The 2011 Census: Estimating the Population Alexa Courtney

Overview Background –New topics on questionnaire –Internet Data Collection Delivery of data Overview of 2011 processing –Discuss “downstream” processes

2011 Census 27 th March 2011 –Census Rehearsal: 11 th October 2009 Most complex UK Census –More questions and topics than previous censuses –Range of delivery and completion options Similar to 2001 – keep what worked Some new/modified methodologies –Operational and statistical Wide range of outputs

Questionnaire New topics –Citizenship –Second address –National identity –Language Full census returns from short-term migrants –In UK for 3 months or more –Identified through intention to stay question

Questionnaire completion Internet completion being offered for first time –Internet Access Code provided on front of paper questionnaire Offers opportunities to improve data quality and reduce respondent burden –Automatic routing –Validation rules –Use of radio buttons No unnecessary changes from paper questionnaire to minimise modal bias Advantages and disadvantages –Reduces amount of editing required –Increases possibility of multiple responses

Data delivery Can be split into three groups –Questionnaires returned within 6 weeks of Census day Majority of data Fully processed across UK Matched to CCS –Questionnaires returned within 10 weeks of Census day Fully processed in England, Wales & Northern Ireland –Questionnaires returned more than 10 weeks after Census day May be used in coverage adjustment

Removing false persons Problem identified in 2001 Census –Records created in error Pages crossed out Dust on scanner “Two of Five” rule Name (from individual questions) or Date of Birth AND One of: Name (from individual questions), Date of Birth, Sex, Marital Status, or Name (from household members table)‏ Important for data quality and matching

Multiple response resolution Overcount Several types of multiple response –Two questionnaires from same household Two paper questionnaires Paper and Internet –Person on same questionnaire twice (or more!)‏ –Person on Household and Individual questionnaire –Person on Household and Internet questionnaire Needs to be a quick process

Multiple response resolution Duplicate households identified when receipted –Questionnaire tracking for England, Wales & Northern Ireland –Matched questionnaire IDs and address in Scotland Resolved by matching people within household –Key variables: Name (or soundex), Date of Birth, Sex –If Age <30, name must match exactly Minimise risk of matching twins –If no people match, two household records created –If any people match, questionnaires merged

Multiple response resolution Merging questionnaires –“Most complete” response kept –Missing variables copied from duplicate record(s) –Priority given to individual questionnaires Process for within postcode multiples –People completing neighbour’s questionnaire –Similar principles for resolution

Filter Rules Based on 2001 Rules Used to identify incorrect/unnecessary responses Deterministic – based on other responses Used to prepare data for main edit & imputation e.g. Person aged <16, economically inactive (student) e.g. Person employed, not looking for work

Edit and Imputation Will use CANCEIS system Resolves inconsistent data –Probabilistic –Programmed with all possible inconsistencies Impute missing data –Based on complete records –Searches for similar donor Ensures complete and consistent data

Output flags Non-standard outputs possible for England & Wales –Use information on Second Residences Population staying in UK 3-12 months identified –Exclusion from standard outputs –Production of specific outputs –England, Wales and Northern Ireland only Considering including this population in coverage adjustment Mark records now to enable easy production of these outputs

Coverage assessment process Estimation Matching Adjustment 2011 Census Quality Assurance Census Coverage Survey

Disclosure Control - Options Necessary to protect confidentiality of respondents Three options were short-listed: –Pre-tabular: Record swapping (pre-tabular) –Small number of records swapped across areas –Adds uncertainty to “unique” records Over-imputation (pre-tabular) –Some variables deleted and re-imputed –Post-tabular: Invariant ABS Cell Perturbation (IACP) –Small counts can be altered –Two stage process to ensure “additivity”

Disclosure Control – Chosen Methodology Pre-tabular method recommended –User preference for consistency between tables –IACP method rejected Record swapping chosen instead of over-imputation –No persons or data items removed –Outputs at national level and high geographies unaffected

Outputs Main base will be Usual Residents –All people living in UK for 12 months or more –Consistent across UK First outputs – September 2012 –Other standard outputs by Spring 2013 ONS producing non-standard outputs –e.g. Weekday population, Majority of time –Consultation to decide exactly what Outputs on short-term migrant population –All people living in UK for 3-12 months –England, Wales and Northern Ireland

Questions?