• General principles of objective methods
• Key issues relating to use of objective methods
• Types of objective methods for dietary assessment

1. Objective methods do not rely on written or verbal responses from a study participant (or their proxy) but instead record phenomena from which dimensions of diet, physical activity, or anthropometry can be inferred.
2. The phenomena can be anthropometric, physiological, kinematic, biochemical, or environmental in nature.
3. Technology is often used to capture these variables directly.
4. Direct observation can also capture some of these phenomena. In terms of classification, this method sits between subjective and objective methods since the observers make subjective records, but they are typically independent of the person under study. However, when the observer is a proxy-reporter (e.g. teacher or parent), observations may not be fully independent of the person under study.
5. 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.

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 assess the validity of other subjective or objective methods.
3. Objective methods can be costly, intrusive, and 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 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.
6. Depending on the instrument used, an individual’s recognition of being recorded may alter their 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.

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

1. Direct observation
2. Duplicate diets
3. Nutritional biomarkers