Para avaliar a validade do peso auto-referido em inquéritos de prevalência de obesidade, o mesmo foi comparado com o peso medido de 659 adultos, residentes em Porto Alegre, RS, Brasil, em 1986-87. Ambos os pesos foram obtidos por entrevistador, na casa do participante, na mesma ocasião. A média das diferenças entre peso auto-referido e peso medido foi pequena (-0,06 +/-3,16 kg; média +/- desvio padrão) e a correlação entre eles alta (r=0,97). Sessenta e dois por cento dos participantes referiram seu peso com erro < 2 kg, 87% com erro menor do que 4 kg e 95% com erro < 6 kg. Indivíduos de baixo peso hiperestimaram seu peso, o oposto ocorrendo com indivíduos obesos (p; 30) por peso referido, foi de 10% e a por peso medido, de 11%. Concluindo, a validade do peso auto-referido é aceitável para inquéritos de prevalência realizados em contextos similares.In order to evaluate the validity of self-reported weight for use in obesity prevalence surveys, self-reported weight was compared to measured weight for 659 adults living in the Porto Alegre county, RS Brazil in 1986-87, both weights being obtained by a technician in the individual's home on the same visit. The mean difference between self-reported and measured weight was small (-0.06 +/- 3.16 kg; mean +/- standard deviation), and the correlation between reported and measured weight was high (r=0.97). Sixty-two percent of participants reported their weight with an error of < 2 kg, 87% with an error of < 4 kg, and 95% with an error of < 6 kg. Underweight individuals overestimated their weight, while obese individuals underestimated theirs (p; 30) by reported weight was 10%, by measured weight 11%. Thus, the validity of reported weight is acceptable for surveys of the prevalence of ponderosity in similar settings

Duncan, Bruce B.

Pellanda, Lúcia

Polanczyk, Carísi A.

Schmidt, Maria I.

Tavares, Mário

Zimmer, Paulo M.

English

In order to evaluate the validity of self-reported weight for use in obesity prevalence surveys, self-reported weight was compared to measured weight for 659 adults living in the Porto Alegre county, RS Brazil in 1986-87, both weights being obtained by a technician in the individual's home on the same visit. The mean difference between self-reported and measured weight was small (-0.06 +/- 3.16 kg; mean +/- standard deviation), and the correlation between reported and measured weight was high (r=0.97). Sixty-two percent of participants reported their weight with an error of < 2 kg, 87% with an error of < 4 kg, and 95% with an error of < 6 kg. Underweight individuals overestimated their weight, while obese individuals underestimated theirs (p; 30) by reported weight was 10%, by measured weight 11%. Thus, the validity of reported weight is acceptable for surveys of the prevalence of ponderosity in similar settings.Para avaliar a validade do peso auto-referido em inquéritos de prevalência de obesidade, o mesmo foi comparado com o peso medido de 659 adultos, residentes em Porto Alegre, RS, Brasil, em 1986-87. Ambos os pesos foram obtidos por entrevistador, na casa do participante, na mesma ocasião. A média das diferenças entre peso auto-referido e peso medido foi pequena (-0,06 +/-3,16 kg; média +/- desvio padrão) e a correlação entre eles alta (r=0,97). Sessenta e dois por cento dos participantes referiram seu peso com erro < 2 kg, 87% com erro menor do que 4 kg e 95% com erro < 6 kg. Indivíduos de baixo peso hiperestimaram seu peso, o oposto ocorrendo com indivíduos obesos (p; 30) por peso referido, foi de 10% e a por peso medido, de 11%. Concluindo, a validade do peso auto-referido é aceitável para inquéritos de prevalência realizados em contextos similares

Cadernos Espinosanos (E-Journal)

Validity of self-reported weight - A study of urban brazilian adults*Maria I. Schmidt**, Bruce B. Duncan**, Mário Tavares***, Carísi A. Polanczyk****, LúciaPellanda****, Paulo M. Zimmer****SCHMIDT, M.I. et al. Validity of self-reported weight - A study of urban brazilian adults. Rev.Saúde Pública, 27: 271-6, 1993. In order to evaluate the validity of self-reported weight for use inobesity prevalence surveys, self-reported weight was compared to measured weight for 659 adultsliving in the Porto Alegre county, RS Brazil in 1986-87, both weights being obtained by atechnician in the individual's home on the same visit. The mean difference between self-reportedand measured weight was small (-0.06 +/- 3.16 kg; mean +/- standard deviation), and thecorrelation between reported and measured weight was high (r=0.97). Sixty-two percent ofparticipants reported their weight with an error of < 2 kg, 87% with an error of < 4 kg, and 95%with an error of < 6 kg. Underweight individuals overestimated their weight, while obeseindividuals underestimated theirs (p<0.05). Men tended to overestimate their weight and womenunderestimate theirs, this difference between sexes being statistically significant (p=0.04). Theoverall prevalence of underweight (body mass index < 20) by reported weight was 11%, bymeasured weight 13%; the overall prevalence of obesity (body mass index [] 30) by reportedweight was 10%, by measured weight 11%. Thus, the validity of reported weight is acceptable forsurveys of the prevalence of ponderosity in similar settings.Keywords: Obesity, epidemiology. Body weight. Validity.IntroductionSelf-reported weight, if accurate, can be veryuseful in public health studies, obviating the needfor in the field weight measurements. However thevalidity of this information has not been uniformlyaccepted6,8,9,11,13,14.A literature search covering the last ten yearsrevealed no study evaluating the accuracy of self-reported weight in an adult Brazilian population.The purpose of this study is to evaluate the va-lidity of reported personal weight in a survey of659 adults living in Porto Alegre, RS, in 1986-87,by comparing the self-reported weight with ameasured weight, both obtained in the individual'shousehold during the same interview.MethodologyData were collected in the Porto Alegre county(Brazil) as part of a Pan-American Health Organi-* Research supported by the Pan-American Health Organ-ization, CNPq (Conselho Nacional de DesenvolvimentoCientífico e Tecnológico) and FAPERGS (FundaçãoAmparo à Pesquisa do Estado do Rio Grande do Sul).** Department of Social Medicine of the Faculty of Medi-cine (UFRG) Porto Alegre, RS-Brazil*** Médico**** Clinic Hospital of Porto Alegre-Porto Alegre, RS-BrazilReprints: M.I. Schmidt - Rua Ramiro Barcelos, 2600/414 -90035-003 - Porto Alegre, RS - Brazilzation multicenter study of non-communicabledisease risk factors10. The methodology used inthis study has been reported in greater detail previ-ously3. In brief, a sample of 1,157 men and wom-en between 15 and 64 years of age were inter-viewed in 1986 and 1987 in randomly selectedhouseholds in selected census zones of the city.Weight was measured during the chronologicalsecond half of the data collection. During this peri-od, 699 individuals were interviewed. Of these, 39did not have a self-reported weight, and one did nothave his height recorded. Thus, analysis here wereperformed on the remaining 659 individuals. Smalldifferences in the total number of individuals incategories of race and educational level reflect twomissing values for each of these variables.Educational achievement, measured by thenumber of grades completed, was categorized onthe basis of Brazilian educational levels. Race wasassessed by the interviewer and, for the purposesof this study, the categories of black and "mulato"(a mixed white and black appearance) weregrouped together as black.Weight was always self-reported before beingmeasured. Measurement was performed to thenearest kilogram using a portable bathroom scalewith individuals dressed lightly with shoes re-moved; height was assessed to the nearest centi-meter using a metallic tape. Scales were regularlycalibrated according to a pre-established protocol.Categories of ponderosity were defined using arecommended2,5,15 body mass index (BMI: weightin kilograms/height in meters squared) cutpointsof underweight (BMI < 20), normal (20 < = BMI< 25), overweight (25 <= BMI < 30), and obese(BMI > 30).Error in the reporting of weight was calculatedas the difference between self-reported and meas-ured weight, negative differences thus represent-ing underestimation and positive differences over-estimation. The statistical significance of thesedifferences was tested by paired t-tests. The corre-lation of self-reported weight with measuredweight was expressed using Pearson's correlationcoefficient1. The association of various factorswith under and overreporting of weight was evalu-ated through multiple linear regression modellingof the differences between reported and measuredweight as a function of these factors7.Error in the classification of individuals as un-derweight, overweight and obese on the basis ofself-reported weight was evaluated through contin-gency tables comparing self-reported weight withmeasured weight classifications. The sensitivityand specificity of using self-reported weight inthis process was calculated along with appropriateconfidence intervals4. Sex-specific linear regres-sions were used to evaluate the degree to whichtrue (measured) weight can be predicted on the ba-sis of reported weight7.ResultsAs seen in Table 1, 62% of the 659 individualsstudied were women, slightly more than half under35 years of age, 72% white, and most with either aprimary or secondary school educational achieve-ment. Thirteen percent were underweight, 48% ofnormal ponderosity, 28% overweight and 11%obese.The overall comparison of self-reported andmeasured weight (Table 1, bottom) showed thetwo measurements to be remarkably similar. Themean of the differences between the weights ob-tained by the two techniques was -0.06 kg, withstandard deviation of 3.16 kg. The correlation be-tween the two, for the whole sample, was r=0.97.Figure 1 illustrates the overall distribution ofthe differences between self-reported and meas-ured weight. Approximately 62% of the sample re-ported their weight with an error of less than 2 kg.The largest underreporting overall was 28 kg (1case, a 54 - year old, obese woman); the largestoverreporting 12 kg (1 case, a 54 - year old, over-weight woman). But large differences were rare.In fact, only 13% of the sample reported differenc-es equal to or greater than 4 kg, and only 5% dif-ferences equal to or greater than 6 kg.Table 1 also presents comparisons of self-reported and measured weight across categories ofsex, age, race, educational attainment, and ponde-rosity. Correlations between self-reported andmeasured weight are high in all categories consid-ered, ranging from 0.88 to 0.99. Examining thedifferences between self-reported and measuredweight, one finds very similar results for men andwomen, perhaps with a small tendency for men tooverreport (mean = 0.30 kg, p < 0.1) and for wom-en to underreport (mean = -0.29 kg, p < 0.1) theiractual weight. Even smaller differences are foundacross age and race categories. Greater differencesare found when examining the data across educa-tional and ponderosity categories, although statisti-cally significant differences were found only forthe latter. Underweight individuals overreporttheir weight on average 1.2 kg (p<0.05); over-weight individuals tend to underreport their weighton average 0.5 kg (p<0.1), and obese individualsunderreport their weight by 1.3 kg (p < 0.05).As men and women, in general, err in differentdirections in their reporting of weight, as do un-derweight as compared to overweight individuals,mean differences between reported and measuredweight were calculated separately for men andwomen by ponderosity category. Among men, theunderweight overreported their weight by 1.2 kg,those with normal ponderosity by 0.42 kg, and theoverweight by 0.22 kg, while obese men underre-ported their weight by 1.4 kg. Among women, theunderweight overreported their weight by 1.2 kg,while those with normal poderosity underreportedtheir weight by 0.07 kg, the overweight by 1 kgand the obese by 1.2 kg.In multiple linear regression modelling ofweight difference, after controlling for measuredweight, only height, age and sex associated withsignificantly altered reported weight. In thismodel, for each centimeter of height, the differ-ence between reported and measured weight was0.05 kg greater (p<0.01); for each year of age0.02 kg greater (p=0.02). Males reported aweight 0.67 kg lower than that reported by fe-males (p=0.04). Neither race, educationalachievement nor second-order interaction termswere significantly associated with discrepant re-ported weight in these analysis.Figure 2 demonstrates the distribution of differ-ences between self-reported and measured weightacross categories of ponderosity. Consistent withthe data in Table 1, this figure shows that the leanoverreport their weight while the overweight un-derreport theirs. Additionally, the figure showsthat the larger absolute differences tended to be re-ported by the overweight and the obese.In order to evaluate the error associated withself-reported weights for the classification of indi-viduals in different categories of ponderosity, sen-sitivity and specificity were evaluated using meas-ured weight as a gold standard. Table 2 shows thatthe error was small and had as a main effect thegeneration of lower sensitivity indices.Sensitivity was the lowest for the underweightclassification - 28% of those underweight bymeasurement were not so by reported weight.False positive rates were very low for all catego-ries, as indicated by the high specificity seen.To evaluate the impact of these false negativeand false positive rates in the estimation on the fre-quency of ponderosity categories, Table 3 com-pares prevalences calculated using self-reportedweight with those determined using measuredweight. As was to be expected, given the fact thatthe sensitivities were considerably lower than thespecificities, the net result was one of underestima-tion of the frequency of abnormal poderosity. Theeffect was small, 1-2%, for the abnormal categories.Measured weight was predicted, in women, bythe equation: Measured weight (kg) = 2.10 + 0.97reported weight (kg) with R 2 =95. In men, theequation was: Measured weight (kg) = -0.58 +1.00 reported weight (kg), with R2=0.98. In thesesex-specific models, no other factor significantlypredicted measured weight.DiscussionThe validity of self-reported weight of adults15-64 years living in Porto Alegre, Brazil, in1986-7, appears to be very similar to that reportedfor adults living in the United States and Fin-land6,8,9,11,13,14. Although comparability betweenstudies is somewhat limited by differences instudy design or analysis, the following statementscan be made. In this study, differences betweenreported and measured weight greater than orequal to 4 kg were seen in 13% of the sample,compared with 11% in the Finnish study6. In thePorto Alegre study, differences greater than orequal to 2 kg occurred in 38% of the sample, andgreater than or equal to 3 kg in 22%, in compari-son with differences greater than or equal to 5 lbs(2.3 kg) in 37% of the Minnesota Lipid ResearchClinic11 sample. The remaining studies8,9,13,14 ei-ther were not population-based or did not reportdata on the distribution of these differences.In a society where few homes possess a weight-scale, this finding of relatively similar accuracy ofweight reporting may be in part explained bywidespread use of drug store scales, and, perhapsto a lesser extent, to fairly frequent medical con-tact in Porto Alegre.It is also important to note that these data, col-lected in households at the same moment in time,derive from a community-based study rather thanfrom one with a more restricted base, such as thatof participants or volunteer potential participantsin clinical studies or of applicants for life insu-rance6,8,9,11,13,14. Thus, it is less likely that the par-ticipants in this study would have specific reasonsfor misrepresenting their own weight or havingunusually recently measured it, or that their weightshould have changed during the time lapse that oc-curred between its report and measurement.Before discussing the implications of the differ-ences encountered between self-reported andmeasured weight, it is important to note their po-tential causes. The cause could be a problem ofknowledge, that is to say, the person does notknow his current weight. Or the problem could berelated to the person's body image. Here, regard-less of how regularly or accurately he measureshis weight, the person erroneously forms an ideaof his weight in accordance with an internally de-sired body image. Finally, the problem could be atthe level of weight reporting - even a person withaccurate knowledge, consistent with his body im-age, may be led by factors such as cultural normsor personal psychological needs to report hisweight inaccurately to an interviewer.The finding here presented - that self-reportedweight is biased towards what appears to be a cul-turally ideal or a desirable body weight - is alsoconsistent with the literature6,8,9,11,13,14: over-weight individuals underreport while underweightindividuals overreport their body weight. Whetherthe bias occurs at the level of body image, or atthat of reporting, cannot be safely judged from thisstudy nor from the previous ones. However, it isunlikely that mere differences in acquired knowl-edge of measured weight can explain the differen-tial biases seen here by sex and ponderosity cate-gory. The fact that men tended to overreport theirweight while women to underreport theirs proba-bly reflects gender specificity in cultural ideals ofweight, be they acting at the level of body imageor misreporting. In this regard, results here aresimilar to those of the Lipid Research Clinic popu-lation in the USA11. Notably however, women inthe USA sample tended to underreport across allcategories of ponderosity, whereas in Porto Ale-gre, this underreporting occurred in significantlyonly for the overweight and the obese. This differ-ence may well reflect some cross-cultural varia-tion in the concept of "ideal weight".Other factors associated with discrepancy inself-reported weight, consistent with the literature,were height and age. These associations were ofsmaller magnitude: for example, for each decadeincrease in age, overreporting increased by 0.2 kg;for each 10 cm increase in height, 0.5 kg.What are the implications of these findings forfield studies conducted in similar settings? If thepurpose of such a study is to estimate the weightof groups of individuals, error in populationweight parameter estimates should be small. Fur-thermore, it is possible to use the equations herepresented to adjust the self-reported weight. Simi-larly, if the purpose is to describe prevalences of cat-egories of ponderosity based on self-reportedweight, the error in underestimation should also besmall. It is possible to use the sensitivity and speci-ficity here presented to produce an adjusted preva-lence for each ponderosity category 12.It must be noted, however, that these data donot address the effect or weight mis-reporting inanalysis whose objective is to evaluate the associa-tion of weight with other variables.Finally, it was concluded that the validity ofself-reported weight is sufficient to permit its usein surveys of the prevalence of obesity and otherweight categories in settings similar to the onehere studied.SCHMIDT, M.I. et al. [Validade do peso auto-referido.Estudo de população urbana de adultos, Brasil]. Rev,Saúde Pública, 27: 271-6, 1993. Para avaliar a validadedo peso auto-referido em inquéritos de prevalência deobesidade, o mesmo foi comparado com o peso medidode 659 adultos, residentes em Porto Alegre, RS, Brasil,em 1986-87. Ambos os pesos foram obtidos por entre-vistador, na casa do participante, na mesma ocasião. Amédia das diferenças entre peso auto-referido e pesomedido foi pequena (-0,06 +/-3,16 kg; média +/- desviopadrão) e a correlação entre eles alta (r=0,97). Sessentae dois por cento dos participantes referiram seu pesocom erro < 2 kg, 87% com erro menor do que 4 kg e95% com erro < 6 kg. Indivíduos de baixo peso hiperes-timaram seu peso, o oposto ocorrendo com indivíduosobesos (p<0,05). Os homens tendiam a superestimar seupeso, o oposto ocorrendo com as mulheres (p=0,04). Aprevalência geral de baixo peso (índice de massa corpo-ral (IMC) < 20) por peso auto-referido foi de 11% e apor peso medido, de 13%; a prevalência geral de obesi-dade (IMC > 30) por peso referido, foi de 10% e a porpeso medido, de 11%. Concluindo, a validade do pesoauto-referido é aceitável para inquéritos de prevalênciarealizados em contextos similares.Descritores: Obesidade, epidemiologia. Peso corporal.Validez.References1. BERQUÓ, E.S.; SOUZA, J.M.P. de; GOTLIEB.S.L.D. Bio-estatística. São Paulo, Ed. Pedagógica e UniversitáriaLtda., 1981.2. BRAY, G.A. Classification and evaluation of the obesities.Med. Clin. N. Am., 73:161-84, 1989.3. DUNCAN, B.B.; SCHMIDT, M.I; POLANCZYK, C.A.;HOMRICH, C.S.; ROSA, R.S.; ACHUTTI, A.C. Fa-tores de risco para doenças não-transmissíveis emárea metropolitana na região sul do Brasil: Prevalên-cia e simultaneidade. Rev. Saúde. Pública, 27: 43-8,1983.4. FLETCHER, R.H; FLETCHER, S.W.; WAGNER, E.H. Epi-demiologia clínica, bases científicas da conduta médi-ca. Porto Alegre, Ed. Artes Médicas, 1989.5. GARROW, J.S. Índices of adiposity. Nutr. Abstr. Rev. Ser.A,. 5 3: 6 9 7 -7 0 8, 19 83.6. JALKANEN, L.; TUOMILEHTO, J.; TANSKANEN, A.;PUSKA, P. Accuracy of self-reported body weightcompared to measured body weight, a population sur-vey. Scand. J. Soc. Med., 15:191-8,1987.7. KLEINBAUM, D.G.; KUPPER, L.L.; MULLER, R.E. Ap-plied regression analysis and other multvariable meth-ods. Boston, PWS - KENT, 1988.8. PAGANINI-HILL, A. & ROSS, R.K. Reliability of recall ofdrug usage an other health-related information. Am. J.Epidemiol, 116:114-22, 1982.9. PALTA, M.; PRINEAS, RJ.; BERMAN, R.; HANNAN, P.Comparison of self-reported and measured height andweight. Am.J. Epidemiol., 115:223-30, 1982.10. PAN AMERICAN HEALTH ORGANIZATION. Health ofadults and the elderly. In: Health conditions in theAmericas. Washington, Pan American Health Organiza-tion, 1990. (Scientific publication no. 524)P.11. PIRIE, P.; JACOBS, D.; JEFFREY, R.; HANNAN, P. Dis-tortion in self-reported height and weight data. J. Am.Diet. Assoc.78: 601-6, 1981.12. ROGAN, W.J. & GLADEN, B. Estimating prevalencefrom the results of a screening test. Am. J. Epidemiol.,107:71-6, 1978.13. SCHLICHTING, P.; HOILUND-CARLSEN, P.F.;QUAADE, F. Comparison of self-reported height andweight with controlled height and weight in women andmen. Int. J. Obesity, 5:67-76, 1981.14. STUNKARD, A J. & ALBAUM, J.M. The accuracy of self-,reported weights. Am.J. Clin.Nutr. ,34:1593-9,1981.15. WHO STUDY GROUP ON DIET, NUTRITION AND PRE-VENTION OF NONCOMMUNICABLE DISEASES,Geneva, 1989. Report. Geneva, 1990. (Who TechnicalReport Series, 797).Recebido para publicação em 17.12.1992Aprovado para publicação em 4.6.1993

Validade do peso auto-referido: estudo de população urbana de adultos, Brasil

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Validity of self-reported weight: a study of urban brazilian adults

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