Which is the better “Likert scale” to be used in a heuristic checklist to be given to the evaluators to identify usability problems? Is it a 5 points or 7 points scale and why? The options extend from strongly disagree to strongly agree. I found some papers used 5 and others used 7 levels, but without mentioning why they choose that.
Answer
Research suggests more up to 11 or so is better. So 7>5. (http://www.measuringu.com/blog/scale-points.php) but not by a large enough degree to matter too much.
The short answer is that 7-point scales are a little better than 5-points—but not by much. The psychometric literature suggests that having more scale points is better but there is a diminishing return after around 11 points (Nunnally 1978). Having seven points tends to be a good balance between having enough points of discrimination without having to maintain too many response options. So what are the consequences of this?
This research also I believe neglects the impact of seeing all these extra buttons on the screen when you have 20 7-point questions vs. 20 5-point questions; it could lead the testee to lose concentration and start answering willy-nilly.
The big debate is really one of 5 vs 6. Give the user a safe haven or force them to take a side; I would lean towards odd here, if the user cares little enough that they want to choose a middle option then is their slightly bad/slightly good choice really so reliable or will it just skew the data?
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