Luke Coppen noted that a previous Nineteen Sixty-four post included a Catholic population estimate for Great Britain that was larger than what many would expect. As he correctly identified this was related to the World Values Survey (WVS) estimate that was part of a multi-survey calculation. The WVS results were reviewed and a the problem was identified. This was related to the way in which "invalid responses" (either NA or don't know) are coded, treated, and reported. In a number of countries, including Great Britain this affected Catholic population estimates. In a now updated post CARA utilizes all WVS respondents, regardless of the NA/DK or invalid designations. Countries most affected by this have significant numbers of people who are not a member of a religion. The WVS question specifically measures membership, rather than affiliation or identity, and this can create, in some countries, a significant number of NA/DK responses.
The graph below shows the trend in the Catholic population percentage for Great Britain from 1964 to 2007. This has grown larger in recent decades and measures just under 10% representing 5.9 million individuals in 2005. This is nearly 800,000 fewer Catholics than the Church estimated in the Annuarium Statisticum Ecclesiae (ASE) 2005.
The number of NA/DK respondents in the WVS data for Great Britain has grown over the years. In 1981, there was virtually no difference between the Catholic population estimate when these respondents were included or excluded (11.6% when excluded and 10.5% when included). There were also no large differences in these estimates in 1990 or 1999. However, significant differences did occur in the 2006 survey. When these respondents are included the WVS estimates become consistent with the ISSP and British Election Studies observations.
