Date: 6 August 2014
Theme: Population
User update: reviewing the methods used to estimate internal migration at local authority level in England and Wales ONS is reviewing the methods it uses to estimate annual internal migration at local authority (LA) level in England and Wales. If improvements can be developed, they will be implemented in the mid‐2015 LA population estimates (published mid‐2016) alongside a revised back series to 2011 where data allow. We are pleased to update users on this project: Key points: 
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The current process for estimating internal migration at LA level has several known issues which ONS would like to address. An improvement project is in the process of investigating the feasibility of using additional administrative data, and the application of statistical modelling techniques, to improve the quality of internal migration estimates. 1. Existing method and issues Internal migration between LAs in England and Wales is currently estimated using a methodology based on a combination of administrative data sources. We essentially look for changes in address by NHS number on two cuts of England and Wales patient registers (PR), one year apart. These data are supported by information on those enrolled in higher education which helps adjust for students, many of whom do not update their patient records when moving to attend courses. The current methodology has been used since the last Census. Annual England and Wales LA population estimates take account of internal migration estimates when they are updated each year. 2011 Census reconciliation research found differences for specific age‐sex groups at LA level between 2011 population estimates rolled forward from Census 2001, and Census 2011 estimates. Further work has indicated that error in internal migration, especially for young adults, is one of the likely sources of these differences. These errors result from: 1.1. Delays in patient registration 
People may delay updating their patient record when they move, or not update it at all before moving again. This means some moves are missed and others are estimated to have happened later than was actually the case.  These delays in registration can lead to over and under estimation in population estimates when flows are large, and inflow is significantly larger than outflow (or vice versa). This is more likely for younger adults and more likely for some types of LAs than others.  This issue can have a particular impact just after a Census. If an individual moved before Census day and Census captured them at their new location, but they updated their patient record after Census User update: Reviewing the methods used to estimate LA internal migration
day, we would ‘double count’ the move. For example, we reduce the Census base of the LA they appeared to move away from (given the PR comparison) by one, even though Census did not capture them there. 1.2 The moves a PR based comparison misses 
A comparison of PR between one year and the next may miss moves made by people who are born, who die, or move more than once during the year. For example, if someone moves but then dies during the year, they may be removed from the PR before the second cut is taken. If this happens, the move they made will be missed because they (and their location) do not appear on both cuts of the PR that are then compared. As their death will be registered in, and affect the population estimates of the LA they moved to, we need to capture that they moved there first as well. 
Currently adjustments are made to account for these missed moves and are based on another health based data source, the NHS central register. This records movement more regularly and therefore captures many of the moves an annual PR comparison misses, but the data are only available on an outdated geography that is larger than LA level. This means blanket adjustments for all LAs within each larger area are made, and these perform better for some LAs than others. 2. Where we are now There are three main ways that, in theory, ONS could improve the method: 2.1 Editing address details Improving the quality and coverage of PR address information may help tackle errors caused when people do not update their patient record when they move. This could be done by: 
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Ensuring the PR data in 2011 (only) matches the Census data as closely as possible by matching the two together and using the Census address in preference to the PR address when they are different (according to certain criteria). Improving how we use Higher Education Statistics Agency (HESA) data to edit PR based address information for students. Investigating whether other data sources now available to ONS can be linked to PR data, and whether the address information on those sources appears more up to date for problem age‐sex groups. We are in the process of investigating the feasibility and potential impact of these interventions. 2.2 Better accounting for missed moves As discussed, the adjustments made for those who are born and then move, those who move and then die, and those who move more than once during the year need to be improved. There are two ways this could be done: 
Using record level births and deaths registration data more directly, adding them (and where the event happened) to the relevant PR data to ensure individuals appear on both cuts of the PR that are then compared. This would still not account for multiple moves during the year. 2 User update: Reviewing the methods used to estimate LA internal migration
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Another health based data source, Personal Demographics Service (PDS)1 data, may be able to tell us about moves more regularly, and unlike the NHS central register, show these moves at the LA level. We are currently analysing the quality of a large PDS data extract, and are exploring whether access to future extracts can be secured. 2.3 Statistical modelling techniques Where movement of certain age‐sex groups appear to be most poorly captured by administrative data, these moves could be modelled completely. For example, Census 2011 collected data about individuals’ address the previous year, providing good information about internal migration between March 2010 and March 2011. If these data were considered robust for the problem age‐sex groups, this information could be combined with administrative data in a model. However, any model dependent on Census data assumes internal migration patterns as captured by Census will be broadly representative of patterns through the decade. We are currently assessing the feasibility of this approach in collaboration with academic contacts. 3. Next steps The research will assess the feasibility and likely impact of the interventions described with a view to improving the quality of the internal migration estimates. We aim to implement any methodological changes, including a revised back series where possible, when the mid‐2015 England and Wales LA population estimates are published in June 2016. Users will be kept updated. 1
http://systems.hscic.gov.uk/demographics/pds 3