data were collected at six times of day from a total of 48 subjects working at a self-paced version of a visual. signal identification task. Speed of inspectton and confidence of report both tended to increase over the day, while signal detectability (d’) declined. The response criterion (In 8) remained invariant with time of day. To determine whether variations in speed were being offset by those in accuracy, the efficiency indices d’/T and d’/fi were derived. Analyses showed that efficiency was stable over test times: 8:00, 11:OO. 77:OO. 20:O0. indicating the operation of a speed-accuracy trade-off: but efficiency dropped significantly at 14:00 (possibly a post-lunch dip) and again at 23:O0. at the end of the day. The shift in the trading relation between speed and accuracy was in the direction of faster but less careful performance as the day progressed; the possibility that this is due to accumulated fatigue rather than to time of day per se, is discussed.
Introduction As a result of the recent impetus given to research on circadian rhythms in human performance (Colquhoun 1971) it is now widely recognized that the level of functioning on various kinds of task may fluctuate during the course of the day. In many instances it is thought that these performance fluctuations reflect diurnal changes in arousal, which is held to be low in the early morning, and to increase over the day, reaching a peak in the evening (Colquhoun 1971, 1982). However, in other instances performance does not follow the presumed course of arousal; instead it seems to be influenced by motivational factors, such as the degree of effort the subject is willing to put into his performance, in conjunction with the task’s sensitivity to the effects of such com-
* Mailing address: A. Craig. MRC Perceptual and Cognitive Performance Unit, Experimental Psychology, University of Sussex. Falmer, Brighton BNl 9QG, UK.
OOOl-6918/85/$3.30
0 1985, Elsevier Science Publishers
B.V. (North-Holland)
Laboratory
of
pensatory effort (Colyuhoun 1971, 1981, 1982: Hockey and Colquhoun 1972). In a recent extension of this notion, Monk and Leng (1982) provided tentative support for the argument that a time of day effect on performance might solely reflect a modification of the subject’s approach to the task in hand. and not involve any improvement or deterioration in his actual efficiency. The present study focusses on strategic changes in a signal detection task, and pursues the suggestion of Hockey and Colquhoun (1972) that the adoption of a signal detection theory methodology might enable one to distinguish between the effects of time-of-day on motivational state and on “ true” efficiency. Signal detectability, as indexed by the statistic d’, has previously been found to be relatively impervious to changes with time of day. Using a visual discrimination task, Craig (1979) had demonstrated the invariance of d’ between a test administered at 8:00 and another at 20:00 h. The same task was subsequently used by Craig et al. (1981a) in test times at 11:OO and 13:00 h. in a study of the post-lunch dip in discrimination efficiency. Apart from a 10 percent reduction in efficiency consequent upon the ingestion of food, the detectability levels obtained at 11:OO h, and at 13:OO h when no food was consumed. did not differ from those previously found by Craig (1979). Craig et al. (1981 b) examined five sets of vigilance data for the occurrence of diurnal variation in auditory signal detectability, and found only a single example showing a marginal improvement in d’ over the day; and in that instance performance had been tested sequentially over the day, so that there was a possibility that the increment in d’ was simply a practice effect. More recently, Kerkhof (1982) has reported evidence of a slight improvement over the day in auditory signal detectability, when practice effects were balanced out (although a statistically significant improvement was limited to those subjects classified as “Eveningtypes”, on the basis of the Horne and Ostberg 1977 questionnaire). The general conclusion to be drawn from these studies is that signal detectability is likely to remain invariant with time of day (with the exception of a post-lunch dip). but may on occasion show some modest improvement. However, all the studies mentioned involved tasks in which presentation was paced by the experimenter at a moderate rate. being nowhere in excess of thirty events per minute (cf. Davies and Parasuraman 1981). In these circumstances, a considerable shift across
the range of attitudes (between conservatism and risk) to the task can be tolerated without being evident in the obtained measures of performance, particularly if latencies are not measured. Some indications of shifting attitudes do, however, exist. With the vigilance tasks in which signal probability was characteristically low, so that a non-central criterion (lnp # 0) was adopted, Craig et al. (1981b), found a general increase over the day in hits and false alarms, indicative of a criterion shift towards the risky end of the continuum (i.e. /? declined over the day). Subjects behaved as though they were requiring progressively less sensory evidence before reporting the presence of a signal. Craig (1979) and Kerkhof (1982) both used equiprobable signal and non-signal events, conducive to the employment of a central criterion setting (In /I = 0), and hence likely to be rather insensitive to modest changes in attitude. Nevertheless, although the intermediate criterion for reporting “signal” or “non-signal” did not shift, Craig (1979) whose subjects could make their reports at either high or low levels of certainty, did observe a slight shift in the confidence with which these reports were made, greater confidence being reported in the evening than in the morning; in the absence of a detectability change, this shift might be taken to imply that confident judgments were being made on the basis of less evidence. The foregoing provides reasonable grounds for speculating that operating position along a speed-accuracy trade-off function varies with time of day. The speculation is pursued here by the obvious technique of employing a self-paced task.
Method This particular task was administered as one of a battery of six perceptual-motor tasks, approximately equivalent in demand, presented in a balanced order within each one-hour session, in a more comprehensive time of day study that is to be reported at a later date. Tusk muteriul A self-paced version of the task used by Craig (1979) was developed. The required discrimination was between two black discs, one of which was 10 percent larger in area than the other. The discs (diameters = 10, 10.5 mm) were printed on white cards (141 x 89 mm), one disc only appearing on each card. Packs of 50 cards were prepared;
these contained an average of 10 (range: 7- 13) discs of one size. 40 of the other. C’ards were numbered. as were corresponding ro-s in a response sheet where Ss recorded their judgments on a four-category scale (“certain big”. “big”. “small”. “certain small”) like the one used by Craig (1979). Each S was provided with a stop-watch. and recorded the time to complete the set of 50 judgments. under instructions to work quickly hut without sacrificing accuracy. Practice packs contained t&o appropriately labelled demonstration cards. a set of 10 cards (5 big and 5 small) to he identified, with immediate feedback available from the reverse side of each card. and a further set of 20 cards which were drawn from the same population as the test packs, and for which post-block feedback was provided. These practice packs were worked through at the start of each session.
A total of 48 students. 14 male, 34 female. median age 19 years (interquartile range = 2 years). took part in the study. being paid a small honorarium for volunteering their services. All were normally resident on the university campus, close to the laboratory. They were randomly assigned to six groups of 8 Ss each.
Performance was sampled six times over the day, at three-hourly intervals: 8:O0. 11 :OO. 14:OO. 17:O0. 20:O0. 23:O0. Each group of Ss attended once a day. for six days, at a different time on each day. Test order was balanced over days using a design credited to Williams (1949) (see also Cochran and Cox 1957: 133 et sequ.) and based on the latin square. whereby each test time was preceded once by each of the remaining times. For completeness. in assessing any asymmetric treatment residuals - residual effects of prior treatments on performance under current treatments - that could prove so critical in time of day research. Ss returned for a seventh test day, at the same time as on their sixth day. Thus each test time was preceded by each time. including itself. thereby ensuring that direct (treatment) and residual effects are orthogonal. This design provides an important check on the critical assumption that current performance at any time is not differentially affected by the time at which the previous test was given. Paradoxically. this critical assumption has never been tested in any previous time to day study. For half the Ss in each group, the larger disc was the more frequently occurring. for the others the smaller disc occurred more frequently: and whichever an S experienced at the start. on his first practice session, he experienced throughout the study. All testing was conducted in a sound attenuated (40 dB) temperature controlled (22.7 i 1,O”C; 57.5 f 5.5 percent relative humidity) laboratory. with experimenter present on all occasions.
For the analyses, the less frequently occurring disc was taken as the “signal”. and. disregarding their certainty, correct affirmative reports were defined as “hits”. incorrect
119
‘\
‘0
1.0
CONFIDENCE
I,, 0800
1100 TIME
2.6
F
i
_ _c,-
,
,
1400
1700
OF
(
2000
,_7
z
2300
DAY
Fig. 1. Variation in performance as a function of time of testing
ones as “false alarms”. Estimates of d’ and /3 were then obtained from the scored hits and false alarms at this intermediate criterion setting. In addition, following the example of Craig (1979), an index of confidence was obtained by computing the proportional usage of the “certain” report categories (negative reports as well as affirmative ones). A fourth measure, inspection time - the time taken to work through the set of cards, inspecting each disc and recording each judgment - was also recorded. These four separate indexes - d’, P(analyzed as In /3). report confidence and inspection time - were subject to analysis of variance, appropriate to the latin square design. (It may be noted at the outset, and no further reference will be made on this point, that no evidence was found, for any measure, of a difference due to the particular disc, big or small, designated as “signal”.) The initial analysis (of variance) of these data from days 2-7 of testing, indicated an absence of differential residual effects due to time of day (p > 0.20. all cases); in other words, performance during a test session is not influenced by the time at which the preceding test was given. This provides a rational basis for confidence that any time of day effects for the core test days, 1-6, are not artefacts. The ANOVA results are summarized in table 1, and the findings are illustrated in fig. 1. As is evident, signal detectability declined over the day while inspection time
1
Table
Results of ANOVA.
on performance
Effect
meawreh:
F-ratios (c// = 5. 230) and significance
level\
MClQlre
due to
Time taken
d’
In P
2.49 h
0.73 L
1.97j’
2.94 ’
4.30 I’
14.X6 ”
4: (“certain”
report)
(set) Time of day Day.\ “
(=
practice)
5.21 I’ 43.16 a
p < 0.001
h p < 0.05
’ non significant p i 0.10 c p i 0.025 d
improved. and report confidence tended to increase; the criterion measure. Ing, remained invariant with time of day. It is interesting to note that, judging by the pattern of results shown in table 1 and fig. 1. the processes determining the placement of the intermediate criterion Infi (i.e. the division between reporting an item as “big” or as “small”) appear to be independent of those involved in deciding to use the “certain” report category. and also of those involved in determining inspection time. It seems clear that the results depicted in fig. 1 reflect variations along a speed-accuracy trade-off function. with speed and confidence increasing together at the expense of a detectability decrement: although it seems equally clear that there are genuine efficiency deficits at 14:OO. and again at 23:O0. Detectability has been variously described as a direct function of inspection time. or of the square root of time (Vickers 1979). As can be seen in table 2. it made little material difference which of these relations was accepted. The time of day functions for are remarkably similar, and ANOVA’s coupled with the indices d’/T and d’/$? post-hoc comparisons conducted on these derived efficiency measures yielded identical conclusions: levels do not differ between 8:O0. 11:OO. 17:00, 20:00, but are substantially lower at 14:00 and at 23:00 ( p < 0.025, Duncan’s multiple range test). These findings agree with the previous evidence obtained with paced tasks. including the occurrence of the post-lunch dip. The only exceptional finding seems to be the efficiency drop that occurs at the end of the day, and which has not been demonstrated previously.
Table 2 Variation
in detectability
per unit inspection time, as a function of time of testing.
Time 8:OO Efficiency
of test 11:oo
14:oo
17:oo
20:oo
23:00
measure
d’/T
0.0205
0.0208
0.0183
0.0212
0.0219
0.0195
d’/JT
0.2394
0.2368
0.2181
0.2419
0.2417
0.2231
Conclusions This study confirms the value of adopting a signal detection theory methodology in pursuing time-of-day research, as had been suggested by Hockey and Colquhoun (1972). The data reported provide fairly conclusive evidence that the trade-off between inspection time and signal detectability is influenced by time of day. The results therefore support and extend the notions of Monk and Leng (1982) that time of day effects on performance might reflect changes in attitude or approach to the task in hand; and by confirming this view, the present study implies that approaches which regard performance as varying with time of day along a simple continuum of efficiency are inappropriate. The question of interest is whether operating location on the trade-off function is itself a reliable function of time of day. The present results and those of Monk and Leng (1982) are in accord in that for quite different tasks, both show a similar trend over the day from an attitude of conservatism towards one of risk. But such a shift could easily derive from accumulated fatigue and a decline in readiness to work at a task (Vidacek 1982); so it will be important to distinguish between effects due to time of day, and those due to time awake. The post-lunch dip that was in evidence, corroborating the earlier finding by Craig et al. (1981a) with the paced task, also seems worth pursuing in future research, as does the late night drop in efficiency found here. The obvious question in both instances is whether the effect is due to time of day per se, or merely to specific circumstances that happen to coincide with these times.
References Cochran, W.G. and G.M. Cox, 1957. Experimental designs (2nd cd.). New York: Wiley. Colquhoun, W.P.. 1971. ‘Circadian variations in mental efficiency’. In: W.P. Colquhoun (ed.). Biological rhythms and human performance. London: Academic Press. Colquhoun, W.P.. 1981. ‘Rhythms in performance’. In: J. Aschoff (cd.). Handbook of behavioral neurobiology, Vol. 4. New York: Plenum. pp. 333-348. Colquhoun, W.P.. 1982. ‘Biological rhythms and performance’. In: W.B. Webb (ed.). Biological rhythms, sleep, and performance. Chichester: Wiley. pp. 59986. Craig. A., 1979. Discrimination. temperature and time of day. Human Factors 21. 61-68. Craig, A.. K. Baer and A. Diekmann. 19Xla. The effects of lunch on sensory-perceptual functioning in man. International Archives of Occupational and Environmental Health 49, 105-114. Craig, A.. R.T. Wilkinson and W.P. Colquhoun, 1981b. Diurnal variation in vigilance efficiency. Ergonomics 24, 641-651.
Davies.
D.K.
Hockey.
and K. Parasuramnn.
G.R.J.
In: W.P. London: Home.
19X1. The psychology of vigilance.
and W.P. Colquhoun.
Colquhoun
(cd.).
Aspects of human
English Universities
J. and 0.
1972. ‘Diurnal
variation
effxiency:
London:
Academic
in human performance: diurnal
rhythm
Press.
a rcviw’.
and loss of \lcep.
Press.
OAtberg. 1977.
Individual
differences
in human circadian
rhythms.
Biologlcnl
Psychology 5. 179- 180. Kerkhof,
G.A..
1982. Event-related
for morning-type Monk,
T.H.
potentials and auditory signal detection:
and evening-type
and V.C.
subjects. Psychophysiology
their diurnal wrlation
19. 94-103.
Leng. 1982. Time of day effects in simple repetitive
tahks: some posible
mechanisms. Acta Psychologica 51. 207-221. Vicktrrh. D.. 1979. Decision processes in visual perception. Vidacek. NIOSH Williams.
S.. 1982.
Health
and safety implications
Foreign Research Agreement E.J..
treatments.
1949.
Experimental
Australian
London:
of diurnal
Academic
variability
Press.
In tolerance
to stress.
No. 02-016-C.
designs balanced
Journal of Scientifx
for the estimation
Research AZ. 149%16X.
of residual
effects
01
SPEED-ACCURACY
TRADE-OFF
Angus CRAIG and Ruth CONDON UWWS//J($ Su.ssrs.UK Accepted
115
58 (1985) 115-122
AND TIME OF DAY *
April 1984
Data were collected at six times of day from a total of 48 subjects working at a self-paced version of a visual. signal identification task. Speed of inspectton and confidence of report both tended to increase over the day, while signal detectability (d’) declined. The response criterion (In 8) remained invariant with time of day. To determine whether variations in speed were being offset by those in accuracy, the efficiency indices d’/T and d’/fi were derived. Analyses showed that efficiency was stable over test times: 8:00, 11:OO. 77:OO. 20:O0. indicating the operation of a speed-accuracy trade-off: but efficiency dropped significantly at 14:00 (possibly a post-lunch dip) and again at 23:O0. at the end of the day. The shift in the trading relation between speed and accuracy was in the direction of faster but less careful performance as the day progressed; the possibility that this is due to accumulated fatigue rather than to time of day per se, is discussed.
Introduction As a result of the recent impetus given to research on circadian rhythms in human performance (Colquhoun 1971) it is now widely recognized that the level of functioning on various kinds of task may fluctuate during the course of the day. In many instances it is thought that these performance fluctuations reflect diurnal changes in arousal, which is held to be low in the early morning, and to increase over the day, reaching a peak in the evening (Colquhoun 1971, 1982). However, in other instances performance does not follow the presumed course of arousal; instead it seems to be influenced by motivational factors, such as the degree of effort the subject is willing to put into his performance, in conjunction with the task’s sensitivity to the effects of such com-
* Mailing address: A. Craig. MRC Perceptual and Cognitive Performance Unit, Experimental Psychology, University of Sussex. Falmer, Brighton BNl 9QG, UK.
OOOl-6918/85/$3.30
0 1985, Elsevier Science Publishers
B.V. (North-Holland)
Laboratory
of
pensatory effort (Colyuhoun 1971, 1981, 1982: Hockey and Colquhoun 1972). In a recent extension of this notion, Monk and Leng (1982) provided tentative support for the argument that a time of day effect on performance might solely reflect a modification of the subject’s approach to the task in hand. and not involve any improvement or deterioration in his actual efficiency. The present study focusses on strategic changes in a signal detection task, and pursues the suggestion of Hockey and Colquhoun (1972) that the adoption of a signal detection theory methodology might enable one to distinguish between the effects of time-of-day on motivational state and on “ true” efficiency. Signal detectability, as indexed by the statistic d’, has previously been found to be relatively impervious to changes with time of day. Using a visual discrimination task, Craig (1979) had demonstrated the invariance of d’ between a test administered at 8:00 and another at 20:00 h. The same task was subsequently used by Craig et al. (1981a) in test times at 11:OO and 13:00 h. in a study of the post-lunch dip in discrimination efficiency. Apart from a 10 percent reduction in efficiency consequent upon the ingestion of food, the detectability levels obtained at 11:OO h, and at 13:OO h when no food was consumed. did not differ from those previously found by Craig (1979). Craig et al. (1981 b) examined five sets of vigilance data for the occurrence of diurnal variation in auditory signal detectability, and found only a single example showing a marginal improvement in d’ over the day; and in that instance performance had been tested sequentially over the day, so that there was a possibility that the increment in d’ was simply a practice effect. More recently, Kerkhof (1982) has reported evidence of a slight improvement over the day in auditory signal detectability, when practice effects were balanced out (although a statistically significant improvement was limited to those subjects classified as “Eveningtypes”, on the basis of the Horne and Ostberg 1977 questionnaire). The general conclusion to be drawn from these studies is that signal detectability is likely to remain invariant with time of day (with the exception of a post-lunch dip). but may on occasion show some modest improvement. However, all the studies mentioned involved tasks in which presentation was paced by the experimenter at a moderate rate. being nowhere in excess of thirty events per minute (cf. Davies and Parasuraman 1981). In these circumstances, a considerable shift across
the range of attitudes (between conservatism and risk) to the task can be tolerated without being evident in the obtained measures of performance, particularly if latencies are not measured. Some indications of shifting attitudes do, however, exist. With the vigilance tasks in which signal probability was characteristically low, so that a non-central criterion (lnp # 0) was adopted, Craig et al. (1981b), found a general increase over the day in hits and false alarms, indicative of a criterion shift towards the risky end of the continuum (i.e. /? declined over the day). Subjects behaved as though they were requiring progressively less sensory evidence before reporting the presence of a signal. Craig (1979) and Kerkhof (1982) both used equiprobable signal and non-signal events, conducive to the employment of a central criterion setting (In /I = 0), and hence likely to be rather insensitive to modest changes in attitude. Nevertheless, although the intermediate criterion for reporting “signal” or “non-signal” did not shift, Craig (1979) whose subjects could make their reports at either high or low levels of certainty, did observe a slight shift in the confidence with which these reports were made, greater confidence being reported in the evening than in the morning; in the absence of a detectability change, this shift might be taken to imply that confident judgments were being made on the basis of less evidence. The foregoing provides reasonable grounds for speculating that operating position along a speed-accuracy trade-off function varies with time of day. The speculation is pursued here by the obvious technique of employing a self-paced task.
Method This particular task was administered as one of a battery of six perceptual-motor tasks, approximately equivalent in demand, presented in a balanced order within each one-hour session, in a more comprehensive time of day study that is to be reported at a later date. Tusk muteriul A self-paced version of the task used by Craig (1979) was developed. The required discrimination was between two black discs, one of which was 10 percent larger in area than the other. The discs (diameters = 10, 10.5 mm) were printed on white cards (141 x 89 mm), one disc only appearing on each card. Packs of 50 cards were prepared;
these contained an average of 10 (range: 7- 13) discs of one size. 40 of the other. C’ards were numbered. as were corresponding ro-s in a response sheet where Ss recorded their judgments on a four-category scale (“certain big”. “big”. “small”. “certain small”) like the one used by Craig (1979). Each S was provided with a stop-watch. and recorded the time to complete the set of 50 judgments. under instructions to work quickly hut without sacrificing accuracy. Practice packs contained t&o appropriately labelled demonstration cards. a set of 10 cards (5 big and 5 small) to he identified, with immediate feedback available from the reverse side of each card. and a further set of 20 cards which were drawn from the same population as the test packs, and for which post-block feedback was provided. These practice packs were worked through at the start of each session.
A total of 48 students. 14 male, 34 female. median age 19 years (interquartile range = 2 years). took part in the study. being paid a small honorarium for volunteering their services. All were normally resident on the university campus, close to the laboratory. They were randomly assigned to six groups of 8 Ss each.
Performance was sampled six times over the day, at three-hourly intervals: 8:O0. 11 :OO. 14:OO. 17:O0. 20:O0. 23:O0. Each group of Ss attended once a day. for six days, at a different time on each day. Test order was balanced over days using a design credited to Williams (1949) (see also Cochran and Cox 1957: 133 et sequ.) and based on the latin square. whereby each test time was preceded once by each of the remaining times. For completeness. in assessing any asymmetric treatment residuals - residual effects of prior treatments on performance under current treatments - that could prove so critical in time of day research. Ss returned for a seventh test day, at the same time as on their sixth day. Thus each test time was preceded by each time. including itself. thereby ensuring that direct (treatment) and residual effects are orthogonal. This design provides an important check on the critical assumption that current performance at any time is not differentially affected by the time at which the previous test was given. Paradoxically. this critical assumption has never been tested in any previous time to day study. For half the Ss in each group, the larger disc was the more frequently occurring. for the others the smaller disc occurred more frequently: and whichever an S experienced at the start. on his first practice session, he experienced throughout the study. All testing was conducted in a sound attenuated (40 dB) temperature controlled (22.7 i 1,O”C; 57.5 f 5.5 percent relative humidity) laboratory. with experimenter present on all occasions.
For the analyses, the less frequently occurring disc was taken as the “signal”. and. disregarding their certainty, correct affirmative reports were defined as “hits”. incorrect
119
‘\
‘0
1.0
CONFIDENCE
I,, 0800
1100 TIME
2.6
F
i
_ _c,-
,
,
1400
1700
OF
(
2000
,_7
z
2300
DAY
Fig. 1. Variation in performance as a function of time of testing
ones as “false alarms”. Estimates of d’ and /3 were then obtained from the scored hits and false alarms at this intermediate criterion setting. In addition, following the example of Craig (1979), an index of confidence was obtained by computing the proportional usage of the “certain” report categories (negative reports as well as affirmative ones). A fourth measure, inspection time - the time taken to work through the set of cards, inspecting each disc and recording each judgment - was also recorded. These four separate indexes - d’, P(analyzed as In /3). report confidence and inspection time - were subject to analysis of variance, appropriate to the latin square design. (It may be noted at the outset, and no further reference will be made on this point, that no evidence was found, for any measure, of a difference due to the particular disc, big or small, designated as “signal”.) The initial analysis (of variance) of these data from days 2-7 of testing, indicated an absence of differential residual effects due to time of day (p > 0.20. all cases); in other words, performance during a test session is not influenced by the time at which the preceding test was given. This provides a rational basis for confidence that any time of day effects for the core test days, 1-6, are not artefacts. The ANOVA results are summarized in table 1, and the findings are illustrated in fig. 1. As is evident, signal detectability declined over the day while inspection time
1
Table
Results of ANOVA.
on performance
Effect
meawreh:
F-ratios (c// = 5. 230) and significance
level\
MClQlre
due to
Time taken
d’
In P
2.49 h
0.73 L
1.97j’
2.94 ’
4.30 I’
14.X6 ”
4: (“certain”
report)
(set) Time of day Day.\ “
(=
practice)
5.21 I’ 43.16 a
p < 0.001
h p < 0.05
’ non significant p i 0.10 c p i 0.025 d
improved. and report confidence tended to increase; the criterion measure. Ing, remained invariant with time of day. It is interesting to note that, judging by the pattern of results shown in table 1 and fig. 1. the processes determining the placement of the intermediate criterion Infi (i.e. the division between reporting an item as “big” or as “small”) appear to be independent of those involved in deciding to use the “certain” report category. and also of those involved in determining inspection time. It seems clear that the results depicted in fig. 1 reflect variations along a speed-accuracy trade-off function. with speed and confidence increasing together at the expense of a detectability decrement: although it seems equally clear that there are genuine efficiency deficits at 14:OO. and again at 23:O0. Detectability has been variously described as a direct function of inspection time. or of the square root of time (Vickers 1979). As can be seen in table 2. it made little material difference which of these relations was accepted. The time of day functions for are remarkably similar, and ANOVA’s coupled with the indices d’/T and d’/$? post-hoc comparisons conducted on these derived efficiency measures yielded identical conclusions: levels do not differ between 8:O0. 11:OO. 17:00, 20:00, but are substantially lower at 14:00 and at 23:00 ( p < 0.025, Duncan’s multiple range test). These findings agree with the previous evidence obtained with paced tasks. including the occurrence of the post-lunch dip. The only exceptional finding seems to be the efficiency drop that occurs at the end of the day, and which has not been demonstrated previously.
Table 2 Variation
in detectability
per unit inspection time, as a function of time of testing.
Time 8:OO Efficiency
of test 11:oo
14:oo
17:oo
20:oo
23:00
measure
d’/T
0.0205
0.0208
0.0183
0.0212
0.0219
0.0195
d’/JT
0.2394
0.2368
0.2181
0.2419
0.2417
0.2231
Conclusions This study confirms the value of adopting a signal detection theory methodology in pursuing time-of-day research, as had been suggested by Hockey and Colquhoun (1972). The data reported provide fairly conclusive evidence that the trade-off between inspection time and signal detectability is influenced by time of day. The results therefore support and extend the notions of Monk and Leng (1982) that time of day effects on performance might reflect changes in attitude or approach to the task in hand; and by confirming this view, the present study implies that approaches which regard performance as varying with time of day along a simple continuum of efficiency are inappropriate. The question of interest is whether operating location on the trade-off function is itself a reliable function of time of day. The present results and those of Monk and Leng (1982) are in accord in that for quite different tasks, both show a similar trend over the day from an attitude of conservatism towards one of risk. But such a shift could easily derive from accumulated fatigue and a decline in readiness to work at a task (Vidacek 1982); so it will be important to distinguish between effects due to time of day, and those due to time awake. The post-lunch dip that was in evidence, corroborating the earlier finding by Craig et al. (1981a) with the paced task, also seems worth pursuing in future research, as does the late night drop in efficiency found here. The obvious question in both instances is whether the effect is due to time of day per se, or merely to specific circumstances that happen to coincide with these times.
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