Like the index of dissimilarity, the index of isolation ranges from 0 to 1, with 1 being the highest level of segregation possible. Both the index of dissimilarity and the index of isolation are based on the definition of relevant groups, which is left up to the judgement of the researcher (Watson, 2009). This study has defined two groups: poor (pupils belonging to the first per capita family income quintile) and non-poor (all other students). The centile gap is not based on the definition of groups but uses income information to calculate the average percentile differences between those attending each organizational unit and the median percentile in each. If students were perfectly integrated by income across State and private schools, the index would take the value 0. At the other extreme, if students were perfectly segregated, the index would take the value 1.