Projections of the Aboriginal and Torres Strait Islander population based on the 2001 Census included an 'unexplained growth' component to account for changes in the Aboriginal and Torres Strait Islander population between censuses which could not be attributed to demographic factors (that is, the difference could not be fully accounted for by natural increase over the intercensal period). For example, a change in the identification rate. The unexplained growth assumption used in the 'High' series of the 2001-based projections was a 1.6% increase per year in the Aboriginal and Torres Strait Islander population of Australia (see Experimental Estimates and Projections, Aboriginal and Torres Strait Islander Australians, 1991 to 2009, cat. no. 3238.0).
Analysis on the change in the Aboriginal and Torres Strait Islander population between 2001 and 2006, as derived from the 2001 and 2006 Censuses respectively, indicated that the growth over this period could be attributed almost entirely to demographic factors and it was agreed that it was very unlikely to see future unexplained growth. Therefore, a zero assumption was made regarding unexplained growth for projections based on the 2006 Census.
Detailed analysis on the change in Aboriginal and Torres Strait Islander population between 2006 and 2011 can be found in Census of Population and Housing: Understanding the Increase in Aboriginal and Torres Strait Islander Counts, 2006-2011 (cat. no. 2077.0). A panel of experts, brought together to advise and review the ABS methods used in this release, advised that they did not believe that another change in identification of this magnitude would be seen in the next 15 years. The ABS is unable to determine whether any possible change would result in an increase or decrease to identification and therefore, zero unexplained growth has been assumed for this series of projections.