TEDS Data Dictionary

9 Year Study

Contents of this page:


The 9 Year study data were collected by means of booklets:

  • Parent booklets.
    Parent-reported data relating to the family and to the twins.
  • Child booklets.
    Twin self-reported data, including cognitive tests.
  • Teacher booklets.
    Teacher-reported data relating to the twins.

The measures used in the booklets are described in full in a separate page.

The 9 Year data were collected between 2002 and 2004 from TEDS families with twins born between January 1994 and August 1995. Data collection was timed to coincide with the school year in which the twins reached their 9th birthdays.

The sample

The sample for the 9 Year Study was restricted to just two cohorts:

  • Cohort 1: twins born between January and August 1994 (data collected 2002/03)
  • Cohort 2: twins born between September 1994 and August 1995 (data collected 2003/04)

Hence, the latter part of the 1995 cohort, and the whole of the 1996 cohort, were omitted from this study.

The study sample included all TEDS families except for the following: (a) those that had withdrawn from TEDS; (b) those with address problems, who could not be traced; (c) families in which either twin had a severe medical condition; and (d) for cohort 1 only, families with overseas addresses.

Initial contact (by consent form) was made with 7531 of the 9411 families in the cohorts specified above, from the original TEDS sample from ONS. Hence there were around 1900 families that were not contacted. Of these, roughly 850 had withdrawn, roughly 750 were known address problems, roughly 250 were excluded on medical grounds, and roughly 40 were excluded because they lived overseas.

The data returns for the 9 Year study are summarised in a separate page. There are further pages comparing samples and returns for different TEDS studies.

Data collection

Data collection periods were linked to the school year (September to August) in which twins reached the age of 9 years. The reason for this was to ensure that all twins were in the same school year when the teacher data were collected. Hence, families were divided into school cohorts according to when their twins were born (as described above). This simplified administration of the study, because all families in a cohort could be contacted at once. Twin ages vary from roughly eight and a half to nearly ten years, at the time when the data were collected.

Initial contact with families was made by sending them an information pack together with a consent form. The consent form was printed on a postage-paid postcard (unfortunately no electronic copy of the consent form has been retained). Parents were given the option of opting out of the study if they did not want to take part. On the same consent form, parents were also asked to give consent for TEDS to contact the twins' teachers, and were asked to provide contact details for the teachers. Parents were sent up to three written reminders if they did not return their consent forms promptly.

Each family that returned written consent was then sent a pack of three booklets: a parent booklet (pdf), and a child booklet (pdf) for each twin. Up to four written reminders were sent to families that did not return their booklets promptly. In cohort 1, a telephone reminder was also used (after the written reminders) for families that had consented and received vouchers, but had not returned their booklets.

All of the large-scale mailings were handled externally by Group Sigma: the initial consent mailing to families, and the subsequent mailings of booklets both to families and to teachers. In addition, in cohort 1, Group Sigma handled the returns of the consents and booklets. The consent postcard, and the freepost envelopes for the booklets, were addressed to Group Sigma, who logged the returns as they received them, and sent regular updates to TEDS to enable us to keep track of progress. This sometimes caused problems in communications with families, because the TEDS records were not always up to date. Therefore, for cohort 2 the consents and family booklets were returned directly to TEDS instead; the booklets were subsequently sent to Group Sigma for scanning.

In cohort 1, vouchers (to the value of £5 for each twin) were sent to families along with the booklets, as an inducement to encourage the twins to complete their booklets. This procedure was reviewed at the end of the cohort, because nearly 400 families had failed to return their booklets even though they had received the vouchers. Hence in cohort 2, the vouchers were sent to the twins as a reward, only after they had completed and returned the booklets.

The teacher study included all families that had given consent and provided contact details for the twins' teachers and schools. Teacher booklets (pdf) were sent directly to teachers, and were not seen by the families themselves. Each teacher also received a return envelope addressed to Group Sigma. Up to three written reminders were sent to teachers that had not returned their questionnaires promptly.

The measures used in the parent, child and teacher booklets are described in detail on another page.

Data entry

General data entry issues (for all studies) are described in a separate page. Data from the 9 Year booklets were taken up electronically by optical scanning. The layout and design of the booklets had been designed with this in mind. Scanning was handled externally by Group Sigma, a commercial company.

In cohort 1 (as explained above), the booklets were mailed directly to Group Sigma by families and by teachers. In cohort 2, the booklets were mailed first to TEDS, and then delivered to Group Sigma in batches. In both cohorts, the booklets were scanned in large batches. After scanning each batch of booklets, Group Sigma returned the data in plain text files (the data files are described in detail in another page).

The parent booklet contains several free text fields, which could not be scanned; the verbatim text from these fields was instead entered manually by Group Sigma into Excel spreadsheets. The Excel files for each batch of booklets were returned to TEDS alongside the text files containing the scanned data.

The twin booklet contains the "story", consisting entirely of free text. Before scanning, the story page was removed from the booklet, and the twin's ID and name were written on it. The story pages were later entered independently of the rest of the twin booklet. Data entry was carried out initially by Group Sigma staff, and later by staff employed by TEDS. Each story was entered manually into a plain text file. Each file was saved (with the .txt file extension) with a file name consisting of the TwinID. At the time of data entry, some initial coding was also carried out, with the codes incorporated alongside the text within the file. The coding rules were complex and are not described further here.

There are annotated versions of the parent, twin and teacher bookets (pdfs). These documents show, in green, the scan positions used for each field in the Group Sigma raw data files. They also show, in blue, the field names and value codes that have been used in the cleaned raw data.

The primary electronic raw data, as provided by Group Sigma, were returned in plain text files (from scanning) and Excel files (parent verbatims). The story data, manually entered and coded, were returned in plain text files. All these original raw data files have all been retained for reference. After data cleaning, the booklet data (but not the story data) were aggregated and stored in a single Access database file. This Access database is now treated as the master copy or source of all 9 Year data to be used in analysis. All the raw data files are described in full in 9 Year raw data files.