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DAPA Measurement Toolkit

Objective methods introduction

  • Objective methods do not rely on written or verbal responses from the individual (or his/her proxy), but instead record phenomena from which dimensions of diet, physical activity or anthropometry can be inferred.
  • The phenomena can be anthropometric, physiological, kinematic, biochemical, or environmental in nature.
  • Technology is often used to capture these variables directly.
  • Direct observation can also capture some of these phenomena. This method sits on the boundary between subjective and objective methods, since although observers make subjective records, they are typically independent from the person under study. However, when the observer is a proxy-reporter (e.g. teacher or parent), observations may not be fully independent from the person under study.
  • The initial raw measurement by the tool is normally then subject to a sequence of inferential steps which result in estimates of diet, physical activity or anthropometry.
  • Objective methods minimise issues relating to respondent bias, such as recall errors and social desirability.
  • 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.
  • 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.
  • Objective methods may require specialised training.
  • Participant’s consent is essential, as always. Depending on the mode of assessment (e.g. blood drawing, video recording, prolonged observation), willingness to participate may vary and be a source of selection bias.
  • Depending on the instrument used, individual’s recognition of being recorded may alter behaviours, a ‘reactivity bias’ that may be related to social desirability.
  • In addition, objective measures each possess their own limitations and no single "gold-standard" exists.

To capture dietary exposure via objective methods, it is important to clarify a specific aim of dietary assessment, respondent burdens, expense, and quality control. A specific aim could be:

  • To capture an average of dietary exposure in a population (e.g. in a national survey)
  • To rank individuals based on a habitual dietary exposure in a study population (epidemiological research)
  • To determine absolute intakes of certain food or nutrient intakes (epidemiological or dietetics research)

A tool that could be useful for one specific aim may not be useful at all for another aim. Respondent burdens are to be considered. Taking a blood specimen to assay objective biomarkers is quick, but causes pain and discomfort. Video-recording an individual’s eating during a meal time may be more unpleasant than he or she thought at the time of signing a consent form.

Objective recording is often expensive. Both data collection and processing can be costly, and special training is needed to quantify levels of exposure. Quality control of objective methods is also needed for example by conducting the same assessment with a controlled sample. Because of the challenge to implement objective assessment, the approach is taken often in a small scale and often used for validation of subjective methods then used in much larger studies.

Objective methods for dietary assessment can be broadly grouped into the following categories:

  • Direct observation: recording what and how much people are consuming. The target can be a specific individual, a specific family, and a specific dining occasion (e.g. cafeteria).
  • Duplicate diets: recording or assessing individuals’ diets, collecting or preparing as identical meals as possible, and assessing the chemical compositions.
  • Biomarkers: collecting bio-specimens and assaying chemical compositions that reflect dietary exposure, at least partly.

Details are documented separately for each approach.