in 1991 had switched to another group by 2001. They suggested that conscious changes in affiliation explained little of this instability, whereas unreliability of the question was significant, partly due to the ambiguous nature of the categories used and partly due to imprecision in the imputation of missing values. It has also been argued that the wording of the ethnicity question in the 2001 census, "What is your ethnic group?", embodies "an essential being ethnic" as opposed to "a constructed belonging to an ethnicity". The latter would be reflected in a question such as "choose one box to best describe your ethnic group", which was subsequently added in the 2011 census. Sociologist Steven Vertovec argues that "much public discourse and service provision is still based on a limited set of Census categories", and that "these categories do not begin to convey the extent and modes of diversity existing within the population today". User consultation undertaken by the Office for National Statistics (ONS) for the purpose of planning the 2011 census in England and Wales found that most of the respondents from all ethnic groups that took part in the testing felt comfortable with the use of the terms "Black" and "White". However, some participants suggested that these colour terms were confusing and unacceptable, did not adequately describe an individual's ethnic group, did not reflect his or her true skin colour, and were stereotypical and outdated terms. The heading "Black or Black British", which was used in 2001, was changed to "Black/African/Caribbean/Black British" for the 2011 census. As with earlier censuses, individuals who did not identify as "Black", "White" or "Asian" could instead write in their own ethnic group under "Other ethnic group". Persons with multiple ancestries could indicate their respective ethnic backgrounds under a "Mixed or multiple ethnic groups" tick box and write-in area. Between 2004 and 2008, the General Register Office for Scotland (GOS) conducted official consultation, research and question testing for the purpose of planning the 2011 Scottish census, with key evidence informing the new classification drawn from similar workshops carried out by the Office for National Statistics, the Welsh Assembly Government (WAG), and the Northern Ireland Statistics and Research Agency (NISRA). The GOS found that "Black" was a polarising term for many focus group participants and interviewees. Some participants opposed the use of such terms, while others supported them. Opposition to the term "Black" was strongest among