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 users 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 _Geography_Policy.aspttp:// _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 Jan11 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