Output Consultation Plans and Statistical Disclosure Control Strategy

developments

Angele Storey and Jane Longhurst ONS

Background

• Census users require a range of different outputs• SARs are a key output• Detailed information may raise confidentiality

issues• Requirements to protect data:

– Public trust and cooperation– Legal rights and obligations– National and International standards for statistics

• To balance confidentiality requirements with maximising utility to meet user needs

• Need comprehensive understanding of user needs and then

Update on…..

• User consultation

• Progress on Microdata

• Statistical Development Control (SDC) developments

User consultation–tabular output

• Current activities are focussing on understanding user’s high level output requirements for tabular output and microdata samples

• 38 in-depth interviews conducted with users drawn from across census user communities

• On-line survey planned to test emerging themes with wider user base

User consultation–tabular output cont…

• Areas that are being explored with users in the interviews and on-line consultation include

– Balance of additivity, consistency, accuracy in tables– Level and complexity of outputs used and required– Strength of user requirement for flexible output and level

of flexibility required– Metadata use and requirements– Access, dissemination methods and media

Census Personas as aid to output design

2011 Census output geography

• ONS consultation on small area geography for England and Wales: Nov ’06 to Feb ’07

http://www.statistics.gov.uk/about/consultations/Small_Area_Geography_Policy.asp

Conclusions:– High degree of stability at both OA and SOA level– Minimal changes after Census 2011 limited to less than

5% of the OAs nationally– changes made by simple mergers and splits

• OA/SOA hierarchy will be primary output geography for 2011 Census output

Progress on microdata

• Census Microdata Strategy Working Group (CMSWG) set up May 2007

• Includes representatives from CCSR, GROS and NISRA

• Responsible for determining strategy for specification and production of 2011 Census microdata samples

• First step – to explore high level user requirements for 2011 Census microdata samples

• CCSR : Presenting initial findings from survey this pm

• Survey findings will be considered at next meeting of CMSWG

Key future dates in microdata work programme

• April ’08 – Feb ’09

- CCSR consultation with users on content of microdata samples

- CCSR work with ONS to develop draft of statistical specification

• Feb ’09 – June ’09

- CCSR consult formally on statistical specification of microdata samples

• Dec ’10

- Final agreed specifications for microdata samples• Jan’11 onwards

- Development, production, testing and delivery stages

Approach to developing SDC policy and methodology for 2011 Census output

• SDC for 2011 Census outputs is a major concern

for users

• Different SDC methodologies were adopted for

standard tabular 2001 Census outputs across the

UK

• Late addition of small cell adjustment by

ONS/NISRA resulted in high level of user confusion

and dissatisfaction

• Publicised commitment to aim for a common UK

SDC methodology for all 2011 Census outputs

Approach continued..

• Phase 1 (March ’06 – Jan ’07)– UK agreement of key SDC policy issues

• Phase 2 (Jan ’07 – Sept ’08) – Evaluation of all methods complying with agreed SDC policy

position in terms of risk/utility framework and feasibility of implementation

• Phase 3 (Sept ’08 – April ’09)– Recommendations and UK agreement of SDC

methodologies for 2011 Census tabular outputs• Phase 4 (Jan ’09 onwards)

– Evaluate and develop SDC methods for microdata, future work on output specification, system specification, development and testing

SDC progress

• RsG agreed high level SDC policy ? 2006

• UK SDC working group established to take forward

work

• UKCDMAC subgroup established to QA work

• Phase 2 (focus on tabular outputs):

– Evaluation of 2001

– High level qualitative evaluation and short-listing of SDC

methods for tabular output

– Detailed quantitative evaluation of short-list

Qualitative criteria

• Primary criteria

– Additivity and consistency

– Over user acceptability

– Protection against differencing

– Feasibility of implementation

• Secondary criteria

– Impact on microdata releases

– Simple to understand

– Easy to account for in analyses

SDC short-list

• Record swapping

• Over-imputation

• ABS Cell Perturbation method

• Small cell adjustment with record swapping (to

provide comparison with 2001)

Quantitative evaluation

• Evaluate short-list

• Range of tables using 2001 Census data

• Balance between risk and utility

• Combine quantitative results with qualitative criteria

• Aligned with work on Outputs, Downstream

Processing and Geography

• Continued communication and consultation with

users

Timetable

• Phase 1 (March ’06 – Jan ’07)– UK agreement of key SDC policy issues

• Phase 2 (Jan ’07 – Sept ’08) – Evaluation of all methods complying with agreed SDC policy

position in terms of risk/utility framework and feasibility of implementation

• Phase 3 (Sept ’08 – Spring/Summer ’09)– Recommendations and UK agreement of SDC

methodologies for 2011 Census tabular outputs• Phase 4 (Feb ’09 onwards)

– Evaluate and develop SDC methods for microdata, future work on output specification, system specification, development and testing

Risk Assessment for microdata

• Disclosure risk depends on records that are unique

in the sample and in the population

• Evaluate risk using scenarios and quantitative risk

measures

Risk assessment for microdata

• Disclosure risk scenarios– Assumptions concerning prior knowledge of intruder and

information available to him, e.g. private database, journalist, nosy neighbour

– Identify key variables - indirectly identifying variables– Use this process to decide what needs to be protected

against– Need to update scenarios

• Quantitative risk measures– Percentage of population uniques that are in the file– Identify high risk records

SDC for Microdata

• Perturbative methods– PRAM (implemented for 2001 Census SAR)– Record swapping– Adding noise

• Non-perturbative methods– Recoding (implemented for 2001 Census SAR)– Suppression– Sub-sampling

• Mixed strategies• Different methods will be evaluated• Need to recognise interdependence with SDC

method for tabular outputs

Access Options

• ‘End user licence’ access to low (but not zero) risk

data

• ‘Special licence’ access to detailed but anonymised

data

• On-site laboratory access - ‘safe centre’

• Remote access/execution

Impact of legislation

• Statistics and Registration Services Act – April

2008

• Personal information held by the Board must not be

disclosed unless an exemption holds

– E.g. to an approved researcher

• ONS approved researcher working group

• ONS SDC microdata standards working group

Summary

• Ongoing progress made for 2011 Census

• Output consultation

– Tables

– Microdata

– Geography

• Statistical disclosure control

– Short-list of SDC methods for protecting tables

– Quantitative evaluation of short-list

– Future work on microdata