1. Spatial Interaction: An Audit of Population Flow Data in the UK
Adam Dennett, Oliver Duke-Williams and John Stillwell
School of Geography, University of Leeds
Presentation of the 4th International Conference on Population Geographies, Chinese University of Hong Kong, July 2007

2. Outline of presentation
Introduction: relevant background on interaction data and CIDER. Why the need for an audit of interaction data?
Audit of Interaction Data Sources: a whistle-stop tour of interaction data in the UK. What datasets are available and what are the characteristics, advantages, shortcomings and accessibility issues associated?
What are the recommendations coming out of the audit? What will be the implications for CIDER?

3. Introduction – CIDER
CIDER: the Centre for Interaction Data Estimation and Research
Based now, principally, at the University of Leeds though software runs at Manchester
Data Support Unit: part of the ESRC-funded UK Census Programme

4. Access to Census Data and the Census Data Support Units
Public access to key statistics, census area statistics and standard tables through National Statistics and NOMIS
Provide essentially the same data for 2001, although CDU gives access to data from 1981 and 1991 as well.
Census Registration Service (University of Essex)
Based at at the University of Manchester. Provides access to census aggregate outputs from 1981 to 2001 through the interface.
Based at Edina at the University of Edinburgh provides access to digital boundary data associated with census outputs, as well as look-up tables for geographical conversion.
Based principally in the School of Geography at the University of Leeds. Provides access to interaction datasets through the interface.
SARs for small samples of households and individuals are supported by the Cathie Marsh Centre for Census and Survey Research (CCSR) based at the University of Manchester.
CeLSIUS, based at the London School of Hygiene and Tropical Medicine, provides access to the Longitudinal Study dataset, comprising linked records for 1% of the population of England and Wales from 1971.
Based at the University of St-Andrews, the Scottish LS is a replica of the England and Wales LS, although samples 5.3% of the Scottish Population.
Explain Interaction data to do with FLOWS of migrants and commuters 4

5. Introduction - CIDER
Currently we currently administer interaction (flow) data from the 1981, 1991 and 2001 Censuses
Migration
Commuting

6. Introduction – CIDER Data Sets and Geographies
2001 Census: Special Workplace Statistics (SWS) (Levels 1, 2 & 3)
2001 Census: Special Travel Statistics (STS) (Scotland Levels 1,2 & 3 and Level 2 Scottish postal sectors)
2001 Census: Special Migration Statistics (SMS) (Levels 1,2 & 3)
Also comparable datasets from 1991 and 1981
As well as the standard District, Ward and OA geographies available, different aggregations of these basic units, as well as various bespoke geographies are available for different data years
Other Geogs include 100 Zones (FHSAs), 1991 counties and countries, LLSOAs MLSOAs, foreign origins

7. Introduction - WICID
Can select information either by geography or data first.
Output is in a variety of formats including CSV, html etc…

8. Introduction – CIDER’s Ongoing Objectives
CIDER’s objectives of relevance to this presentation:
To expand the system to incorporate a range of UK interaction data sets from outside of the census
To gather/estimate further UK census-based data sets and include them in the system
To undertake research based on the current and future interaction data sets held within the WICID software system

9. Interaction Datasets in the UK: An Audit
Purpose of the Audit:
Before adding new datasets to WICID, we need to know what’s out there!
To identify and evaluate sources of interaction data in the UK that might compliment the current census datasets held in WICID
To make recommendations relating to the inclusion of the most useful datasets in a new, expanded version of WICID
Whilst detailed and comprehensive, UK census datasets have the obvious limitation of being decennial – other datasets, whilst maybe lacking data coverage of census are temporally more regular.
Datasets collated on a more frequent basis provide an opportunity for more complete temporal coverage.
Whilst other datasets are in existence to study internal migration, their ease of access to researchers is limited. WICID allows for a flexible, query building approach which facilitates easy access to the information people want through selection of Origins, Destinations and a range of disaggregated variables – age, sex etc… Inclusion of additional datasets in wicid will add real value to the service.

10. Interaction Datasets in the UK: An Audit
Interaction Data from the Census
Interaction Data from administrative data sources
Interaction Data from survey data sources
To be comprehensive need to briefly review census data

11. Sources of Interaction Data: Census Data Sources

12. Census Tables KS24: Migration (All people)
ST008: Resident type by age and sex and migration
ST009: Age of household reference person (HRP) and number of dependent children by migration of households
ST010: Household composition by migration of households
TT033: Migration (People): All people in the area and those who have moved from the area in the past year, within the UK
Within standard census tables we are able to access some interaction data
Data in these tables only gives us counts of migrants

13. Census Tables Detail from Key Statistics Table 24 at district level
Interaction data contained in the main census tables only provide a detailed geographical breakdown of either the origin units or the destination units but not both.

14. Origin Destination Statistics
Data available at 3 geographical levels: district, ward and output area
Level 3 (OA) data has been severely compromised due to the Small Cell Adjustment Method (SCAM) which has been implemented to preserve confidentiality
Data already held by WICID
Key points to note are that WICID holds data as origin / destination matrices.

15. Commissioned Tables
Customised output from the 2001 Census may be commissioned from ONS Customer Services – costs involved
All commissioned tables, once paid for by original customer are then listed and made available for free by ONS on their website
Customised output from the 2001 Census may be commissioned from ONS Customer Services when particular cross-tabulations are not available from the standard tables, but commissioned tables incur charges to recover staff and material costs.
Once a table has been delivered and paid for by a customer, it is listed on the ONS website ( and becomes available to all users free of charge on request from Census Customer Services.

16. Samples of Anonymised Records (SARs)
SARs have been available since the 1991 Census
In 1991 and 2001 a selection of different SAR files were produced, broadly based around either Individuals (2-3% sample) or Households (1% sample)
Opportunity to cross-tabulate variables not available in the main census tables
Of more use for studying migrants than commuters

17. Samples of Anonymised Records (SARs)
Issues:
Usefulness of licensed SAR files is still limited when studying interaction as even with relatively detailed destination geographies, origins are still coded very coarsely
No spatial coding of workplace location
The 2001 Controlled Access Microdata Samples (CAMS) offer more opportunity to study interaction with both origin and destination coded at LAD level, however CAMs small sample size restricts analysis of migrants to only a few variables

18. Longitudinal Studies (LS)
Three major census based longitudinal studies:
ONS Longitudinal Study of England and Wales
Scottish Longitudinal Study
Northern Ireland Longitudinal Study
There are also a range of non-census birth cohort studies:
The MRC National Survey of Health and Development (NSHD)
The National Child Development Study (NCDS)
1970 British Cohort Study (BCS1970)
Millennium Cohort Study
New UK Household Longitudinal Study
Specific migrations associated with certain points in life – move to Uni/first job, start of family, retirement etc…
Changing questions in birth cohort studies might focus on health, income, marital status, etc…

19. Longitudinal Studies (LS)
Census based longitudinal studies tend to ask the same questions with each wave. Birth cohort studies often ask a core of repeated questions with a selection of questions which change with each wave
Very useful for studying migration patterns over the lifecourse
Issues:
Potentially disclosive nature of longitudinal data means access is often strictly controlled
Spatial information is limited

20. Sources of Interaction Data: Administrative Data Sources

21. NHSCR Data for England and Wales
No compulsory system in the UK to record movement of population
National Health Service Central Register (NHSCR) records patients who move and change their doctor
From this proxy data, estimates can be made of national migratory movements
This complicates the measurement of population movements between censuses.
From this Proxy data, estimates can be made of national migratory movements.
CIDER in negotiations with ONS to secure post ’98 data

22. NHSCR data for England and Wales
Advantages NHSCR data has over the census
Measure of all moves that take place during a period – unlike census
They include students who register with a doctor or health service facility in the area of their further or higher education institution (unlike pre 2001 censuses)
Their quarterly availability means that they provide a continuous record of migration over time
Comprehensive as individuals are given an NHS record at birth and those in private health schemes usually maintain their NHS registration during their lives
1. unlike census data which misses multiple migrations, migrants who are born in the period and migrants who die during the period;

23. NHSCR Data for England and Wales
Disadvantages NHSCR data has when compared with the census
Only age and sex of attributes for individuals
The propensity to register with a new doctor will vary between different migrant subgroups, e.g. young males and pregnant women
They only provide data on flows between relatively coarse spatial units (FHSAs/HAs)
Some short-distance moves missed
Individual members of armed forces are only recorded as entries to and exits from the services - only the origin of recruits and the destination of those discharged are recorded
Long-stay psychiatric patients and prisoners are excluded altogether
Some individuals, notably young adult males, may not register with a doctor until they need to, which may be several months after they have moved, whilst others such as the elderly or pregnant women are likely to register immediately if not before they have made a move
Output data from the population census can be obtained at ward and enumeration district level
Some short distance moves that happen to cross an FHSA boundary will not be recorded because the individuals may not bother to register with a new doctor;

24. NHSCR Data for England and Wales
NHSCR data availability
Individual anonymised records or Primary Unit Data (PUD) have been available since the early 1980s
The TIMMIG system (TIMe series MIGration) developed at Leeds (Rees and Duke-Williams, 1993) merged quarterly data into annual blocks to create time series from 1975 to 1998
NHSCR data availability post-1998 is more problematic
Between 1998 and change from data being produced at the FHSA geography to the new HA geography.
Regional flows are freely available from the ONS, however flows between Health Authorities require special request to the ONS

25. NHSCR Data for Scotland and Northern Ireland
Little if any attempt to produce a consistent set of migration estimates across the whole of the UK
At best ONS produces flows to and from Scotland and Northern Ireland as a whole and HAs in England and Wales
Whilst more detailed in and out flow estimates have been produced, flow matrices for areas both within the countries and between these countries and the rest of the UK are not made available by ONS, NISRA or GRO Scotland
Ideas for separate CIDER project here – creation of set of migration estimates for the whole of UK.

26. Pupil Level Annual School Census (PLASC)
Records, amongst other things, the home location of pupils and the schools they attend - potential for studying school commute
Results from the PLASC are held by the DfES - the National Pupil Database (established in 2002) contains much of this information
There is potential to track pupils and their movements over time, however there is no indication of a system that is being put in place to process this interaction data from the PLASC
Harland and Stillwell (2007) demonstrated with plasc data from Leeds that it is possible to study ‘journey to school’ disaggregated by other NPD variables.

27. Higher Education Statistics Agency (HESA) data
The Higher Education Statistics Agency (HESA) - collects and provides data on students and staff in HEIs as well destinations for HE graduates
There are ‘student’ and ‘first destination’ datasets
HESA holds postcodes for the permanent residence of students prior to entry to an HEI and the postcode of the administrative headquarters of the HEI attended
Potential for studying student migrations in great detail

28. Administrative Interaction Data Sources – Hospital Episode Statistics (HES) data
HES data contain information on each hospital episode experienced by a patient
Data are available for every financial year from onwards
Huge variety variables for each patient including location of residence and treatment
Potential for studying ‘commute to hospital’
and each record holds around 100 personal, medical and administrative details of each patient admitted to hospital in England. This includes geographical information about the location of treatment and where the patient lived.

29. Administrative Interaction Data Sources: Other Sources
Worker Registration Scheme
National Insurance Number Statistics (NINo)
Home Office asylum seekers and visitor switchers
Estimates of migration between the UK and the Irish Republic
Home Office
Eurostat
United Nations
Customer databases

30. Sources of Interaction Data: Survey Data Sources

31. Labour Force Survey (LFS - Great Britain and Northern Ireland)
GB LFS running since 1973 (in its current format since 1992). Surveys households on a range of socio-demographic variables
Primary unit data available to download from the Economic and Social Data Service (ESDS) enabling users to create their own inter-regional flow matrices

32. Labour Force Survey (Great Britain)
Example of inter-regional flow matrix derived from the LFS

33. Labour Force Survey (Great Britain)
A special licence dataset is also available which is referenced to local authority level, although access is very restricted
It is also possible to derive an immigration matrix of foreign origins to regions
NI LFS broadly comparable to GB LFS although far less useful for deriving flows

34. International Passenger Survey (IPS)
Touted as ‘the richest source of information on international migration’, the IPS samples around 250,000 people entering and leaving the main British air and sea ports
IPS is the primary source of information for the Government’s Total International Migration (TIM) estimates
Issues:
Seasonally variable sample survey
Misses many asylum seekers and their dependents
Only provides information on the intentions of respondents rather than their final actions
Anyone using primary unit data from the ESDS needs to be especially wary of seasonal adjustment issues
Criticised for being seasonally variable
Despite being used for TIM - In recent years attention has been drawn to the significant underestimation of Eastern European Migrants.

35. General Household Survey (GHS) and Northern Ireland Continuous Household Survey (CHS)
In existence since 1971
Sample usually 8,000-10,000 households, (around 15,000-20,000 respondents)
Has always featured a question on the amount of time each respondent has lived at a current address
Issues:
It is not possible to derive internal migration matrices from any questions in GHS
The Northern Ireland Continuous Household Survey (CHS) is related to the GHS, although 1983 is the only year where any measure of population migration can be derived
The sample size changes slightly year-on-year
The GHS has always asked a question relating, in some way, to the amount of time each respondent has lived at a current address, although it is not possible to derive internal migration matrices from this question. Only destination rather than origin.

36. Integrated Household Survey (formerly Continuous Population Survey)
Fieldwork for the Integrated Household Survey (IHS) is scheduled to start in January 2008
This survey will integrate several other ONS surveys including the Labour Force Survey (LFS), General Household Survey (GHS), Expenditure and Food Survey (EFS), the Annual Population Survey (formerly the NeSS Survey) and Omnibus Survey (OMN)
Reasonable to assume similar measures of migration to those derived from the LFS will be available from the HIS

37. National Travel Survey (NTS)
First commissioned in 1965/66, the National Travel Survey (NTS) has, since then, provided periodic snapshots of British travel behaviour
Data are only available from 1972 onwards, with continuous data only available from 1988
Currently sampling around 16,000 addresses in Great Britain annually, Primary unit data are available to download for selected periods since 1972 from the ESDS
Inter-regional matrices can be derived from the primary unit data

38. Recommendations coming out of the audit…

39. Recommendations coming out of the Audit…
Data should be included in WICID from the following four sources:
2001 Census: the large and more complex matrices of migration and commuting flows commissioned from ONS that have national coverage at district and sub-district spatial scales
NHSCR: annual flows, from 1975 to 1998, of NHSCR patient re-registration movements between 100 FHSA-based zones, disaggregated by age and sex; and annual flows, from 1998/99 onwards, of NHS patients movements between HAs, disaggregated by age and sex
Generally, with the census there are already online query and extraction systems in place, so CIDER does not wish to replicate these existing census services. In the case of some of the large commissioned tables – extensive data and spatial coverage means that it will be useful to add these datasets to wicid.
CIDER already holds NHSCR data for for set of 100 zones based on FHSA geography. As a relatively reliable source of year-on-year migration data, it would be very useful to include this data in WICID. ONS have expressed their willingness to release post 1998 data for CIDER to use, however due to new HA geography, work will need to be done to create a continuous time series from 1975.
Student migrations to HE institutions are important both in terms of their magnitude and impact

40. Recommendations coming out of the Audit…
HESA: annual flows, from 2001 onwards, of student movements between MLSOA of parental domicile and HEI, disaggregated by various characteristics
NHS IC: annual flows, from 2001 onwards, of hospital patients from LLSOA or MLSOA of residence to hospital, disaggregated by various attributes

41. Implications for CIDER
CIDER are currently in negotiation with the custodians of our targeted data sets to see if incorporation of the data into WICID is possible
All current indications are positive, however due to the differing availability and cost of the data, it is likely that the acquisition and incorporation of some data will happen before others
For some datasets, CIDER may need to secure additional funding before we are able to include it in the WICID system

42. Thank you
Adam Dennett,
Centre for Interaction Data Estimation and Research,
School of Geography,
University of Leeds
For the full audit: