Conversion of ABS population projections to enrolment projections

Updated: 17 February 2011

The Australian Bureau of Statistics (ABS) have calculated projections of the population of Australian residents aged 18 years and over for each Census Collection District (CD) starting with a base at 30 June 2003 annually through to 30 June 2009. To allow baseline comparison with latest electoral roll counts, interpolation was used to derive 22 November 2004 population. The 30 April 2009 population projections were also calculated by interpolating between 30 June figures.

For most CDs it was assumed that the proportional relationship between electoral enrolments and resident population aged 18+ will continue. Accordingly, the population projections were converted to enrolment projections as follows:

P2004           =       ABS projection of residents aged 18+ at 22 November 2004
P2009           =       ABS projection of residents aged 18+ at 30 April 2009
E2004           =       Enrolled persons at 22 November 2004
E2009           =       Projected enrolled persons at 30 April 2009

E2009           =       (E2004 / P2004) * P2009

For example, a Census Collection District's figures may be:

P2004           =       471
P2009           =       498
E2004           =       411

E2009           =       (411 / 471) * 498
                    =       435

In a minority of CDs where enrolments were greater than the baseline population projection, it was assumed that electoral enrolments will grow by the same amount as the population of Australian residents aged 18+, i.e.:

E2009           =       E2004  + (P2009  - P2004)

For example, a Census Collection District's figures may be:

P2004           =       146
P2009           =       228
E2004           =       150

E2009           =       150 + (228-146)
                    =       232

Some CDs with very high growth have low enrolment : population ratios due to lags in occupancy and/or change in enrolment address. These ratios were adjusted upwards as the lags work out over time, adjusting to the degree necessary to maintain the overall Territory enrolment ratio.