J R Soc Med 2003;96:497-498
doi:10.1258/jrsm.96.10.497
© 2003 Royal Society of Medicine
The accuracy of guestimates
Iain McLean MPhil
C Mary Anderson BM BCh 1
Cath White MB ChB 1
Academic Unit of Obstetrics and Gynaecology, University of
Manchester
1
St Mary's Sexual Assault Referral Centre, St Mary's Hospital, Manchester,
UK
Correspondence to: Iain McLean, Research Assistant, St Mary's Sexual Assault
Referral Centre, St Mary's Hospital, Whitworth Park, Manchester M13 0JH, UK
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SUMMARY
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At times a clinician must describe the size of a lesion when
measurement
presents difficulties. A ‘guestimate’
may then be recorded. We
tested the accuracy of forensic clinicians
(10 physicians and 1 nurse) in
guestimating the sizes of ten
test objects, comparing their performance with
13 individuals
from other professions. The participants were permitted to
handle
the objects but not to use a measuring device.
For three of the objects (a balloon, a scratch and a coin) the entire
sample of participants significantly overestimated size, by 13-22%. Both
participant groups overestimated the size of a scratch, the clinicians being
more accurate but not significantly so.
Guestimates should be avoided unless the use of a ruler or tape measure
will be against the interests of the client.
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INTRODUCTION
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Forensic clinicians, rather than use a scale or rule, sometimes
make an
educated guess of the size of a lesion based on experience
and knowledge of
their own finger size or hand span. The level
of accuracy in such
‘guestimates’ is unknown but
may have a bearing on the subsequent
records as evidence for
court. At a research meeting of the St Mary's Sexual
Assault
Referral Centre it emerged that guestimates of the diameter
of a 1p
coin could be grossly erroneous. There are issues of
visual perception here.
‘Size constancy’ is the
visual processing phenomenon by which an
object appears to
be of a constant size irrespective of distance from the
viewer.
1 Errors in
the neural procedures (assessing angle and distance)
will affect the accuracy
and precision (reliability) of size
judgments. These operations are not
entirely fixed, and practice
in estimating size can assist accuracy and
precision, since
recognition of particular sizes of objects will mitigate the
effect of errors in the sensory encoding
processes.
2 Repeated
forensic examination of injuries, including recording of their
dimensions,
would provide such practice. We investigated whether
such skills are to be
trusted, looking primarily at guestimates
of length.
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METHODS
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24 people volunteered to take part—11 forensic clinicians
and 13
others (nurses, counsellors, crisis workers, administrators,
lawyers). Ages
ranged from 25 to 72 years, and all but 2 participants
(both in the comparison
group) were female.
Ten test items were used ranging from 9 mm to 185 mm in length. Each was
placed on a background made from irregular shaped white paper. A specially
prepared form was used to record the guestimates. Anonymity was guaranteed for
participants, and confidential feedback on performances was offered. The test
items were spaced at least two feet apart, and arranged in a random sequence.
Participants were allowed to handle them but not to use any calibrated device.
They were not to confer or see each others' results. Guestimates for all 10
were to be recorded within 5 minutes.
Some non-clinical participants were not able to guestimate in mm or cm as
requested but used inches and fractions of inches. These were converted to the
metric system and rounded up to the nearest mm. All data were coded in
millimetres and entered into SPSS and Excel for statistical analyses.
Within-subject differences in the responses of the participants (together and
as separate groups) from the true item lengths were investigated by one-sample
t-tests, which are tolerant to departures from normal distributions.
Between-subject differences between the responses of the two groups were
investigated by the Mann-Whitney U-test, a non-parametric test accommodating
unequal sample sizes and data distribution.
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RESULTS
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240 observations were made, with no missing data.
Table 1 shows
the results for
all participants. The size of three of the
ten objects (balloon, scratch and
10 cent coin) was significantly
overestimated. Both participant groups
overestimated the size
of the scratch. The degree of error was not related to
item
size.
The forensic clinicians did not outperform the other participants. They
tended to be more accurate for small objects and less accurate for large
objects (Figure 1).
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DISCUSSION
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The inaccuracy of the forensic clinicians reached statistical
significance
for only one item, the 135 mm scratch. However,
the same was true of the group
of other professionals, so forensic
clinicians do not seem especially blessed
with skills of guestimation.
The trends in the data do suggest they are more
accurate with
smaller items, which may reflect the type of injuries observed
and recorded by the participant clinicians, who examine survivors
of sexual
assault.
Although this was an artificial exercise and the test conditions were not
the same as a forensic examination, the inaccuracy of guestimates probably
reflects reality. It may be that some forensic clinicians have an inherent
skill for guestimation and others improve with practice. For both, the taking
of true measurements is advisable when the record of injury may ultimately be
used in a prosecution. Where use of a measuring device is deemed against the
best interests of the client and examination, it is as well to be aware of
one's limitations.
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REFERENCES
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- McKee SP, Smallman HS. Size and speed constancy. In: Walsh V,
Kulikowski J, eds. Perceptual Constancy: Why Things Look as They
Do. Cambridge: Cambridge University Press, 1998: 373
-408
- McKee SP, Welch L. The precision of size constancy.
Vision Res 1992; 32:1447
-60[Medline]
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January 1, 2004;
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49 - 49.
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