Vol. 20, No. 2, pp. 206-210
Printed in Great Britain
Journal of Public Health Medicine
Body mass index: a comparison between
self-reported and measured height and weight
Anthony Hill and Julian Roberts
Abstract
Background Body mass index is used to measure obesity in
individuals and to monitor trends in population obesity.
Some observers use self-reported height and weight to
assess body mass index; others use measured parameters.
This paper reports on a study to compare body mass index
when calculated from self-reported and measured heights
and weights.
Methods A randomized postal questionnaire survey and
follow-up clinical measurement study were carried out in a
geographically defined population in the rural South West of
England; subjects were 6000 residents of Somerset health
district aged 16-64 years selected from the Family Health
Services Authority register.
Results The response rate for the postal questionnaire was
57.6 per cent. A total of 73.3 per cent of responders agreed to
clinical measurements; 84 per cent of measured volunteers
had over-reported their height and 74 per cent underreported their weight. The difference between body mass
indices based on self-reported and measured values of
height and weight is highly statistically significant for the
whole population and for most age-sex and body mass
groups.
Conclusions Self-reported heights and weights are unreliable
and if used for monitoring health targets should be treated
with caution.
Keywords: self-reported, measured, BMI
Introduction
Many lifestyle surveys have been undertaken in the United
Kingdom in recent years. Most have been postal questionnaires
as this is an appropriate way to collect most lifestyle data, but
these are subject to a variety of biases including non-response
bias and a systematic reporting bias for some variables such as
height and weight1"3
Some monitoring of obesity targets is done by using body
mass indices calculated from measured height and weight. This
is particularly the case with national surveys.67 However, many
local surveys use only self-reported height and weight
Somerset Public Health Department carried out lifestyle
surveys of its residents in 1987 and 1992. The second of these
self-completed questionnaire surveys had an additional component where measurements were made of height, weight and
blood pressure. This report shows the variation between the
distribution of body mass index using self-reported and
measured height and weight
The calculation of body mass index from individual heights
and weights is widely used to provide an indication of
obesity.8 In the past it has been accepted that there should be
different thresholds for men and women for each of the
categories, with women broadly requiring lower body mass
indices for inclusion into each of the categories.9 More
recently, The health of the nation10 set a target to 'reduce the
percentage of men and women aged 16-64 who are obese by
at least 25 per cent for men and at least 33 per cent for women
by 2005'. It defined obesity as a body mass index of over 30
for both men and women. The common thresholds are used in
this paper.
Methods
In 1992 a sample of 6000 residents of Somerset health district
were taken from the computerized Family Health Services
Authority register. The sample was of 16-64-year-old residents
using systematic selection from a randomly selected starting
point
The survey was conducted using a postal questionnaire.
Included in the questionnaire were the questions on selfreported height and weight Responders were given the
opportunity to use either metric or imperial units and to report
in whole or part units. In 1992 those completing the
questionnaire were invited to attend for measurement of
height, weight and blood pressure. These took place between
one and four months after the questionnaire was completed, in a
clinic setting at convenient locations, and were conducted by a
small team of nurses trained for the purpose. A standard
protocol was used based on the Welsh Heart Health Survey.11
South Humber Health Authority, Health Place, Wrawby Road, Brigg, North
Lincolnshire DN20 8GS.
Anthony HOI, Director of Public Health
Somerset Health Authority, Wellsprings Road, Taunton, Somerset TA2 7PQ.
Julian Roberts, Health Targets Monitoring Officer
Address correspondence to Dr A. Hill.
© Oxford University Press 1998
207
BODY MASS INDEX
Table 1 Numbers (and percentages given in parentheses) in body mass index categories using self-reported and measured
weights and heights
Mala
Numbers
Female
Self-reported
Underweight
Normal weight
Overweight
Obese
63 (6.3)
500 (49.7)
376 (37.3)
68 (6.8)
Total
1007(100.1)
Measured
Total
Measured
Self-reported
Measured
Self-reported
37 (3.7)
364(36.1)
470 (46.7)
136(13.5)
131 (10.5)
724 (57.9)
298 (23.8)
98 (7.8)
65 (5.2)
631 (50.4)
393(31.4)
162(12.9)
194(8.6)
1224(54.2)
674 (29.8)
166(7.4)
1007(100)
1251 (100)
1251 (99 9)
2258(100)
102 (4.5)
995(44.1)
863 (38.2)
298(13.2)
2258(100)
Percentages may not add to 100 because of rounding.
Body mass index was calculated from both self-reported and
measured heights and weights.
Results
Theresponserate to the 1992 postal questionnaire was 57.6 per
cent. For the clinical measurement study 73.3 per cent of the
questionnaire responders agreed to take part Further attrition of
the sample resulted in a final 38.4 per cent of the original
sample.
The differences between body mass indices using selfreported and measured weights and heights and common
thresholds for males and females are shown in Table 1. This
provides an indication that self-reporting of height and weight
may be substantially unreliable.
As body mass index is derived solely from the heights and
weights of people, underestimates of body mass index must
arise from either over-reporting of height and/or underreporting of weight Figures 1 and 2 indicate the extent of
these tendencies. Although the plots show that there are degrees
of both under-reporting of height (the points to the left of the
vertical zero line on Fig. 1), and over-reporting of weight (the
points to the left of the vertical zero line on Fig. 2), the converse
is more often the case. Indeed, 84 per cent of the measured
volunteers had over-reported their height. The mean of the
difference between self-reported and measured height was
7.46 cm (95 per cent CI 4.80, 10.01). A smaller proportion of
the sample misreported their weight, with 74 per cent of those
who had their weight measured having previously underreported their weight, but generally by small amounts. The
mean of this difference was 0.85 kg (95 per cent CI -0.30,
2.00).
The mean difference in body mass indices based on selfreported and measured values was 1.29 (95 per cent CI 1.23,
1.34). This difference was associated with the body mass index
based on measured values and, to a lesser extent with age group.
The understating of body mass index increases as measured
body mass index increases, by about 0.1 for every unit, and as
age increases, by about 0.05 for each step from a ten-year age
group to the next. There is no association with gender. The
statistical significance between body mass indices based on
self-reported and measured values for age-sex groups and
body weight groups is shown in Table 2. It is worth noting
that numbers are small for all underweight groups and for obese
16-24-year-olds.
AM
140
190
120
180.
• •i
.-.••jKg?.,
17O
100
•
•
160
60
140
40
-10
0
10
20
Height difference (self reported-measured)
Figure 1 Measured height (cm) by difference (cm) between
self-reported and measured height.
i
•Mm
80
150
130
-20
•
•
20
-20
-10
u
m
0
10
m
20
Weight difference (measured-self reported)
Figure 2 Measured weight (kg) by difference (kg) between
self-reported and measured weights.
Table 2 Difference in means between measured and self-reported body mass indices for age-sex and body weight groups
16-24 years
25-34 years
M
F
rvalue
0.16
0.82
1.64
2.74
0.52
5.24
4.08
2.36
Underweight
Normal
Overweight
Obese
SIg.
j+
M
Dfff. in
means
rvalue Sig.
Diff. in
means
0.47
0.92
1.24
1.68
3.00
9.79
5.38
2.68
0.42
0.95
1.40
1.95
++
++
++
+
45-54 years
F
rvalue
1.29
8.78
8.98
3.82
SIg
••
M
Dfff. in
means
rvalue
SIg.
0.17
0.79
1.30
1.76
0.81
8.62
11.13
6.32
•+
55-64 years
F
Dfff. in
means
rvalue
-0.35
1.02
1.28
1.92
-1.32
10.30
12.93
8.86
SIg.
Diff. in
means
rvalue
-H-
0.66
0.78
1.25
2.04
2.83
8.82
11.83
6.31
Sig.
JOURNAL OF PUBLIC
Dfff. in
means
35-44 years
All ages
X
tn
r-
M
F
M
F
M +F
Dfff. in
means
rvalue
Dfff. In
means
rvalue
Sig.
Diff. in
means
rvalue
Sig
Dfff. in
means
rvalue
Dfff. in
means
-0.77
0.74
1.49
1.99
-1.51
5.93
11.54
10.34
0.02
0.79
1.38
2.18
0.06
8.69
14.87
13.70
—
++
++
++
1.20
1.07
1.67
2.30
9.10
8.23
20.20
10.06
++
0.47
1.04
1.54
2.16
0.78
8.39
12.59
7.58
ae
, p<0.05; ++, p<0.01.
Sig.
: :
0.29
0.87
1.43
2.07
rvalue
3.10
25.93
36.34
22.23
Sig.
MEDKJINE
Underweight
Normal
Overweight
Obese
SIg.
BODY MASS INDEX
Discussion
The 1992 Lifestyle Survey took as its starting point the general
approach and methods of its 1987 forerunner. It therefore relied
primarily on a postal, self-completion questionnaire. The
drawbacks of using this method are well established. First,
there is the possibility that those who do not return
questionnaires have different characteristics from those who
do. Second, it is possible that self-reporting, particularly of
certain characteristics, may be unreliable.
In relation to information on height and weight it may be
that these twin problems of non-response and self-reporting are
accentuated. For instance, people with 'undesirable' physical
proportions may ignore altogether requests for personal
information, and those that do provide information may 'stretch
the truth' to convey a more 'desirable' self-image.2
The response rate of 38.4 per cent of the original sample is
low. This volunteer cohort self-selected twice - first to respond
to the postal questionnaire and then to attend for clinical
measurements. As a result of this, considerable caution is
needed in interpreting the results, especially in view of the
lower response rate from younger age groups and to a lesser
extent, men. Therepresentativenessof the postal questionnaire
responders has been reported elsewhere.12 The time lapse
between self reporting and measurement reading is a further
reason for caution.
The related Hypertension Survey, which measured actual
heights and weights of volunteers who had taken part in the
Lifestyle Survey, allowed some exploration of the impact of
self-reporting bias. Our study clearly shows over-reporting of
height and under-reporting of weight by both men and women.
This differs from the Welsh Heart Health Survey,13 where
weight wasreportedwithout significant bias in men. It is similar
to, but more marked than thefindingsof Ziebland et al.,2 Pirie
et al? and Palta et al.4
The difference between body mass indices based on selfreported and measured values is highly statistically significant
for the whole population and for nearly all age-sex groups and
body mass index categories. Where the difference is not
statistically significant the numbers of subjects are very small.
This finding is clearly of importance in population studies and
health target monitoring but is also of relevance to clinicians
faced with an individual patient.
The problem of obesity is usually tackled as a component of
coronary heart disease prevention programmes. Accurate, local
data are crucial for a more specific, focused approach to obesity
reduction in high-risk sub-groups within the population.
The over-reporting of height, under-reporting of weight and
consequential underestimates of body mass index are important
in setting and monitoring health targets such as those associated
with the Health of the nation. Local targets in Somerset were set
in 1992 using self-reported levels from the 1987 Lifestyle
Survey.
On the basis of common thresholds for men and women, the
209
baselinefigurefor obesity in women was 7.3 per cent (obtained
in the Somerset Lifestyle Survey of 1987) and the 2005 target
was 4.9 per cent - a 33 per cent reduction as for national
targets. At the time this was considered to be a favourable
situation locally, but comparing like with like would almost
certainly have led to a greater and earlier investment by the
Health Authority in programmes to reduce obesity in the
population.
This report emphasizes the care which needs to be taken in
making comparisons between local targets and surveys for
obesity which may be based on self-reported values and
national targets which are monitored using measured values.
The values appear to be statistically different in Somerset and
variations considerably greater than we expected.
Acknowledgements
We thank our colleagues involved in the Somerset Lifestyle
Survey, the residents of Somerset who took part, Mrs
J. Clarkson for statistical advice, and Mrs L. Fry and Ms M.
Kim-Hooton for typing the manuscript.
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Accepted on 12 January 1998