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
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Abstract |
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Methods All 3182 surviving members of the Boyd Orr cohort were sent postal questionnaires in 19971998 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
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Introduction |
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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.911 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.
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Methods |
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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 adiposityBMI (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 removedso 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.
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Results |
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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 2. 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 2
). 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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For all measures, except leg length, the correlations between self-reported and measured values were generally over 0.90 (Table 3).
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Discussion |
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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.1923 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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Acknowledgments |
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