Diet, physical activity and anthropometry have multiple complex dimensions and as a result many instruments have been designed for a variety of purposes. Subjective methods rely on the information about these dimensions firstly being stored in the memory and the individual being able or willing to report these details accurately.
Therefore, an important disadvantage of subjective methods is their potential to be influenced by biases such as recall bias and social desirability bias. However, subjective methods generally have the following important advantages:
It is possible to capture detailed data using subjective methods, but there is typically a trade-off between detail and participant burden.
Subjective methods of anthropometric assessment are typically used for practical reasons. Different aspects of anthropometry are required for different types of study and various tools to measure these are available. Methods can aim to measure current or historical body dimensions.
Subjectively reported data are highly correlated with objectively measured dimensions but still prone to errors, which may vary by:
For instance, larger individuals tend to over-report height and under-report weight, resulting in an underestimated body mass index. When using subjective methods, careful interpretation of the data needs to be considered, and it is recommended that the validity of subjectively measured data is assessed against objectively measured values in a subsample of the main study. Correction factors can then be developed and applied to the data in the main study. An example of the stages of inference in predicting an anthropometric target variable through subjective measurement is shown in Figure A.2.1.