AS 2542.2.3:2014 Sensory analysis
6.2 Choice of unipolar or bipolar response scale
The polarity of a scale is defined by the location of the neutral or zero point:
– in a unipolar scale, the neutral or zero point is located at one end of the scale;
– in a bipolar scale, the neutral or zero point is located at the centre of the scale.
A bipolar scale is used when the intensity of a property can differ in either direction from a neutral or ideal value. For example, a bipolar scale may run from “Not sweet enough” to “Too sweet”, whereas a unipolar scale may run from “Not at all sweet” to “Extremely sweet”.
When constructing bipolar scales, an inappropriate choice of scale anchors can produce a scale that does not form a true continuum and has no logical centre point. Avoid the use of anchors that are not based on a single attribute (e.g. “dark brown” to “bright red”) unless they lie on a recognized sequence of stages or grades of the product.
6.3.1 Continuous scale
Assessors may be asked to make numerical responses on a continuous scale, meaning that numbers with fractional parts can be used. Line scales are typically 15 cm (6 in) long, labelled at each end with the extreme values of the attribute being assessed. The assessor responds by marking the line in the position corresponding to the perceived intensity.
The marked position is converted to a number by the analyst. A continuous scale gives assessors an opportunity to express small differences in judgement. On the other hand, the task may seem more difficult than using a category scale and transcription of the data takes longer unless an automatic data acquisition system is available.
6.3.2 Discrete scale In the case of discrete scales, it has been observed that:
– the smaller the number of categories, the greater the end effect, which therefore diminishes the discriminatory capacity of the scale (see [7]);
– assessors with little training consider a discrete (9-point) scale easier to use than a continuous (15 cm) scale (see [8]);