Reliable quantitative data on disabled children has been lacking for a number of reasons. Defining and measuring childhood disability and the circumstances of disabled children and their households present a number of complex theoretical, philosophical and technical issues [9]. These affect both prevalence estimates and related socio-demographic information about the children and their households. The multi-dimensional, dynamic and contested nature of disability may make it inherently difficult to measure (for a detailed discussion of these issues, see [10]). Furthermore, the way that disability is defined determines both the type of data being collected and the process of data-collection. It has been argued that it is by no means obvious or certain who might be regarded or might regard themselves as rightfully being inside or outside the disability category [11]. In addition, the perception of disability as a fixed and distinct status has been called into question as has the related notion of a simple dichotomy between those who are disabled and those who are not [12-14]. In turn, definitions and understandings of disability inevitably shape the range of responses by research participants. The willingness of parents to identify their children as disabled, for example, may vary according to whether the definition used reflects their own definition of disability generally, their perception of any difficulties their child may experience and the implications as they understand them, of defining their child as 'disabled'.

In addition to the issues discussed above, gathering and providing information on disabled children and their households is further complicated by validity and reliability issues. Estimating the prevalence of childhood disability is subject to the same validity and reliability issues affecting the measurement of adult disability. These include the representativeness of the sample and the episodic nature of some disabilities [10]. In addition, there are a range of issues relating specifically to the measurement of disability in children. These include the relative rarity of disability in early childhood, the difficulty of ensuring the inclusion of all disabled children, including those who live away from home for some of the time, and the use of adult specific questions that fail to take account of the child's age and development,[15].

Despite inherent difficulties in defining and measuring childhood disability and the circumstances and characteristics of disabled children, such data is key to the development of appropriate and timely service provision for this group and their families.

A number of key UK government data sources now offer the opportunity to generate up-to-date prevalence estimates and other information on disabled children and their households but their data remain largely unpublished. This paper uses data from a key source, the FRS. The FRS's advantage over other government surveys is that it collects information on children who would be classed as disabled under the Disability Discrimination Act (DDA), 1995 and 2005 and therefore have rights under this legislation. Given the obligations owed to these children by a range of bodies, it is important to know how many there are and their characteristics and circumstances.

Data on the prevalence of child disability and the characteristics and circumstances of disabled children is key to understanding the relationship between impairment and restrictive social conditions, and to informing policy development that aims to both reduce childhood disability and provide appropriate and timely service provision for this group and their families.

The FRS is considered by the Department of Work and Pensions to be its key source of information on disability [18]. It is an annual cross-sectional survey that collects information on the incomes and circumstances of approximately 29,000 private UK households which contain around 16,000 children age 0-18 years.

In order to examine the associations between individual, social and material factors when other variables were controlled for, logistic regression models were fitted on the dependent variable 'DDA defined disability'.

A strength of this study is that it uses data from the FRS, a nationally representative cross-sectional survey with a high response rate and data on a relatively large number of children and young people age 0-18. The overall prevalence estimates generated therefore, are likely to be reliable and valid. As the prevalence of child disability in the UK is relatively low, however, the size of some sub-groups of disabled children was small. In total there were only 90 disabled children from black, minority ethnic or mixed parentage groups. As a result, it was only possible to provide analyses for white UK/other and black/minority ethnic/other groups rather than by individual ethnic group. It is acknowledged that such analyses are limited and fail to provide sufficient information on disabled children from particular ethnic backgrounds. To generate accurate national and local prevalence estimates and data on the circumstances of different ethnic groups in the UK, a substantially larger and/or boost sample is required.

A further strength of the study is both its use of measures of income poverty and of material deprivation. Income measures alone are not necessarily good measures of living standards, particularly at the bottom of the income distribution [21,22]. In using measures of income, lack of socially perceived necessities and debt, this study encompasses a broader range of measures of living standards.

The findings reported here provide up-to-date data on the numbers and circumstances of a nationally representative sample of disabled children and their households in the UK. Using a definition of disability enshrined in the DDA, 952, 741 children (7.3%) in the UK in 2004-5 were reported to be disabled. The overall prevalence estimate of child disability reported in this study is higher than those reported from the FRS for earlier years and for the estimate for the same FRS survey year published by the Department for Work and Pensions. This is explained by use of different definitions of disability across surveys and across years within the same survey. The most recent DDA-related definition of disability was used to generate these analyses. Using this more inclusive measure increased the prevalence estimate by almost two percentage points and 250,000 children above the published estimates for 2003/4. This illustrates how changes to disability definitions and survey question sets within surveys can affect prevalence estimates.

Other recent UK surveys and data sources have produced estimates ranging from 4.5% to 16% [23], thus the prevalence estimate generated by our study falls mid-range. Mooney et al [24], using published figures and information from local authorities have suggested that in England the mean percentage of disabled children was lower, falling between 3% and 5.4%. This lower range estimate may be attributable, at least in part, to the definitions of disability employed and the populations of children included in the data sources, whose primary purpose may not be to capture the whole population of disabled children. Furthermore, prevalence estimates derived from sources relating to private households, as in this study, will not collect data on the small number of children living elsewhere, for example in residential establishments. As a result, they are likely to underestimate the prevalence of childhood disability. This suggests that data users need to understand and be clear about how the estimates they use are derived and population coverage.

The significant association between the age of the child and disability found in this study is consistent with other research [25]. Lower prevalence among younger children is likely to be explained in part, by the fact that a range of conditions do not manifest themselves until later, and some become progressively more activity limiting as the child gets older. Failure by health, education and social care agencies to identify disability early in a child's life however, may also play a part in some cases.

Our findings indicate that disabled children and their households live in different personal circumstances, and substantially more disadvantaged material circumstances than their non-disabled children. Gordon et al's (2000) reanalysis of the Office of Population, Censuses and Surveys (OPCS) disability survey also highlighted the poverty and poor living standards of disabled children in Britain in the 1980s [6]. The analyses presented here suggest that little has changed. Disabled children in the UK today continue to experience income inequality and material and social disadvantage. While an association between poverty and childhood disability is well-established [26], little is known about the precise nature of the relationship between childhood disability and social disadvantage or the extent to which factors such as low income precede or follow impairment. It was clear however, that the household incomes of disabled children and their families were, on average, lower than those of non-disabled children and that they experienced higher levels of debt and social deprivation. This is likely to be attributable to a number of factors. Households with disability children have a greater dependence on social security benefits and are faced with the additional financial costs associated with caring for a disabled child [27-30]. It has been estimated that in the UK, families need incomes that are 10% - 18% higher than similar families with non-disabled children to have the same living standard [31]. Higher levels of low income, debt and social deprivation are likely to be linked to the higher prevalence of lone-parenthood and parental disability in households with disabled children [32,33].

This study highlighted that one third of disabled children lived in lone parent households, a similar proportion to that reported by Emerson and Hatton, 2007 [30] but a considerably higher proportion than that identified by the OPCS disability survey, which reported that 19% of disabled children lived with a lone parent [6,34]. The association between childhood disability and lone parenthood persisted when social disadvantage and other factors were controlled for.

An association between lone parenthood and childhood disability has been reported elsewhere in the UK [35,36] and in other countries, including the United States [37,38]. Reasons for this observed relationship between lone parenthood and disability are however, unclear and further research is needed. Higher divorce rates among parents of disabled children, lower rates of repartnering and a higher prevalence of births of disabled children to lone mothers are possible explanations. While studies, in general, have found higher divorce rates among parents of disabled children than non-disabled children, some studies suggest that this is not universally the case, with divorce rates among parents of children with Downs syndrome occurring proportionally less often than among other parents [39]. While further research is need to explain the observed relationship between of lone parenthood and disability, given the further association of these with poverty and material deprivation, and evidence of high parental workloads linked with caring for some disabled children, the difficulties faced by these families need to be addressed through services and public policy.

Our findings indicate that disabled children in the UK are more likely than non-disabled children to live in rented accommodation. This confirms the findings from other research [30,40]. Other studies indicated lack of space, and poor access inside and outside the home as commonly reported problems for families with disabled children [41,40,25]. Together these data underline the need for national and local policies and services that seek to address the housing needs of disabled children and their households.

A significant finding of this paper is evidence of a clustering of child and adult disability. First, disabled children were more likely to live with disabled siblings and other disabled children than non-disabled children. One quarter of children with a DDA defined disability lived with one or more siblings who also had a DDA-defined disability. To date, information on the number of UK disabled children in any one family unit has been unclear. The OPCS survey did not collect data on the number of disabled children living in a family or household. Gordon et al (2000) have suggested that estimates from other studies for the number of families caring for two or more disabled children range between 4% and 11%. A number of these studies however were based on data from families who had made successful applications to the Family Fund Trust. Such studies are unlikely to be representative of the wider population of disabled children because not all families of disabled children apply for assistance from the Trust and in addition, its eligibility criteria preclude access by middle and higher income families [6]. Second, we found a relatively high prevalence of parental disability among parents of disabled children. The association between child and adult disability persisted when social disadvantage and other factors were controlled for. Although an association between child and adult disability in the same household has been described [42], the research did not control for the impact of confounding factors, as we did. In addition, the proportion of disabled children living with disabled adults was found to be smaller than was evident in our study.