Each family in the sample was sent three booklets: a parent booklet plus two copies of the child booklet (pdfs). The parent booklet included a consent form on the first page. The measures used in the booklets are described in detail on another page. The measures were entirely parent-reported, although the child booklet included cognitive tests that were administered by the parents on the children.
The booklets were designed to be completed when the twins were precisely 2 years old; they were therefore sent to families at or just before the twins' 2nd birthdays. This involved regular mailings of booklets (generally once per month) during 1996 and 1997.
Regular reminders were sent to families who did not return the booklets promptly. Up to 7 reminders were sent, over a period up to 11 months after the original booklets were sent to each family.
General data entry issues are described in another page. For the 2 Year study, in the 1994 cohort, data from the booklets were entered by TEDS staff into a Microsoft Access database. In the 1995 cohort, data entry was handled externally by NOP Numbers, a commercial company; it is not clear what type of software system they used for data entry, but the entered data were returned in Excel spreadsheets.
Data entry staff, both in TEDS and at NOP, carried out basic coding of the raw data by converting tick boxes to numeric code values - see 2 Year raw data item coding for the parent and child booklets (pdfs). The Drawing task items in the child booklet were scored by TEDS staff before the booklets were sent to NOP. There is no surviving manual of coding instructions for data entry staff, other than paper versions of the pdf documents linked above.
Entry of verbatim text data (from the parent booklet) differed between the two cohorts. In the 1994 cohort, all verbatim text data was entered and recorded in the Access database. However in the 1995 cohort, the only verbatim text items entered (in Excel) were the family contact details (from the consent form), the twin names (raw items AN28 and AN29), and contact details for a relative or friend (from the final page of the booklet). In both cohorts, a coded numeric item was entered for each text item in the parent booklet, with value 1 if a text response had been made by the parent, or value -77 if not. These coded items are shown in the parent booklet raw item coding (pdf).
The original raw data, as entered and before cleaning, still exist in Access and Excel files. These data have been cleaned and aggregated together, and stored in a single Access database file (see 2 Year raw data files for further details). This latter Access database is now treated as the master copy or source of all 2 Year data to be used in analysis. The ways in which the raw data have been cleaned is described in another page.