Introduction to Objective Methods

  1. Objective methods do not rely on written or verbal responses from the individual under study but instead record phenomena from which anthropometric dimensions can be inferred
  2. Technology is often used to capture these variables directly
  3. The initial raw measurement by the instrument is normally then subject to a sequence of inferential steps which result in anthropometric estimates
  1. Objective methods minimise issues relating to respondent bias, such as recall errors and social desirability
  2. As a result, these methods can provide more accurate estimates of diet, physical activity and anthropometry with less complex error structure; they are often used as criterion methods to demonstrate the validity of subjective methods, or other objective methods
  3. Some objective methods can be costly, intrusive, plus burdensome in terms of time and effort for the participant and researcher, sometimes rendering them more difficult to apply to large epidemiological settings
  4. Objective methods may require specialised training
  5. Participant’s consent is essential, as always. Depending on the mode of assessment (e.g. use of calipers, submersion etc.), willingness to participate may vary and be a source of selection bias
  6. Depending on the instrument used, individual’s recognition of being recorded may alter behaviours, a ‘reactivity bias’ that may be related to social desirability
  7. In addition, objective measures each possess their own limitations and no single "gold standard" exists or not feasible in most settings (e.g. chemical analysis of the human cadaver)
  1. Objective methods for anthropometry and body composition vary greatly in terms of the dimensions assessed and the mechanisms/technologies used
  2. Vary from simple and cheap tape measure to expensive full body scanning (e.g. MRI or DEXA)
  3. Also great variation in suitability for use in field – ability to transport easily etc
  4. Often use well established:
    1. Principles (e.g. Archimedes in under water weighing)
    2. Equations (e.g. population specific equations)
    3. Assumptions (e.g. body component constants such as body water content used for calculating body fat)
  5. Estimates of body composition are only inferred from the initial measurement of a physiological or biochemical phenomenon; the inferences which follow contribute towards the overall method. The relationships between measured values and body composition estimates used to make such inferences (e.g. % body fat) may vary between individuals or populations, meaning population and age-specific methods may be required to increase validity of assessment
  6. Some simple anthropometric measures require no additional processing and can be used in the raw format (e.g. height or weight)
  7. Anthropometry conducted in a laboratory or clinical setting normally provide estimates also valid in outside that location; this is in contrast with laboratory assessment of diet or physical activity, which may not provide valid estimates of free-living behaviour
  8. Although ideally maximised, acceptable standards of accuracy and methodological rigour may vary according to the purpose of the assessment, with differences likely across laboratory, clinical and field settings

Objective methods for anthropometric assessment can be used to measure the following:

  1. Weight
  2. Height
  3. Waist and hip circumference
  4. Birth weight
  5. Body shape/image
  6. Body composition (e.g. fat distribution, visceral fat, lean mass)

The following considerations should be addressed when using objective methods for anthropometric assessment:

  1. Environment: have a safe, secure and private environment for the volunteer whenever possible. A calm environment is very important when measuring infants and toddlers. Distraction techniques like the use of bubbles are useful to occupy young children. Minimise the risk of interruptions and have no more people present than are strictly necessary.  Ensure conversations remain confidential and are not overheard by ‘passers-by’.  Consider whether the gender of the observer or field worker is likely to increase anxiety in certain population groups.
  2. Introduction: inform the volunteer of who you are and your role in the measurements. Volunteers should be informed prior to measurements with adequate and clear information to allow them time to consider, and the opportunity to ask questions.
  3. Information: at the point of measurement, explain the procedure clearly in advance to the volunteer, providing a rationale for the measurement when appropriate. If possible provide a demonstration or show the equipment. Demonstration of the measurement can help when measuring children as they may be more concerned of the procedure than an adult.
  4. Interaction: More than almost any other anthropometric variable, body weight is associated with feelings of self-worth, body dissatisfaction and social anxiety. This is especially true for those who are very thin, obese and adolescents.  For these individuals, the process of being weighed is likely to induce acute emotional distress. Consider starting with the least sensitive or invasive measurements (e.g. height). This will help develop a rapport with the participant and minimise anxiety and distress.  Take care to use neutral language, avoiding judgemental terms such as ‘fat, overweight, obese, skinny, thin’.