*Geek Box: Visual Analogue Scales
Visual analogue scales [VAS] are common psychometric tools in both psychology and nutrition research, used to measure a particular preference, attitude, or characteristic that may have a range of potential values. For example, hunger and fullness are both subjective, and may differ across a range of responses depending on time of day, fed or fasted, time since last meal, etc.
The simple form of VAS used is a straight, horizontal line, commonly around 100mm [10cm] in length. The far left will generally represent the lowest end of the variable being measured, i.e., with fullness 0mm could be ‘not at all’ while 100mm could be ‘extremely’. Participants are asked to make a vertical line with a pen/pencil/marker across the horizontal measurement line, at a point which represents for them how they feel in response to that question.
This is then turned into data the old-fashioned way: by getting out a ruler, and measuring the point at which the participant made their line crossing the horizontal measurement line. For example, on a 100mm [10cm] hunger scale it could be 70.6mm [7.6cm] – this is therefore the data point for that time. Now, imagine that was pre-meal, and then you measure the participant again 30mins later: now it could be 20.3mm [2.3cm], indicating a shift to a more satiated state following the meal.
VAS can be useful for certain measures that exist on a subjective continuum, rather than using, for example, Likert scales where participants could have a 1-5 score of pre-defined values, e.g., ‘mild’, ‘moderate’, ‘severe’. In this regard, VAS can be more sensitive to smaller, incremental changes in a particular measure than other categorical scales They are also very simple to use and time-efficient for research purposes. They do have certain disadvantages or things to consider, however.
They are primarily subjective, which is not limitation per se, but an important point to bear in mind when the outcome is a variable that could also have physiological measures. They are also validated in very specific contexts, which may not always be generalisable to every circumstance in which they are used. So it is important to think about the validation of a particular VAS, the context in which that validation occurred, and how that context relates to its use in another study.