Estimates of diet, physical activity or anthropometry should be as close to the true value as possible. The process of estimating this true value involves several stages of inference. Errors can be introduced at each of these stages, weakening the inference, and resulting in estimated values which do not fully represent the true values. An example of these stages of inference is shown in Figure C.1.1, which is split into two levels:
The data acquisition level, which can be further divided into:
The time-independent level, which can be further divided into:
Figure C.1.1 Stages of inference involved in predicting the true value by measurement using the tool and overall method. Adapted from [ 2 ].
A tool, sometimes an ‘instrument’, refers to the mechanism by which data are initially acquired in the data acquisition phase (see Figure C.1.1 above). Tools produce measurements.
There are often different tools which fall under the same category, for example, there are different examples of a food frequency questionnaire, such as the Harvard or Willett questionnaire  and the Block questionnaire . Similarly, there are variations in the technical specifications of different accelerometer models used to measure physical activity.
A method is a term given to the combination of the tools used in the data acquisition level, and how the measurements are used to derive estimates during the time-independent level.
Measurements vs. estimates
A tool produces a measurement (e.g. of number of servings of fruit per day), whereas a given method provides an estimate of a target variable of interest (e.g. vitamin C intake) through a series of inferential steps. In some circumstances, the measurement is used in its raw form (e.g. height measurement), and the estimate and measurement are the same.
The way in which measurements by tools are used to derive estimates of the target variable of interest varies by study and sometimes within a study. A single tool can consequently be a component of multiple different methods that produce different target variables.
Additional information as components of methods
Additional information, not captured by the tool itself, can also form a part of the overall method. This can include:
Tools vs. brands
Some tools are developed by companies and assigned brand and/or model names. For example, there are many different brands of accelerometers for measuring physical activity, and each of these brands might have various models with different features. Similarly, some anthropometric tools are branded (e.g. Seca), as are some dietary analysis software products.