Visual Analogue Scale

Original Editor - Venus Pagare

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Introduction

A Visual Analogue Scale (VAS) is a measurement instrument that tries to measure a characteristic or attitude that is believed to range across a continuum of values and cannot easily be directly measured. It is often used in epidemiologic and clinical research to measure the intensity or frequency of various symptoms. For example, the amount of pain that a patient feels ranges across a continuum from none to an extreme amount of pain. From the patient's perspective this spectrum appears continuous ± their pain does not take discrete jumps, as a categorization of none, mild, moderate and severe would suggest. It was to capture this idea of an underlying continuum that the VAS was devised.

Structure and Orientation of the Scale

VAS can be presented in a number of ways,including:

• scales with a middle point,graduations or numbers (numerical rating scales),
• meter-shaped scales (curvilinear analogue scales),
• "box-scales,"scales consisting of circles equidistant from each other(one of which the subject has to mark),and
• scales with descriptive terms at intervals along a line (graphic rating scales or Likert scales.

Numerical rating scales or number scales consist of numbers without a line,although the term is also sometimes used to refer to graphic rating scales.

The most simple VAS is a straight horizontal line of fixed length, usually 100 mm. The ends are defined as the extreme limits of the parameter to be measured (symptom,pain,health)orientated from the left (worst) to the right (best). In some studies,horizontal scales are orientated from right to left ,and many investigators use vertical VAS. No difference between horizontal and vertical VAS has been shown in a survey involving 100 subjects but other authors have suggested that the two orientations differ with regard to the number of possible angles of view. Reproducibility has been shown to vary along a vertical 100-mm VAS and along a horizontal VAS. The choice of terms to define the anchors of a scale has also been described as important.

Administration

• They are generally completed by patients themselves but are sometimes used to elicit opinions from health professionals.
  
•  The patient marks on the line the point that they feel represents their perception of their current state.
• The VAS score is determined by measuring in millimetres from the left hand end of the line to the point that the patient marks.

Merits and Demerits

• VAS is more sensitive to small changes than are simple descriptive ordinal scales in which symptoms are rated,for example, as mild or slight,moderate,or severe to agonizing.
• These scales are of most value when looking at change within individuals
• However,  assessment is clearly highly subjective
• Are of less value for comparing across a group of individuals at one time point.
• It could be argued that a VAS is trying to produce interval/ratio data out of subjective values that are at best ordinal.

Thus, some caution is required in handling such data.