Subjectively assessed waist and hip circumferences are often used in large-scale epidemiological studies to describe body fat distribution when the objective measurements are not feasible.
Individuals self-report their waist and hip circumferences or it is reported on their behalf by a proxy reporter. Examples of frequently used questions:
Accuracy of the subjective waist and hip circumferences may be higher if participants are provided with clear instructions and a standard tape measure; otherwise participants may report circumferences based on their clothing sizes, which is less accurate. Detailed instructions such as taking the measurement after breathing out normally, with the stomach relaxed and not pulled in may also contribute to greater accuracy.
Selection of data source
Data can be collected by interview, by self-administered questionnaire, or by proxy reporter (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 sub-sample of the main study cohort may provide useful information to quantify the errors/bias; corrections can then be carried out at the analysis stage (see below).
If collecting repeated subjective data at different time points, their validity should ideally be monitored as this may vary over time.
Units of measurement
Unit of the measurement (e.g. metric units or imperial scale) should be clearly defined to avoid discrepancy in recorded waist and hip. Incorrect conversion may lead to substantial errors. Units should be appropriate for the population of interest, possibly providing a choice of alternatives.
Waist and hip circumferences, and calculated waist to hip ratio (waist divided by hip) are often used in epidemiological studies as estimates of central and peripheral fat and relative fat distribution.
It is possible to apply statistical methods that correct for or take into account errors if relevant validation data are available. Predictive equations for objectively measured circumferences using data from a random sample of the main cohort can be derived and applied to the whole cohort. For height and weight there are available prediction equations, but these are not widely available for waist and hip.
An overview of subjective waist and hip circumference methods is outlined in Table A.2.10.
Table A.2.10 Characteristics of subjective waist and hip circumference 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|
The accuracy of circumference measures varies with body size, age, sex, education and socio-economic status. ‘Figure consciousness’ also appears to affect the reporting of these data, especially in men. Considerations relating to the use of waist and hip methods in specific populations are described in Table A.2.11.
Table A.2.11 Use of subjective waist and hip circumference methods in different populations.
|Infancy and lactation|
|Toddlers and young children|
|Adolescents||Younger people tend to more frequently underreport their waist and hip circumferences compared to older groups.|
|Adults||Men tend to underreport waist circumference more frequently than women.|
|Other||Overweight individuals tend to underreport waist and hip circumferences more frequently compared to leaner individuals.
Underestimation of waist circumference is also more frequent among participants with lower educational level and social class.
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