Rounding Census 2006 Numbers

Question

If I recall properly, the census numbers used to be perturbed during rounding - is this the case with the 2006 Census?

Answer

Below is the description of how random rounding is used and how it applies to data for the 2006 Census. The information was provided by our Census Consultants and comes from pages 6 and 7 of "Data Quality and Confidentiality Standards and Guidelines" for the 2006 Census
(http://www12.statcan.ca/english/census06/reference/notes/DQguidelines/PDF/2006-DQ-Public-Guide-E.pdf).

From pages 6-7

2.2. Random rounding
All counts in census tabulations are subjected to a process called random rounding. Random rounding transforms all raw counts to random rounded counts. This reduces the possibility of identifying individuals within the tabulations. For 2A (100%) data, all counts are rounded to a base
of 5. This means that all 2A counts will end in either 0 or 5. The random rounding algorithm employed controls the results and rounds the unit value
of the count according to a pre-determined frequency.

2B (20%) data require a slightly different random rounding algorithm. All counts greater than 10 are rounded to base 5, as is done for 2A data. Counts less than 10 are rounded to base 10. This means that any 2B counts less than 10 will always be changed to 0 or 10. The table below shows the effect of rounding on 2B counts with a value less than 10.

The random rounding algorithm uses a random seed value to initiate the rounding pattern for tables. In these routines, the method used to seed the
pattern can result in the same count in the same table being rounded up in one execution and rounded down in the next.