The authors of the dataset used the publicly available school level data from the NPD and identified relevant data on the composition of the disadvantaged groups in the school population of full-time equivalent student. In particular, they used six common indicators of disadvantage in schooling that have been identified as potential factors of school stratification by social group: 1) the number of pupils taking free school meals (FSMt), 2) the number known to be eligible for free school meals (FSMe), 3) the number known to have a statement of special educational needs (SENs), 4) the number known to have special needs without a statement (SENn), 5) the number known to have English as a second or additional language (ESL/EAL), and 6) the number classified as white British ethnic origin (NW) The relevant figures for each indicator per school in each year were converted into what has been termed the Gorard Segregation Index (GS) and the Dissimilarity Index (D) at a national level in order to probe into the main characteristics of school segregation: The Gorard Segregation Index (GS) is an index of segregation between schools that refers to “the proportion of potentially disadvantaged students in a school system who would have to exchange schools with another (non-disadvantaged) student for there to be no segregation by disadvantage between schools in that school system (or the area under consideration)” It has the same basis as the disparity ratio used in studies of health. The Dissimilarity Index (D): the residual for D is the absolute value of the result of subtracting the population proportion of non-FSM pupils in each school from the population proportion of FSM pupils in each school; D itself is the sum of these residuals for all schools, then divided by two.This project looked at several different kinds of relative poverty faced by young people in schools could have important implications for theory and practice, and without gathering any new data. In England, eligibility for free school meals (FSM) is a widely used policy indicator for a pupil known to be from a financially disadvantaged. FSM is routinely treated as context for judging both individual- and school–level attainment, as an indicator of school composition, and as the basis for the pupil premium (PP) funding policy. Knowledge of the quality, reach and limitations of FSM as an indicator is therefore fundamental to accurate decision-making in important areas. FSM-eligibility is not a constant characteristic of an individual pupil, in the same way that sex or ethnicity usually are, but linked to the economy, and family circumstances, meaning that pupils might move in and out of FSM-eligibility over their school careers. It is a threshold characteristic, so we have discussed variation within FSM-eligibility. The project has also analysed longitudinal trends of segregation by poverty at school level.

Based on figures available from the School Level Annual Schools Census (SLASC) from the National Pupil Database (NPD) for state secondary schools at the national level in England from 2003 to 2006, the authors calculated school segregation indices the Gorard Segregation Index (GS) and the Dissimilarity Index (D) for all secondary schools available and for each year by the following six indicators of disadvantage: free school meals take-up (FSMt), free school meals eligibility (FSMe), special educational needs with a statement (SENs); special educational needs without a statement (SENn), English as a second or additional language (ESL/EAL) and non-white British ethnic origin (NW). As for GS, each school’s residual for GS is the absolute value of the result of subtracting the population proportion of all students in each school from the population proportion of potentially disadvantaged students (such as those eligible for FSM) in each school. GS itself is the sum of these residuals for all schools, then divided by two. More formally, GS = 0.5 * (Σ|Fi/F − Ti/T|) Where: Fi is the number of disadvantaged children in school i Ti is the total number of children in school i F is the total number of disadvantaged children in the chosen area T is the total number of children in the chosen area. As for D, the residual for D is the absolute value of the result of subtracting the population proportion of non-FSM pupils in each school from the population proportion of FSM pupils in each school; D itself is the sum of these residuals for all schools, then divided by two. More formally, D = 0.5 * (Σ|Fi/F − Ni/N|) Where: Fi is the number of disadvantaged children in school i Ni is the number of non-FSM children in school i F is the total number of disadvantaged children in the chosen area N is the number of non-FSM children in the chosen area.