1 Harvard School of Public Health, Boston
2 Initiative for Global Health, Harvard University, Cambridge, USA
3 Clinical Trials Research Unit, University of Auckland, New Zealand
Correspondence to: Majid Ezzati E-mail: mezzati{at}hsph.harvard.edu
| SUMMARY |
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Design: Statistical analysis of repeated cross-sectional health examination surveys (the National Health and Nutrition Examination Survey [NHANES]) and health surveys (the Behavioral Risk Factor Surveillance System [BRFSS]) in the USA.
Setting: The 50 states of the USA and the District of Columbia.
Results: In the USA, on average, women underreported their weight, but men did not. Young and middle-aged (<65 years) adult men over-reported their height more than women of the same age. In older age groups, over-reporting of height was similar in men and women. Population-level bias in self-reported weight was larger in telephone interviews (BRFSS) than in-person interviews (NHANES). Except in older adults, height was over-reported more often in telephone interviews than in-person interviews. Using corrected weight and height in the year 2000, Mississippi (31%) and Texas (30%) had the highest prevalence of obesity for men; Texas (37%), Louisiana (37%), Mississippi (37%), District of Columbia (37%), Alabama (37%), and South Carolina (36%) for women.
Conclusions: Population-level bias in self-reported weight and height is larger in telephone interviews than in-person interviews. Telephone interviews are a low-cost method for regular, nationally- and sub-nationally representative monitoring of obesity. It is possible to obtain corrected estimates of trends and geographical distributions of obesity from telephone interviews by using systematic analysis which measure weight and height from an independent sample of the same population.
| INTRODUCTION |
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While technically straightforward, measuring weight and height in large nationally and sub-nationally representative samples and on a regular basis (e.g. annually) is costly. For this reason, population-level surveillance and health research regularly rely on self-reported weight and height. Self-reported weight and height data are subject to random error, and, more importantly, systematic reporting bias.9-14 The magnitude of bias has varied across studies based on factors such as age, actual weight and height, and education.9 Some researchers have nonetheless concluded that self-reported height and weight are acceptable, valid, or excellent for population-based studies.11-14 The US Centers for Disease Control and Prevention (CDC), while acknowledging the bias in self-reported weight and height, presents state-level obesity levels and trends based on the Behavior and Risk Factor Surveillance Survey (BRFSS), which uses telephone surveys.7,8
In previous research, bias in self-reported height and weight has been characterized at the individual level, using measured and self-reported data from the same subjects.9-14 Subjects may, however, reduce intentional misreporting of their weights and heights, if they are measured before/after the interview. The `mode' of interview (e.g. telephone versus in-person) can also affect misreporting as respondents may misreport less when in-person methods are used than in telephone interviews. The mode of interview may result in differential participation rates in different health surveys. Therefore, the total bias in self-reported weight and height at the population-level arises from two sources: first, bias in individual reporting behaviour; and, second, systematic differences in participation in different survey modes. Thus, the very data needed for individual-level validation would make the findings inapplicable to population-level data if based solely on self-reported weight and height, especially those given in telephone interviews. The solution to this apparent dilemma is to adjust self-reported weight and height using measured values at the population levels, with the two estimates obtained independently.
We estimated the population-level relationship between measured and self-reported height and weight in the USA using two nationally representative health surveys and health examination surveys: the BRFSS and the National Health and Nutrition Examination Survey (NHANES). We also examined the role of age and sex on bias in self-reported weight and height. We used this relationship to correct self-reported weight and height from telephone surveys and to estimate the corrected trends in national and state-level obesity in the USA. In addition to providing the first unbiased estimates of the levels and trends in state-level obesity in the USA, this report contributes to methods for measurement and surveillance obesity, and other risks and diseases that regularly rely on self-report data, by quantifying the effects of the mode of self-report as a source of bias.
| METHODS |
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The BRFSS is a cross-sectional telephone survey designed and managed by the CDC but administered by state health departments. The BRFSS uses a multistage-cluster design based on random-digit dialing to select a representative sample from each state's non-institutionalized civilian residents aged 18 years or older. Data from each state are pooled to produce nationally representative estimates. The BRFSS questionnaire primarily focuses on personal risk behaviours and exposures. Median state overall response rate for the BRFSS in 2002 was 45%; median Council of American Survey Research Organizations response rate was 58%. Detailed descriptions of the survey methods are available elsewhere18,19 and on-line [http://www.cdc.gov/brfss/].
Statistical analysis
NHANES household, interview, and examination data files were merged using
the unique sequence number given to each participant. Subjects who did not
participate in both the interview and the examination were excluded
(Table 1). Samples were
weighted using the procedure recommended in the BRFSS and NHANES
documentation. Age-sex-specific (5 year age groups between 20 and 79, and 80+)
mean population height, weight, and body mass index (BMI), defined as weight
divided by height-squared (kg/m2), were calculated for both
measured and self-reported variables for NHANES. For BRFSS, age-sex-specific
mean population BMI, height and weight were calculated for each survey year
corresponding to NHANES. Averages of BRFSS survey years corresponding to each
NHANES round were used for comparison with NHANES.
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| RESULTS |
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Bias in self-reported height had a more complex pattern than that of weight. In younger ages (20-44 years), self-reported height was overestimated for both men and women, with larger overestimation for men than women, and in telephone interviews than in-person interviews. After this age, height was still overestimated, but over-estimations for men and women, and in telephone and in-person interviews, gradually converged. The role of age in over-reporting height may be because height declines in older ages. If people measure their height less frequently than their weight, they may report measurements taken from early adulthood. This `unintentional' misreporting would also explain the convergence of height estimates in telephone and in-person interviews.
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Trends in national and state-level obesity in the USA
We used the relationships in Figure
3 to correct individual self-reported height and weight from the
BRFSS (which is conducted annually and is state-representative), and to
estimate corrected BMI and obesity (defined as BMI
30). The corrected
values are those that would be expected if annual state-representative
examination surveys such as NHANES had been conducted.
Figure 4 shows that between
1988 and 2002, the corrected prevalence of obesity among adult Americans
increased from 16.0% to 28.7% for men and 21.5% to 34.5% for women, with a
nearly linear trend (there is an apparent flattening in 2002 but definitive
conclusions require data from subsequent years). In comparison, the prevalence
of obesity was 19.7% for men and 24.5% for women in NHANES III (1988-1994) and
26.6% for men and 32.7% for women in continuous NHANES (1999-2002) (if
corrected BRFSS values are averaged over the same years, the values would be
17.9% for 1988-1994 and 27.3% in 1999-2002 for men and 23.6% for 1988-1994 and
33.2% in 1999-2002 for women). Figure
3 also shows that the difference between corrected and
self-reported obesity showed a greater increase for women than for men, with
self-reported obesity 6.8% for males and 13.3% for females lower than
corrected values in 2002, versus 5.4 and 10.7, respectively, in 1988.
Figure 5 compares the
self-reported and corrected prevalence of obesity in the US states for 1990
and 2000. In 1990, self-reported obesity in all US states was below 18% for
men and women.8
Corrected estimates show obesity prevalence >18% in 14 states for men and
in 44 states for women (including 11 states >24%). After correction, in
1990, states with the highest prevalence of obesity for men were Mississippi
(22%), Hawaii (22%) and Michigan (20%) and for women District of Columbia
(34%), Delaware (27%) and Mississippi (26%); states with the lowest prevalence
of obesity for men were Colorado (9%), Utah (11%) and Washington (12%) and for
women Massachusetts (17%), Colorado (17%) and Minnesota (18%). In 2000,
self-reported obesity was below 24% in all but two states (Mississippi and
Nebraska) for men and in all but three states (Alabama, Mississippi and
District of Columbia) for women: these states had self-reported obesity
prevalence between 24% and 30%. When height and weight were corrected for
self-report bias, men in 39 states had obesity prevalence >24%, including
two states with prevalence
30%Mississippi (30%), and Texas (31%);
women in all states except Colorado had obesity prevalence >24%, including
33 states with prevalence >30% and six states with prevalence
36%the District of Columbia (37%), Texas (37%), Louisiana (37%),
Mississippi (37%), Alabama (37%) and South Carolina (36%). States with the
lowest prevalence of corrected obesity for men in 2000 were Colorado (18%),
District of Columbia (21%) and Montana (21%) and for women Colorado (24%),
Montana (16%) and Massachusetts (27%).
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| DISCUSSION |
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Comparison with other obesity surveillance studies
Previous reports on bias in self-reported weight and
height9-14
had all been based on individual-level data. As a result, these works could
not examine two factors important for population-level monitoring: first,
individual-level misreporting caused by absence of measurement
subsequent/previous to the interview and by the mode of self-report, and
second, differential participation based on the survey mode. Previous reports
on state-level obesity levels and trends in the USA were based on the
BRFSS,7,8
which uses telephone surveys, and hence significantly underestimates true
obesity as seen in Figure
5.
Strengths and weaknesses of the study
Our results are subject to uncertainty because there may be systemic
variation in misreporting across states and social groups, or over time, for
example because of differences and changes in social values related to weight
and height. If such a variation exists, it would create heterogeneity in the
relationship used for correction (Figure
3), not detectable in our data. Repeated cross-sections in
1988-1994 and 1999-2002 did not indicate a systemic change in this
relationship during the analysis period. The evidence on the role of race and
education as determinants of bias was also not conclusive (data not shown),
and smaller than the effects of age and sex.
Self-reported data on weight and height are the only feasible option for large population surveys that are both nationally and sub-nationally representative and conducted on a regular basis (e.g. annual) in most nations (a small number of industrialized countries like the UK and Japan conduct annual measurements, but most are not subnationally representative). The choice for health researchers and practitioners is therefore between using self-reported weight and height, which are known to be subject to large bias, or relying on a correction algorithm like the one presented in this work that reduces bias, albeit with some uncertainty.
Conclusions and future research
The ideal correction to self-reported height and weight data would be from
a study in which subjects initially report their height and weight in
telephone interviews with the expectation that they would not be
measured later; but they are, in fact, subsequently measured (e.g. by asking
to attend a medical examination at the end of the telephone interview). In
such a study, the results (see Figure
3) could be further divided by age or other socio-demographic
factors. This would allow researchers to examine the interactions of such
factors and actual height/weight as determinants of bias, which was not
possible in our analysis. Such a study, to the best of our knowledge, does not
currently exist but would be an ideal addition to the BRFSS. Even if such a
validation study were implemented, the problem of selection would
persistboth in the initial survey and in the validation
phasebecause some people who misreported their height and weight in the
telephone survey would not agree to subsequent measurement. In the absence of
such an ideal validation study, the method used in our
analysiscorrecting self-reported data using height and weight from
telephone surveys on those from health examinations surveysis the best
available option for unbiased estimates of the levels and trends in
state-level obesity in the USA.
| Footnotes |
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This research was supported by a cooperative agreement from the Centers for Disease Control and Prevention (CDC) through the Association of Schools of Public Health (ASPH) Grant No. U36/CCU300430-23, and by the National Institute on Aging (Grant No. PO1-AG17625). The contents of this article are solely the responsibility of the authors and do not necessarily represent the official views of CDC or ASPH.
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