Subjective methods to assess weight and height can be used instead of objective measurement, when that is not practical.
Individuals either self-report their weight/height or it is reported on their behalf by a proxy-reporter.
Examples of frequently used questions to measure weight subjectively:
Examples of frequently used questions to collect subjectively measured height:
Selection of data source
Data can be collected by interview, by self-administered questionnaire, or by an independent observer (e.g. friend or family member). Questions can be administered using pen and paper or an electronic device such as a mobile phone, tablet or computer, either face-to-face or remotely (e.g. by post or internet)
When studies rely on subjective methods, a validation study in a subset of the main study cohort may be informative to quantify potential sources of errors/bias and to calculate correction factors that can be applied at the analysis stage (see below).
If collecting repeated subjective data at different time points e.g. in longitudinal studies, their validity must be monitored by collecting direct measures at regular intervals as validity may vary overtime.
Units of measurement
Unit of the measurement (e.g. metric units or imperial scale) should be clearly indicated. Incorrect conversion may lead to substantial errors. Units should be appropriate for the population of interest, possibly providing a choice of alternatives.
Subjective assessment of weight and height are often used in large-scale population studies, and can be useful options for individuals who are reluctant to be measured.
Caution should be used when interpreting results from subjective assessment of weight and height as misreporting could result in BMI misclassification and lead to inaccurate estimates of the prevalence of overweight and obesity. It is possible to apply statistical methods that correct for or take into account such errors if relevant validation data are available.
Analysis could be performed by ranking study participants in quantiles of BMI calculated from subjective height and weight; the ‘true’ mean or median value of BMI in each quantile can then be calculated from the objectively measured values for a random sample of participants and used to estimate the association of BMI with disease risk.
Adjustment of subjective BMI scores, based on easily gathered socio-demographic characteristics (gender, age, race/ethnicity, marital and pregnancy status, and household income) can also be used. Predictive equations for weight and height using objective data from a random sample of the main cohort can also be derived and applied to the whole cohort. There are published equations for a number of different populations (1, 5, 7, 12). However, as the pattern of over- or under-reporting may be unique to each population, specific population correction factors may be required.
An overview of subjective weight and height methods is outlined in Table A.2.7.
Table A.2.7 Characteristics of subjective weight and height methods.
|Number of participants||High|
|Researcher burden of data collection||Low|
|Researcher burden of coding and data analysis||Low|
|Risk of reactivity bias||No|
|Risk of recall bias||Yes|
|Risk of social desirability bias||Yes|
|Risk of observer bias||Yes|
|Suitability for field use||High|
|Participant literacy required||Yes, if self-administered|
Considerations relating to the use of subjective weight and height methods in specific populations are described in Table A.2.8. Mis-reporting occurs across all level of body sizes.
Table A.2.8 Use of subjective weight and height methods in different populations.
|Infancy and lactation|
|Toddlers and young children|
|Adults||Underestimation of height parameter is generally higher in women than in men, probably due to a social desirability response. Men tend to overestimate height far more frequently than women.|
|Older Adults||Overestimation of height is greater in older populations.|
|Ethnic groups||Perception of body weight in relation to socially-defined weight norms may be different in middle income countries to high income countries. Cultural differences in awareness of body size, different cultural norms for social desirability, or differing views of body image may influence validity.|
|Other||Overweight individuals tend to underreport weight than lighter individuals.
Under-reporting of weight has also been observed in individuals with higher socio-economic status and education level.
Overestimation of height is greater in overweight people.
Shorter and thinner individuals also tend overestimate their heights, whereas tall people underestimate their height.
Overestimation of height has also been observed more frequently in individuals with lower socio-economic status and lower education level.
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