Pew Research Center · GfK Belgium
Pew's "Being Christian in Western Europe" survey: 26,096 adults interviewed by telephone (cellphone and landline) April–August 2017 across 15 Western European countries plus Slovakia, under the direction of GfK Belgium. This version adds three church-tax items (Q13, Q14a, Q14b) Pew fielded later for its 2019 report. Church tax exists in only 9 of the 16 countries surveyed — mandatory in Austria, Denmark, Finland, Germany, Sweden, and Switzerland; voluntary opt-in in Italy, Portugal, and Spain — and Q13 was asked only in those 9.