Van Asbeck, F W A
Post, M W M
Article History
Received: 8 December 2014
Revised: 29 January 2015
Accepted: 4 February 2015
First Online: 10 March 2015
Competing interests
: The first author is chairman of the non-profit foundation ‘Meten is weten’ (Measurement for knowledge) that has provided the instruction website ExternalRef removed. The remaining authors declared no conflict of interest.