While there was error in the five point SUS it is unclear how much of an impact this actually has on the final SUS score since the study used different systems for each scale.While this seems like compelling evidence to always use 7-point over 5 scales there are two tempering factors. In contrast, when a 7-point version of the SUS was used there was no interpolating for any of the 840 ratings. In 2.5% of the 858 responses users provided responses between two points (95% CI between 1.6% to 3.9%). The users were allowed to “interpolate” or pick between points such as 3.5. Is there too much response error with SUS?Ī recent article tested the SUS response error by counting the number of times users couldn’t decide between two points. The most popular usability questionnaire is the System Usability Scale (SUS) which uses ten five-point scales. If this happens a lot then there aren’t enough scale points. For example, if users think a 5 is too high and a 4 is too low they are forced to settle on an option which is higher or lower than they wanted (assuming they can’t pick a 4.5). If there aren’t enough response options users will be forced to choose the next best alternative and this introduces measurement error.
So what are the consequences of this? Users need choices Having seven points tends to be a good balance between having enough points of discrimination without having to maintain too many response options.
The psychometric literature suggests that having more scale points is better but there is a diminishing return after around 11 points ( Nunnally 1978). The short answer is that 7-point scales are a little better than 5-points-but not by much. Is one better? 7-point scales are slightly better Most questionnaires I’ve examined either use five point scales or seven-point scales. If you’ve ever designed a survey or questionnaire you’ve probably wondered how many points the response options should have.
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