"Religious affiliation is a variable of strong interest to religious organisations, social scientists, and can be used as an explanatory variable in studies on topics such as marriage formation and dissolution, fertility and income. In Australia and in Britain, religion is a variable that is used to derive measures of ethnicity. In New Zealand, religious affiliation is of particular significance for data users interested in Māori and in Pacific Island peoples. It is a variable of historical interest as well; religious affiliation has been collected in the New Zealand Census of Population and Dwellings since 1851.
The standard classification of religious affiliation is a hierarchical classification of three levels. Level 1 of the classification has nine categories, level 2 has 40 categories and level 3 has 84 categories – excluding residual categories."