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 individual storing information about these dimensions in the first instance and then being able or willing to report these details accurately.
The responses derived from subjective methods can therefore 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.
Physical activity consists of multiple quantitative and qualitative dimensions; there is no method which can capture each of these simultaneously and with full detail. Consequently, understanding the error associated with subjective methods is difficult because there is a lack of a true field based criterion against which to compare.
Participants (or a proxy-reporter) report physical activity during a specified time period, which can range from as little as one day to a lifetime. Instruments vary greatly in terms of the scope and detail with which they record the different dimensions of physical activity ( frequency, intensity, duration, posture, type, and domain/context).
The information provided by the respondent is used in conjunction with additional information, such as estimations of the energy costs of the activity , to generate physical activity target variables.
An example of the stages of inference in predicting a physical activity target variable through subjective measurement is shown in Figure P.2.1
Figure P.2.1 Example of stages of inference for a subjective method of physical activity assessment. Source: Corder et al., 2008.
Subjective methods for physical activity assessment can be broadly grouped into the following categories: