Rating scales can have many characteristics, including the following:

• Type of scales - The scale can be formatted in a number of ways, including as a Likert scale, a semantic differential scale, a magnitude estimation scale, or a continuum.
• Scale size - The scale size refers both to the overall length of the scale as well as whether there are an odd or even number of options (an odd number would have a middle point). Research suggests between 5-9 scale points, depending on the topic and the skills of the respondents (Krosnick and Fabrigar, 1997).
• Type of labels – Scales can have either some or all of their responses labeled. Labels can be text or numbers. Numbers may suggest regular intervals between response options, but they can also affect the respondents’ selections (respondents tend to avoid negative numbers).

Types of Scales

Likert scales are probably the most common type of scale in usability questionnaires, bipolar with a range from positive to negative responses to a question.

Semantic differential scales have options labeled at each end. The Questionnaire for User Interaction Satisfaction (QUIS) published by the University of Maryland uses this format.

In magnitude estimation, respondents do not use a pre-defined scale, but rather are instructed to provide ratings proportionate to a baseline rating. The only requirement is that the ratings be positive. McGee (2003, 2004) provides details about this method, which he used successfully to evaluate the perceived usability of software.