Documentation of statistics: Business Services in the Sector for Market Research and Public Opinion Polling
The questionnaire, sampling method and calculation has been adjusted a couple of times, in 2013 where data from 2012 was recalculated with a new method
https://www.dst.dk/en/Statistik/dokumentation/documentationofstatistics/business-services-in-the-sector-for-market-research-and-public-opinion-polling
Documentation of statistics