• What is assessed?
• How is the measurement conducted?
• When is this method used?
• How are estimates of weight and height derived?
• Strengths and limitations
• Populations
• Resources required
• Instrument library
• References

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:

1. "How much do you weigh now?"
2. "What is your current weight?"

Examples of frequently used questions to collect subjectively measured height:

1. "How tall are you now?"
2. "What is your current 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)

Validation study

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.

Strengths

1. Low cost
2. Practical tool as it can be included in study questionnaires and be self-administered
3. Information can be obtained via mail, face-to-face or telephone interviews or the Internet
4. Can be obtained retrospectively
5. Low respondent burden
6. Large number of individuals can be approached
7. Non-intrusive
8. It could increase recruitment rates and participation retention, especially by individuals who are reluctant to be measured
9. No fieldwork required if self-administered or data collected online

Limitations

1. Prone to systematic errors in reporting by different body size and socio-demographic characteristics
2. Prone to response bias e.g. social desirability
3. May not be feasible in certain population where recall bias is high (e.g. misreporting of this measure is higher in older individuals when compared to younger individuals).
4. Corrections may be necessary at analysis point

Table A.2.7 Characteristics of subjective weight and height methods.

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.

1. Minimal resources are generally required with subjective methods as field work may not be necessary
2. Questionnaire via paper and pen or electronic device
3. Instructions for completion and return
4. For mailed questionnaires a pre-paid stamped address envelope
5. Trained interviewers for interviewer-administered tools, plus standard operating procedures for interviewers
6. Data entry cleaning code
7. Standard operating procedures for data entry errors/extreme values/data cleaning
8. Statistical knowledge may be required if correction factors are applied to the data

A method specific instrument library is being developed for this section. In the meantime, please refer to the overall instrument library page by clicking here to open in a new page.

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2. Connor Gorber S, Tremblay M, Moher D, Gorber B. A comparison of direct vs. self-report measures for assessing height, weight and body mass index: a systematic review. Obes Rev. 2007;8(4):307-26.
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