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.
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?
At UXNZ 2015 one of the speakers that it doesn't matter, people are focused on the questions and not on the amount of options. She has worked with surveys for over 15 years.
There is a slight physiological difference between an even or odd number of options. An odd number of options gives you a 'safe haven' to go if you don't know or don't care. If there is an even number you have to pick a side. Depending on the type of your question and survey this might be helpful.