How accurately are height, weight and leg length reported by the elderly, and how closely are they related to measurements recorded in childhood?

D Gunnella, L Berneyb, P Hollandc, M Maynarda, D Blaned, S Frankela and G Davey Smitha

a Department of Social Medicine, University of Bristol, Canynge Hall, Whiteladies Road, Bristol BS8 2PR, UK.
b LSE Health, London School of Economics and Political Science, Houghton St., London WC2A 2AE, UK.
c Department of Epidemiology, University of Liverpool, Liverpool L69 3GB, UK.
d Division of Neuroscience, Imperial College of Science, Technology and Medicine, St Dunstan's Road, London W6 8RP, UK.

Reprint requests to: David J Gunnell, Department of Social Medicine, Canynge Hall, Whiteladies Road, Bristol BS8 2PR, UK. E-mail: D.J.Gunnell{at}bristol.ac.uk


    Abstract
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Background This paper examines (1) the accuracy of self-reported height, leg length and weight in a group of subjects aged 56–78; (2) whether recent measurement of height and weight influences the accuracy of self-reporting and (3) associations between childhood and adult height, leg length and BMI measured in old age.

Methods All 3182 surviving members of the Boyd Orr cohort were sent postal questionnaires in 1997–1998 and a sub-sample (294) was also clinically examined.

Results Self-reported height was overestimated and body mass index (BMI), based on reported height and weight, underestimated. The mean difference between self-report and measured values were for height: 2.1 cm in males and 1.7 cm in females; for BMI the difference was –1.3 kg/m2 in males and –1.2 kg/m2 in females. Shorter individuals and older subjects tended to over-report their height more than others. The overweight under-reported their weight to a greater extent. Recent measurement appeared to decrease over-reporting of height but not weight. Correlations between self-report and measured height and BMI were generally over 0.90, but weaker for leg length (r = 0.70 in males and 0.71 in females). Adult height and leg length were quite closely related to their relative values in childhood (correlation coefficients ranged from 0.66 to 0.84), but associations between adult and childhood BMI were weak (r = 0.19 in males and 0.21 in females).

Conclusions Self-reported measures of height and weight may be used in studies of the elderly although systematic reporting errors may bias effect estimates. As overweight individuals tend to under-report and the short and underweight tend to over-report, studies investigating associations of disease with height and weight using self-reported measures will underestimate effects. The weak associations between childhood and adult BMI indicate that associations between childhood adiposity and adult cardiovascular disease found in this cohort may reflect the specific effect of childhood overweight, rather than its persistence into adulthood. This suggests that avoidance of adiposity may be as important in childhood as in adulthood.

Keywords Self-report measures, life course epidemiology, Boyd Orr cohort, height, leg length, BMI

Accepted 26 January 2000


    Introduction
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Whilst a number of studies have assessed the accuracy of self-reported height and weight (see, for example1,2) few have investigated this issue specifically in elderly populations3 and none have examined the accuracy of self-reported leg length measurements. Height and leg length, together with birthweight, act as anthropometric markers for early life exposures and interest in their measurement has increased because of their associations with adult chronic diseases.4–6 However, the extent to which adult anthropometry reflects childhood nutritional status is unclear, as retarded growth in childhood may be compensated for by catch-up growth in adolescence.7

As part of follow-up studies of the Boyd Orr cohort8 we sent postal questionnaires to all traced surviving study members living in Britain in 1997. In a separate investigation into social class differences in health, a sub-sample of these subjects were interviewed and physically examined, some before and some after the questionnaires were sent.9–11 This paper examines (1) the accuracy of self-reported height, leg length and weight (2) whether recent measurement of height and weight influences the accuracy of self-reporting and (3) associations between childhood and adult height, leg length, and body mass index (BMI) measured and reported in early old age.


    Methods
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
The Boyd Orr cohort
The background and sampling frame for the Boyd Orr cohort are described elsewhere.8,12 In brief, the cohort comprises 4999 subjects who as children took part in the Carnegie Survey of Diet and Health in Pre-war Britain.13 As a result of further searches of archived research records and contact with study members the number of identified cohort members has increased by 26 since earlier reports of this study. The original survey was carried out in 16 centres in England and Scotland between 1937–1939 under the direction of Sir John Boyd Orr. Altogether 1352 families with children took part in the study. In all but two of the centres children from the families underwent detailed clinical examinations to assess health and nutritional status. Examinations included measures of height, weight, leg length and shoulder breadth.

Self-reported anthropometry
Between 1997 and 1998 all 3182 traced surviving members of the Boyd Orr cohort were sent health and lifestyle questionnaires. These included a food frequency questionnaire and questions on social circumstances and health. Subjects were asked to report their current height (in feet and inches), weight in light clothing (in stones and pounds) and inside leg measurement (in inches). The questions used were: ‘What is your height without shoes?’ (in feet and inches); ‘What is your inside leg measurement? (in inches) If you do not know please examine a pair of your trousers’; ‘What is your weight in light clothing?’ (in stones and pounds). For the analysis all values have been converted to centimetres and kilograms. After two reminders 1647 completed questionnaires were returned (52% response).

Anthropometric measurements
In parallel with the postal questionnaire survey, a stratified random sample of 294 study members were interviewed in their homes and their height, sitting height and weight were measured by two researchers.11 The sample was restricted to those who were over 4 years old when they were examined in the original 1930s survey and only one subject per family was sampled. Stratification aimed to achieve approximately equal numbers of subjects across the range of household food expenditure categories. Half of those examined were sent questionnaires prior to measurement and half were given questionnaires afterwards. Height was measured to the nearest millimetre using a Harpenden pocket stadiometer with internal spirit level. Sitting height was measured with the subject seated on a hard surface and leg length was calculated by subtracting sitting height from overall height. Weight was measured to the nearest 100 g using Soehnle 7306 digital scales. Subjects were weighed without shoes in indoor clothes. The two researchers carried conversion charts which they used to tell subjects their weight and height converted to imperial measures (stones and pounds for weight; feet and inches for height).

Statistical analysis
Bias in the self-reported measures was estimated by calculating the mean difference between self-report and measured values (self-report minus measured value). As body weight depends on height we calculated a derived measure of adiposity—BMI —(weight/height2) for all study members. Pearson's correlation coefficients were calculated to assess the linear associations between self-report and measured values. As previous studies have shown gender-specific patterns of error in self-reported measures,1 results are presented separately for males and females.

To assess whether recent measurement of height, weight and sitting height in this study influenced error in self-reported values, we analysed data separately depending on whether subjects had completed the questionnaires before or after the physical examinations took place. Unpaired t-tests were used to assess differences in the accuracy of anthropometric measures recorded before and after clinical measurement. Multivariable linear regression analyses were used to investigate factors associated with systematic and random error in the reporting of height, weight and leg length. The factors examined in these models were age, gender, social class and measured height, leg length, weight and BMI. To assess systematic error, the difference between reported and measured anthropometry was used as the dependent variable. To assess random error the difference was again used, but the sign of the difference was removed—so large negative errors were given the same weight as large positive errors3 and factors associated with inaccuracy, rather than systematic error, can be assessed. We also assessed the data by means of Bland-Altman plots (graphs of the difference between self-reported and measured height against the mean of the two) to determine whether differences between the two are related to the magnitude of the measurement.14

Comparisons between childhood and adult anthropometry are based on the actual adulthood measures and z-scores for the childhood measures. The z-score expresses a child's measurement as the number of standard deviations his/her value is from the mean for given age and sex. Because no reliable reference standards for pre-war children's height, leg length and BMI exist, we used internally derived standards to calculate the z-scores.12 As the values for BMI within 6 month age bands were generally positively skewed, the data were transformed using the reciprocal transformation.15 Polynomial regression models were then used to estimate expected values and for each anthropometric index and its standard deviation for males and females separately.


    Results
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Representativeness of questionnaire responders and those who were physically examined
Compared to non-responders, questionnaire respondents tended to come from more affluent childhood social backgrounds (the fathers of 30.8% of responders, compared to 21.7% of non-responders were from social class I–III; P < 0.001) and currently lived in less socioeconomically deprived areas (difference in Townsend score 0.70; P < 0.001). Likewise questionnaire responders tended to be taller as children (difference in height z-score of 0.21 standard deviations; P < 0.001). There were no significant differences between non-responders and responders with respect to age, gender or childhood BMI. The mean age of questionnaire responders (at 1 January 1996) was 65 years (range 56–78 years). Characteristics of the subjects who agreed to take part in the physical examinations have been previously documented.11 They were more likely to be male, taller in childhood and from more affluent childhood backgrounds.

Accuracy of self-reported height, leg length and weight
Altogether 257 subjects (118 males and 139 females) had both examination and self-report measures of height, weight and leg length. Thirty-seven of the 294 who were examined did not return completed questionnaires. The mean differences between reported and measured anthropometry are given in Tables 1 and 2GoGo. Recent measurement of height appears to increase the accuracy of reported height, although this effect was only statistically significant in females. The accuracy of self-reported weight and leg length was unaffected. Both males and females tend to over-report their height and these effects are more marked in shorter subjects (Table 2Go). Leg length was greatly underestimated using self-reported inside leg measurements. This effect is most probably because inside leg measurement is shorter than measured total height minus sitting height. Again over-reporting was greatest in those with short legs.


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Table 1 Mean difference (95% CI) between reported height, leg lengtha, weight and body mass index and measured values
 

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Table 2 Difference between reported measure and actual measure according to tertile of actual measure
 
Weight was under-reported, the effects being most marked in the overweight (Table 2Go). Because of underestimation of the numerator and overestimation of the denominator used to calculate BMI, its value calculated from self-reported measures underestimated its measured value. Again, the greatest error was seen in overweight individuals.

For all measures, except leg length, the correlations between self-reported and measured values were generally over 0.90 (Table 3Go).


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Table 3 Correlation between self-report and measured anthropometry
 
The factors used in multivariable analysis to predict systematic and random error are given in Table 4Go. Inaccuracy and systematic bias in reporting of stature tend to increase with age. Women tend to report their height more accurately than men. Otherwise, significant factors are as expected from the trends seen in Table 2Go. Figure 1 shows Bland-Altman plots to investigate whether differences between self-report and measured values are strongly associated with mean values. These suggest that the effects of systematic bias discussed above are slight. The distributions of self-reported and measured leg length in the Figure appear somewhat unusual—this is likely to be because (1) leg length was self-reported to the nearest inch but measured values were recorded to the nearest mm and (2) the range of leg length values was less than that for overall height.


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Table 4 Linear regression models: factors associated with differences between actual and self-reported anthropometry derived from multivariable linear regression models
 
Associations between z-scores for childhood height, leg length and body mass index and adult measures
In all, 1009 subjects who self-reported their adult height were examined in childhood when aged over 2 years and had information recorded on childhood height. Their mean age when responding to the questionnaire was 65 years. Fewer subjects (727 [72%]) reported their inside leg measurements and 910 reported both their height and weight. Women were more likely not to report their leg length. Fifty-four per cent of questionnaire responders were women but 70% of those who did not report leg length were female (P < 0.001). Table 5Go presents, by 3-year age group, associations between z-scores for childhood height, leg length and BMI and both self-reported and measured adult values. Associations with childhood height are similar for both reported and measured adult height and do not differ greatly with age. Associations with childhood leg length are generally stronger for the measured than reported values. Childhood and adult BMI were weakly associated, particularly in those whose childhood measurements were recorded in later childhood.


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Table 5 Correlation between childhood anthropometry and both self-report and measured adult height, leglength and body mass index (BMI)
 

    Discussion
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Accuracy of self-reported measures
Our findings in relation to the accuracy of self-reported height and weight in this elderly population are in broad agreement with those of other studies.1–3 Older adults and those who are short tend to over-report their height and this effect is more marked in males. The age effect is likely to be due, at least in part, to the loss of adult height resulting from ageing.16 Subjects are most likely to recall their height as measured in early adulthood and, as height loss is greatest in the elderly, the difference between their recalled and actual height will be greatest. Most subjects under-reported their weight, however, this effect was more marked in the overweight. The net effect of the under-reporting of weight and over-reporting of height was that self-reported BMI was less strongly related to actual BMI than was height or weight. The correlations between self-reported and measured values were similar to those reported in other populations. However, in contrast to previous investigations,1,2 we did not find that women under-reported weight to any greater extent than men.

Our findings in relation to leg length were disappointing. Only 72% of respondents reported this measurement. Clearly the values derived for leg length by subtracting sitting height from actual height do not correspond to inside leg length and so the mean differences of 8.8 cm and 6.9 cm in males and females respectively are not surprising. However, such effects should have less effect on the correlations between reported and measured values; but these were relatively low (0.70 in males and 0.71 in females) in relation to those for height (0.92 in males and 0.90 in females). As seen with height, leg length was over-reported in older and shorter individuals. Taken together, these results suggest that, using the techniques we used, self-reported leg length is a less useful proxy for leg length than self-reported height is for height. We asked subjects to check their clothing sizes if they were unsure of their inside leg measurement. A previous analysis of the reliability of measurement of physique based on the uniform sizes of London busmen indicated that this is a relatively reliable means of obtaining data.17

Surprisingly, we found no strong evidence that recent measurement of height and weight greatly improved the accuracy of self-reported values, although it appeared that accuracy of self-reported height increased. This effect is probably because these older subjects are made aware of the fact that their height has diminished since their youth.

Associations between anthropometry in early old age and z-scores for childhood height, leg length and BMI
Body mass index is a marker of the balance between energy intake and energy expenditure around the time the measure is taken, whereas adult height and leg length reflect a combination of genetic endowment and the accumulation of factors influencing childhood growth.7 Until the onset of osteoporosis in later life, values for stature remain relatively fixed, unlike those for BMI which may fluctuate considerably. Previous analyses of this cohort have demonstrated associations between childhood height, leg length and BMI and adult mortality.5,12,18 Similar mortality associations with these anthropometric measures have been found in long-term follow-ups of adults.19–23 We were therefore interested in determining the strength of the associations between childhood and adult anthropometry in this cohort to investigate the extent to which associations seen in adulthood may reflect childhood influences.

Associations between childhood z-scores for height and adult height were relatively strong and in keeping with those reported in previous studies.24,25 There was little difference in the strength of the associations between those based on self-reported or measured height. However, whilst other studies suggest that the accuracy of prediction of adult height from childhood measures increases with age at measurement in childhood, our data did not generally follow the expected pattern. Correlations between the z-scores for childhood height and adult height at age 11+ are probably as influenced by variations in the age of onset of puberty as they are by differences in final height. As seen with height, the strength of association between childhood leg length and measured/self-reported leg length showed little change with age. Associations were considerably weaker for self-reported leg length, compared to estimates based on measured sitting height.

In keeping with previous investigations26,27 we found that the associations between childhood and adult BMI were relatively weak. Casey et al. reported a correlation of 0.41 between BMI in childhood and at age 50 years.26 The correlations seen in our analysis were 0.19 for males and 0.21 for females. In this study we found no evidence that the strength of associations between childhood and adult BMI increased with age in childhood when the measurements were recorded and associations were similar for self-reported and measured BMI. If anything, associations between childhood and adult BMI were weaker in those aged over 11 years, although this is not in keeping with findings reported elsewhere.26,28

Implications for epidemiological research using measures of childhood and adult anthropometry
Self-reported values for adult anthropometry are less accurate than clinical measurements. Furthermore, there is evidence of systematic bias in self-reported height and weight. The direction of these effects means that, in general, studies assessing associations between self-reported adult height, BMI and leg length will give biased (under)estimates of associations between these indicators and mortality. This is because short people, who are at increased mortality risk,22 tend to over-report their height, and the overweight, who are also at increased mortality risk,22 under-report their weight. These biases will be increased by random measurement error as this usually results in attenuation of exposure-disease associations. Poor response rates and error in the reporting of leg length were disappointing. Future studies might consider including a paper tape measure with postal questionnaires asking subjects to record their leg length from the top of the inside of their leg to the floor or ankle bone.

In the analysis presented here the associations between childhood and adult BMI were relatively weak. This suggests that mortality associations found in relation to childhood and adolescent adiposity in this and other cohorts18,23,29 may arise because childhood adiposity has long-term effects on disease risk, rather than childhood BMI simply acting as a marker for overweight in later life.30 This view is supported by the finding in post mortem studies that childhood adiposity is associated with the extent of atherosclerosis.31 This suggests that avoidance of adiposity may be as important in childhood as in adulthood. Studies with detailed anthropometric data collected throughout the life course are required to further investigate mechanisms for the observed associations.



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Figure 1A Bland-Altman plot of difference between self-report and measured height vs. mean of self-report and measured height

 


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Figure 1B Bland-Altman plot of difference between self-report and measured leg length

 


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Figure 1C Bland-Altman plot of difference between self-report and measured weight vs. mean of self-report and measured weight

 


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Figure 1D Bland-Altman plot of difference between self-report and measured BMI vs. mean of self-report and measured BMI

 

    Acknowledgments
 
Professor Philip James, director of The Rowett Research Institute for the use of the archive and in particular Walter Duncan, honorary archivist to the Rowett. The staff at the NHS Central Register at Southport and Edinburgh. Sara Bright for data entry. We also wish to acknowledge all the research workers and subjects who participated in the original survey in 1937–1939. This work was funded by the Economic and Social Research Council's Health Variations Programme (grant: L128251003) and the World Cancer Research Fund.


    References
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
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