Table 1: Variable derivation details.
| Analysis | Dataset | Variable | Coding |
| Linkage rates | NCDS | Lived in England at one or more sweeps between sweep 6 and sweep 9 | Yes, no |
| Proportion of sweeps between sweep 6 and sweep 9 living in England | Proportion; sweeps with missing information excluded from calculations given uncertainty over status | ||
| Selection into response, linkage consent and successful linkage | NCDS | Sex | Male, female |
| Father’s social class at birth of cohort member | Registrar General’s Social Class1: I (professional)/II (managerial and technical), III (non-manual skilled), III (manual skilled), IV (partly-skilled)/V (unskilled) | ||
| Number of persons per room in their home at birth | ≤1,>1to 1.5, >1.5 | ||
| Linkage quality | NCDS | Day patient or in-patient attendance at sweep 8 | “Since [date of last interview/1 January 2000], have you been in a hospital or clinic as a day patient or in-patient, overnight or longer? Do not include visits for routine, ante-natal or maternity care”: Yes, no |
| Self-rated general health at sweep 9 | “In general, would you say your health is excellent, very good, good, fair or poor?”: Excellent, very good, good, fair or poor | ||
| Linked NCDS-HES data (APC) | Day patient or in-patient attendance (corresponding to NCDS sweep 8) | Binary variable indicating whether a cohort member had any HES APC record(s) over the period between the date of last interview prior to the sweep 8 interview or 1 January 2000 (whichever was later) and the date of the sweep 8 interview2. | |
| Day patient or in-patient attendance (corresponding to NCDS sweep 9) | Binary variable indicating whether cohort members had any HES APC record(s) over the five-year period prior to the date of the sweep 9 interview2. | ||
| Linked data population representativeness | Linked NCDS-HES data (APC) | FAE rate | The total number of FAEs across all cohort members (noting that each cohort member could potentially contribute more than one FAE) was identified within each financial year between 1997-98 and 2015-16 and the rate per 1000 cohort members calculated. For example, if there were 1320 FAEs in a given financial year across 6593 eligible cohort members then the FAE rate would be 200 per 1000. Data for the financial year 2016-17 were excluded as complete HES data are not yet available in the NCDS-HES linkage. |
| Population HES APC data | FAE rate | The number of FAEs across the entire population of England are available for 5-year age bands from published reports [31, 32]. The number of FAEs within the 5-year age band corresponding to the current age of the NCDS cohort was extracted for each financial year (2004–05 to 2015-16; unavailable for earlier years). For example, in the financial year 2004-05 the NCDS participants were age 46 years, so the age 45-49 FAE data were extracted. Office for National Statistics mid-year population estimates for England by single year of age were extracted for the relevant years and aggregated to the same age bands [33]. For example, in the financial year 2004-05 the age 45, 46, 47, 48 and 49 2004 mid-year population estimates were aggregated to obtain the estimated population for the 45-49 year age band. FAE rates per 1000 population for each financial year were then calculated as the ratio of the number of FAEs and the aggregated population in each age band. For example, if there were 797,253 FAEs in a given financial year across an age band-specific population of 3,326,036 then the FAE rate would be 240 per 1000. |
APC: Admitted Patient Care; FAE: finished admission episode; HES: Hospital Episode Statistics; NCDS: National Child Development Survey; OP: Outpatients.
1Registrar General’s Social Class – also referred to as Social Class based on Occupation – is an official scheme of social class designation used in British surveys and censuses for much of the twentieth century [34].
2The precise period under consideration was allowed to differ between individuals.