Per page discussions: Online output questionnaire results VIII: Statistical Disclosure Control (SDC)

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Disclosure control

ONS_Brendan 1236166526|%e %b %Y, %H:%M %Z|agohover

Originally posted by BLine 3 Mar 2009.

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.

Last edited on 1236166552|%e %b %Y, %H:%M %Z|agohover By ONS_Brendan + Show more

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ONS_Brendan 1236166958|%e %b %Y, %H:%M %Z|agohover

The small cell adjustment (SCA) method was used to protect the confidentiality of individual respondents in 2001 Census data and does have significant effects on the utility of some tables.

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.

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Richard Price 1236267498|%e %b %Y, %H:%M %Z|agohover

I would also add that whilst the OA to OA has issues when used with very small counts that it does have uses. It is especially valuable if then being aggregated to higher levels of geography where this disturbance is then minimised.

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.

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BLine 1237118167|%e %b %Y, %H:%M %Z|agohover

taking all the different published breakdowns and averaging the totals sounds like a lot of work to get a more unbiased result. But the proposed licencing and access arrangments should improve things considerably. I'm hoping to further develop a bit of MapBasic software to batch generate self containment convex hulls, derived from 'Range Manager' http://solutionsgroup.tripod.com/rangeman.htm but adapted , maybe renamed as 'Migrations Mapper' or something.

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2011 UK Census

Output Consultation