In 2011 this is worth remembering when whatever arrangements are put into place, otherwise a useful dataset may lose it's value if access is unduely restricted.

The origin destination tables, especially at low geographies, will be greatly affected.

The method itself should be unbiased, so that a total of 50 people travelling from OA1 to OA2 would have an expected value of 50, but there is significant variation between the published total (which is a sum of protected counts) either above or below the unperturbed total.

The variation can be seen for OA to OA counts broken down by different variables. For example, using a breakdown by age and sex could give a total of 25, a breakdown by economic activity could give a total of 33. By taking all the different published breakdowns and averaging the totals, one gets a still unbiased estimate and with far less variability from the unprotected count.

For the 2011 Census, we have already proposed that origin destination tables at low geographies are protected not by SDC methods, but by licensing and access arrangements. The precise details of this approach have not yet been finalised.

We try to use the detailed OA to OA origin :destination tables to estimate 'self containment ' patterns for migrations and travel to work patterns, to help in identifying funcitonal housing market areas. At this scale pretty well every cell is '3', which potentialy causes a lot of distortion. Common sense would suggest that longer distance moves/commutes are likely to be smaller numbers.

But if changing the disclosure rules looks like an unwinnable battle, what would help is some work to show how much distortion it actually does cause. I've experimented by changing all '3' cells to 1 or 2, to see the effect, and it is noticeable. But a check against the real numbers at different distances could perhaps give an average variation that could be applied.