1. Methodology background


 National Statistic   
 Frequency  Annual
 How compiled  Based on third party data
 Geographic coverage  England and Wales
 Last revised  20 August 2018

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2. Important points about unexplained deaths in infancy data

  • The Births and Deaths Registration Act (1836) made it a legal requirement for all births and deaths to be registered from 1 July 1837, making them the most complete data source available.

  • The registration of births and deaths is a service carried out by the Local Registration Service in partnership with the General Register Office (GRO) in England and Wales.

  • Data for unexplained deaths in infancy includes both sudden infant deaths and deaths for which the cause remained unascertained after a full investigation. Data are available from 2004.

  • Deaths are those of babies aged under 1 that occurred in a given year and were linked to their corresponding birth records.

  • Coding for cause of death is carried out according to the World Health Organization (WHO) International Classification of Diseases tenth revision (ICD-10) and internationally agreed rules, allowing for international comparisons.

  • Figures in the Unexplained deaths in infancy tables and Unexpected deaths tables are based on deaths occurring in each calendar year; therefore, they will not be directly comparable to those released in Deaths registered in England and Wales (Series DR), which are based on deaths registered in each calendar year.

  • Our unexplained deaths in infancy statistics adhere to the disclosure control guidance for birth and death statistics.

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3. Overview of the output

Our database of unexplained deaths in infancy is used to create the annual release Unexplained deaths in infancy in England and Wales. It includes sudden infant deaths and infant deaths where the causes remained unascertained after a full coroner investigation. The linked data about parents (collected at the birth registration) can be used for analysis by certain risk factors. The risk factors include birthweight, mother’s age at birth of child, mother’s country of birth, marital status, parity (number of previous children) and the parent’s socio-economic classification based on occupation.

Sudden infant deaths are coded to the International Classification of Diseases tenth revision (ICD-10) with “R95”, which is sudden infant death syndrome. Infant deaths where there was any mention of the terms “sudden infant death”, “cot death”, “SIDS”, “crib death” or similar terms in the death certificate are included. Unascertained deaths use the ICD-10 code “R99”, which is used for other ill-defined and unspecified causes of death.

More detailed information is available in the User guide to child mortality statistics.

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4. Output quality

This report provides a range of information that describes the quality of the data and details any points that should be noted when using the output.

We have developed Guidelines for measuring statistical quality; these are based upon the five European Statistical System (ESS) Quality Dimensions. This document addresses the quality dimensions and important quality characteristics, which are:

  • relevance

  • timeliness and punctuality

  • coherence and comparability

  • accuracy and reliability

  • output quality trade-offs

  • assessment of user needs and perceptions

  • accessibility and clarity

More information is provided about these quality dimensions in the following sections.

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5. About the output

Relevance

(The degree to which statistical outputs meet users’ needs in terms of content and coverage.)

The registration of births and deaths occurring in England and Wales is carried out by the Local Registration Service in partnership with the General Register Office (GRO). Information collected at the registration in England and Wales is recorded on the Registration Online (RON) system by registrars. Most of the information is normally supplied by the informant (usually a close relative), while the cause of death is usually obtained from the Medical Certificate of Cause of Death (MCCD), completed by a medical practitioner when the death is certified. More information on births is available in the User guide to birth statistics and more information on deaths is available in the User guide to child mortality statistics and User guide to mortality statistics.

We quality assure births and mortality data for England and Wales. Once published, they are freely available on our website.

All births and deaths that occur in England and Wales must be registered in England and Wales. Births and deaths of those whose usual residence is outside England and Wales are included in total figures for England and Wales but are excluded from any sub-division of England and Wales. Our statistics exclude births and deaths of all residents of England and Wales that occur and are registered outside England and Wales.

The figures in the unexplained deaths in infancy release are those that have occurred in a given year and were linked to their corresponding birth records.

Age at death is broken down into three broad categories:

  • infant (deaths under 1 year)

  • neonatal (deaths under 28 days)

  • postneonatal (deaths between 28 days and one year)

A further distribution of the postneonatal period by months is presented in order to highlight the high concentration of deaths in the earlier months soon after birth.

Unexplained deaths in infancy are published by:

  • age at death and baby’s sex

  • month of occurrence

  • birthweight, presented as a grouped variable to draw distinction between death rates for low (under 2,500 grams) and normal birthweight (2,500 grams and over)

  • age of mother at the time the child was born, providing a useful comparison between younger and older mothers

  • marital status of the parents as recorded at the birth registration (inside marriage/civil partnership or outside marriage) and registration type (joint or sole registration)

  • National Statistics Socio-economic Classification (NS-SEC) as defined by occupation; for further information on socio-economic classification as defined by occupation, refer to section 7.1 of the User guide to child mortality statistics

  • mother’s country of birth; England and Wales and outside England and Wales

In previous years, the numbers of sudden infant deaths were presented by a more detailed breakdown of the country variable. However, because of small numbers in each category corresponding rates were not published since these would not be robust.

There is a great deal of interest in the deaths of apparently healthy infants. The findings of the release help identify some of the risk factors for these infant deaths. Main users of these data include the Lullaby Trust, the Department of Health and Social Care (DHSC), Welsh Government (WG) and independent researchers, including academics.

Timeliness and punctuality

(Timeliness refers to the lapse of time between publication and the period to which the data refer. Punctuality refers to the gap between planned and actual publication dates.)

There is a delay of 18 months between the publication date and the end of the period to which the data refer. This allows the death certification proceedings to be completed for the maximum amount of cases possible. The majority of unexplained deaths are certified by a coroner either with or without an inquest; therefore it takes much longer for these deaths to be registered. Further information on the impact of registration delays is available.

For more details on related releases, the GOV.UK release calendar provides 12 months’ advance notice of release dates. In the unlikely event of a change to the pre-announced release schedule, public attention will be drawn to the change and the reasons for the change will be explained fully at the same time, as set out in the Code of Practice for Statistics.

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6. How the output is created

Since 2004, we have maintained a database of unexplained infant deaths, which includes information collected at birth and death registration of infants. This database is created using a late extract from the standard deaths registrations database. The extract is taken late to allow enough time for registration following certification by a coroner. The deaths are linked to their corresponding birth record where possible to obtain information on social and biological factors of the baby and parents.

Unexplained infant deaths include both sudden and unascertained deaths of babies aged under one. Sudden infant deaths are those coded to the International Classification of Diseases tenth revision (ICD-10) code, “R95” sudden infant death syndrome. This release includes infant deaths where there was any mention of the terms “sudden infant death”, “cot death”, “SIDS”, “crib death” or another similar term anywhere on the death certificate.

Unascertained deaths are those coded to the ICD-10 code, “R99” other ill-defined and unspecified causes of mortality. This release includes infant deaths where the only mention on the death certificate was unascertained death.

The infant mortality rates are calculated using the number of live births that occurred in the same year as the denominator, more information on rates can be found in section 3.3 of the User guide to child mortality statistics.

Rates are not calculated where there are fewer than three deaths in a cell, denoted by (u). It is ONS practice not to calculate rates where there are fewer than three deaths in a cell, as rates based on such low numbers are susceptible to inaccurate interpretation.

Rates which are based on between three and 19 deaths are displayed in tables but are denoted (u) as a warning to the user that their reliability as a measure may be affected by the small number of events.

More detailed information on the main processes used in the compilation of mortality and birth statistics and on the accuracy and quality of the data used are available in the User guide to mortality statistics and the User guide to birth statistics.

The ONS policy on protecting confidentiality in birth and death statistics is available.

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7. Validation and quality assurance

Accuracy and reliability

(The degree of closeness between an estimate and the true value.)

Information recorded at birth and death registration in England and Wales passes through a number of processes before becoming usable for analysis. Our User guide to mortality statistics provides additional information on the collection, processing and quality of mortality data for England and Wales. More specific information relating to stillbirths and infant deaths is available in the User guide to child mortality statistics. The User guide to birth statistics provides detailed information on the registration, collection and quality of births data in England and Wales.

The accuracy of information contained in the draft birth entry is the responsibility of the informant(s), usually the mother, or the mother and father where the registration is a joint one outside marriage. Willfully supplying false information may render the informant(s) liable to prosecution for perjury. It is believed that, in general, the information supplied by the informant(s) is correct.

Occasionally, birth information might be missing from an entry. This can occur for a number of reasons including the informant refusing to give information, or the informant not knowing the information. Under the Population Statistics Act (2012), certain confidential data items are collected at the registration of a birth. If any of these data items are missing, an appropriate value is imputed. More information on the imputation of missing births data can be found in Section 4 of the User guide to birth statistics. The number of birth records missing age of parents’ can be found in Section 5.9. For years prior to 2012, the number of records missing information on previous live-born children can be found in Section 5.13.

For deaths, other than the cause of death (including a stillbirth), additional information is supplied to the registrar by the informant when the death is registered. For deaths certified after inquest, the coroner, police officers or other witnesses may supply this information, which cannot later be checked by the registrar.

When a birth or death is registered, the registration system provides the opportunity for the registrar to make validation checks at the point of registration, therefore improving the quality of the data. We conduct internal consistency checks to eliminate any errors made in the supply and recording of birth and death records. Checks are more frequent on those records with extreme values for key variables (such as age of mother and father), as these have a greater impact on published statistics. A small number of registrations are raised with the General Register Office (GRO) on a monthly basis for verification.

The underlying data for this release comprises of infant deaths that have been successfully linked to their corresponding birth records. Around 2% of infant deaths cannot be linked to a birth record. The main reasons for this are either that a birth record cannot be found, or the birth was registered outside England and Wales. Further information on the linkage process is available in section 7 of a User guide to child mortality statistics.

Coherence and comparability

(Coherence is the degree to which data that are derived from different sources or methods, but refer to the same topic, are similar. Comparability is the degree to which data can be compared over time and domain, for example, geographic level.)

Figures in the Unexplained deaths in infancy tables include both sudden infant deaths and unascertained deaths. Sudden infant deaths are coded to the International Classification of Diseases tenth revision (ICD-10) code “R95” sudden infant death syndrome, which includes any mention of the terms “sudden infant death”, “cot death”, “SIDS”, “crib death” or similar terms on the death certificate. The unascertained deaths use the ICD-10 code “R99”, which is used for other ill-defined and unspecified causes of death. Figures in the Unexpected deaths tables have been produced according to The Lullaby Trust's definition of unexplained infant deaths that were unexpected, that is, unexplained infant deaths that were referred to a coroner; the tables also include deaths based on final underlying cause of death.

Figures in the unexplained deaths in infancy and unexpected deaths are based on the year the death occurred in. Therefore, these figures will not be directly comparable to those released in Deaths registered in England and Wales (Series DR) as these figures are based on final underlying cause and the year the death was registered in.

Based on evidence that the terms “sudden infant death” and “unascertained” are used interchangeably by coroners certifying these deaths and research showing that the characteristics of infants dying of these two causes are very similar, we decided to include both groups in any analysis of unexplained infant deaths from 2005.

We are the only provider of statistics on unexplained deaths in infancy for England and Wales. There are secondary users of these statistics who report the figures provided by us. It is not possible to directly compare these statistics with other countries because of differences in methodology.

Figures for unexplained deaths are available from 1995 onwards and have been calculated using the same methodology and are therefore comparable. Comparable figures for unexplained deaths by various risk factors are available for 2004 onwards.

From the 2012 data year, we changed the method used for reporting National Statistics Socio-Economic Classification (NS-SEC) for birth statistics. We now use the combined method, which uses the most advantaged NS-SEC of either parent rather than just the father’s NS-SEC. These changes were detailed in Proposed changes to birth statistics by Socio-Economic Classification. The impact of switching to the combined approach was assessed in A combined approach to National Statistics Socio-Economic Classification.

The Human Fertilisation and Embryology Act 2008 contained provisions enabling two females in a same-sex couple to register a birth from 1 September 2009 onwards. In addition, the Marriages (Same Sex Couples) Act 2013 enabled same-sex couples to get married in England and Wales from 29 March 2014. Because of the small numbers of births registered to same-sex couples:

  • births to same-sex couples who are married or in a civil partnership are included with marital births

  • births to same-sex couples who are not married or in a civil partnership are included with births outside marriage

The number of births to same-sex couples is footnoted on relevant tables to assist users. Given the relatively small numbers of births registered to same-sex couples, the impact on statistics is negligible.

Prior to May 2012, when a birth was within marriage, the informant was asked for the number of previous children, by the mother’s husband and any former husband, for both live births and stillbirths. This allowed determination of the registration birth order. If the number of previous live-born children was not given, a value was imputed from a similar record with completely stated and otherwise matching particulars.

Following changes to the Population (Statistics) Act 1938 in May 2012, information is now collected at all birth registrations on the total numbers of previous live births and previous stillbirths that the mother has had (not just those with the current or former husband). This has simplified the question asked by registrars and provides improved coverage. Whether the mother has been previously married is also now collected at all birth registrations. Previously, these data items were only collected from married women. The changes improve the accuracy of birth statistics by birth order and feed into estimates for family size and measures of fertility. A number of published tables were changed; these changes were detailed in How changes to the Population Statistics Act will affect birth statistics.

The difference, between the old and new data, in the proportion of married women reporting previous births is larger than was expected purely from the question change. More information can be found in Quality assurance of new data on birth registrations, as a result of changes to the Population Statistics Act – from May 2012 onwards. This document also provides background to the changes and provides high-level findings from the new data collected in 2012 and 2013. As a result of these findings, changes were made to the Registration Online (RON) system in January 2016 to ensure the birth being registered is excluded from the number of previous children born to the mother; further quality assurance is being conducted on 2016 data.

An investigation of childbearing by registration status in England and Wales, using birth registration data for 2012 and 2013 examines the patterns and characteristics following the improvements to the data collected on previous children and previous marriages at birth registration. The principal characteristics explored in the article relate to whether a woman has been previously married and whether the birth is the mother’s first child or subsequent child.

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8. Concepts and definitions

(Concepts and definitions describe the legislation governing the output and a description of the classifications used in the output.)

The term “sudden infant deaths” was first used as a cause of death in the early 1960s and was defined in 1969 as “the sudden unexpected death of any infant or young child which is unexpected by history and in which a thorough post-mortem examination fails to demonstrate an adequate cause of death”. This definition was revised in 1989 by the US National Institute of Health to include only infants dying suddenly under one year of age. Sudden infant death (SID), sudden unexpected death in infancy (SUDI), sudden infant death syndrome (SIDS) or similar terms are identified by the automatic cause coding system used by ONS and are all coded to the International Classification of Diseases tenth revision (ICD-10) code “R95”. The term “cot death” is used interchangeably with “sudden infant death” and the former is more widely used among the non-technical users of statistics on SIDS.

Unascertained deaths are those for which the cause remains unknown after investigation. The changes that have been identified in the certification practices surrounding sudden infant deaths and unascertained deaths suggest that the distinction between these two causes of death is becoming blurred. While sudden infant deaths were previously recorded as natural and the term “unascertained deaths” was used for deaths of unknown cause, this no longer appears to be the case.

We publish unexplained deaths in infancy statistics by National Statistics Socio-economic Classification (NS-SEC), which is derived using the Standard Occupational Classification (SOC) and employment status. SOC is revised every 10 years. The coding of employment status also changed in 2001 to be consistent with the 2001 Census and SOC2000. Since 2001, NS-SEC has categorised the socio-economic classification of people. The NS-SEC classification is not used internationally; however, it is based on an internationally accepted classification.

A new coding system for country of birth was introduced in 2006 and was used to code mother’s and father’s country of birth. The National Statistics country classification is based on the International Standard Organisation (ISO) 3166-1 Codes for the Representation of Names of Countries and their Subdivisions, adapted to meet data needs of UK National Statistics' users and producers.

Birthweight categories in our statistics conform to low birthweight definitions set by the World Health Organisation (WHO). The lowest category for our published birthweight statistics is under 1,500 grams. Figures are also published for 1,500 to 1,999 grams and 2,000 to 2,499 grams.

The WHO definitions of low birthweights are:

  • low birthweight: less than 2,500 grams

  • very low birthweight: less than 1,500 grams

  • extremely low birthweight: less than 1,000 grams

Births occurring outside marriage or civil partnership may be registered either jointly or solely. A joint registration records details of both parents, and requires them both to be present. A sole registration records only the mother’s details. In a few cases a joint registration is made in the absence of the father or second parent if an affiliation order or statutory declaration is provided. Information from the birth registration is used to determine whether the parents jointly registering a birth outside marriage or civil partnership were usually resident at the same address at the time of registration. Births with both parents at the same address are identified by a single entry for the informant’s usual address, while different addresses are identified by two entries.

For information on the categories of infant deaths based on age (for example neonatal and postneonatal) see Relevance section.

The rates used in the tables are outlined in Section 3.3 of the User guide to child mortality statistics.

The existing provisions for the registration of deaths and the processing, reporting and analysis of mortality can be found in the User guide to mortality statistics.

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9. Other information

Output quality trade-offs

(Trade-offs are the extent to which different dimensions of quality are balanced against each other.)

Checks are carried out quarterly by Office for National Statistics (ONS) on the birth registration data. These checks pick up any suspect records or information within a record that needs to be queried with the General Register Office (GRO). These records are then corrected where appropriate. Any birth records which are still missing mother’s age or father’s age are imputed, see our User guide to birth statistics for more information. Although imputation means that the data are not exact, delay and costs are kept to a minimum.

The information on occupation of the mother and father is coded for a 10% sample of births. This has been shown to give a good approximation to the actual data while being cost effective. Combining occupation with employment status means that a code for socio-economic classification can be derived.

Assessment of user needs and perceptions

(The processes for finding out about uses and users, and their views on the statistical products.)

A user consultation to review infant mortality statistics took place between 20 April and 20 July 2017 and the ONS response to the review is available.

A previous user consultation to review infant mortality statistics took place between 5 July and 16 August 2011 and the ONS response to the review is available.

We published a proposal for changes to birth statistics by socio-economic classification in February 2013. No feedback was received so the outlined changes were implemented.

Proposed changes to child mortality statistics were outlined in the Child Mortality Statistics, 2011 bulletin (published February 2013) and user feedback was requested. Very little feedback was received so the proposed changes were implemented.

User feedback is requested at the bottom of all emails sent by customer service teams within the Vital Statistics Outputs Branch.

We also receive feedback through regular attendance at user group meetings and conferences.

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10. Sources of further information or advice

Accessibility and clarity

(Accessibility is the ease with which users are able to access the data, also reflecting the format in which the data are available and the availability of supporting information. Clarity refers to the quality and sufficiency of the release details, illustrations and accompanying advice.)

The latest figures on Unexplained deaths in infancy can be accessed free of charge on our website. A statistical bulletin containing context and commentary accompanies this release.

Our recommended format for accessible content is a combination of HTML webpages for narrative, charts and graphs, with data being provided in usable formats such as CSV and Excel. Our website also offers users the option to download the narrative in PDF format. In some instances other software may be used, or may be available on request. Available formats for content published on our website but not produced by us, or referenced on our website but stored elsewhere, may vary. For further information, please contact vsob@ons.gov.uk.

For information regarding conditions of access to data, please refer to these links:

Special extracts and tabulations of child mortality data for England and Wales are available to order (subject to legal frameworks, disclosure control, resources and our charging policy, where appropriate). Such enquiries should be made to Vital Statistics Outputs Branch (vsob@ons.gov.uk or telephone: +44 (0)1329 444 110). All user requested data will be published onto the website.

Access to microdata and disclosive data, that is, data which have the potential to identify an individual record, requires the approval of the ONS Microdata Release Procedure (MRP) before the data can be provided.

Useful links

The Vital statistics: population and health reference tables provide annual infant mortality data for the United Kingdom and its constituent countries (based on deaths registered in a year). For data for other UK countries please see the latest infant death statistics for Northern Ireland and the latest infant death statistics for Scotland.

Summary data for infant mortality in England and Wales (based on deaths registered in the year), are available in the Deaths registrations summary tables. A geographical breakdown of infant death numbers and rates by local authority and county (based on deaths registered in a year) is available in Deaths registered in England and Wales by area of usual residence.

The Births summary tables, England and Wales, provide summary statistics for live births in England and Wales.

More general information on the collection, production and quality of mortality data is available in the User guide to mortality statistics.

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