Received: 15 July 2018
Accepted: 14 November 2018
First Online: 17 November 2018
Compliance with ethical standards
: D. Schipp is a self-employed statistician, who has been appointed and financed by Tchibo GmbH for this and other projects.
: The study has been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments.