by Glen O’Neil at http://www.isixsigma.com/offsite.asp?A=Fr&Url=http://intelligentmeasurement.wordpress.com/2007/11/20/likert-scale-surveys-best-practices/ (November 20, 2007)
Actually most studies are not conclusive on the difference between 5 and 7 points on a scale. In other words, both 5 & 7 point would provide you with accurate and reliable responses.
In the article listed below, they conclude that as you add more points, a scale becomes more reliable – but only up to a certain point (higher than 11 is too much for most people). Consequently, they argue that 7 is slightly more reliable than 5.
I would recommend that you use a 7 point scale, particularly if you have other comparative variables that you are going to measure on a 7 point scale – it makes it much easier to compare and avoids having to re-calculate your results.
The article I mentioned:
Alwin, D & Krosnick, J, “The reliability of survey attitude measurement: The influence of questions and respondent attributes”, Sociological Methods Research, 1991; 20; 139