Each family in the sample was sent a questionnaire (pdf), incorporating a consent form on the front page. The measures used in the questionnaire are described in detail on another page. Note that all data were parent-reported.
Questionnaires were mailed out and collected in two 'waves', summarized in the table below:
|Wave:||Wave 1||Wave 2|
|Twin birth dates:||Jan-94 to Aug-94||Sep-94 to Aug-95||Sep-95 to Aug-96||Sep-96 to Dec-96|
|Mailing date:||November 2002||October 2003|
|Approx. twin ages when data returned:||8.2 to 9 years||7.2 to 8.5 years||6.9 to 7.5 years|
|Other materials mailed:||Sent simultaneously with the 9 Year Study consent form.||Sent without any other study materials.||Sent simultaneously with the 7 Year Study parent booklet.|
|Who did the mailing:||Group Sigma (except for overseas and autistic families, who were mailed by TEDS)||TEDS|
As indicated in the table above, for convenience it was decided to mail the 8 Year questionnaire simultaneously with 7 Year and 9 Year study materials for some cohorts. For most families, the mailing of the questionnaires was contracted out to Group Sigma. However, some families were mailed directly from the TEDS office: all overseas families; all families known to have autistic twins; and all families in the final cohort (organised alongside the 7 Year mailing).
In wave 1, the 8 Year questionnaires were returned directly to Group Sigma, and were only returned to TEDS after scanning. Group Sigma were asked to log the questionnaires as they were received, and to send regular updates to TEDS to enable us to keep our records up to date. This proved problematic, so for wave 2 the questionnaires were returned directly to TEDS instead; they were then sent to Group Sigma for scanning at a later date.
Up to 3 written reminders were sent to families who did not return the questionnaires promptly.
General data entry issues (for all studies) are described in a separate page. Data from the 8 Year questionnaire were taken up electronically by optical scanning. The layout and design of the questionnaire had been designed with this in mind. This was handled externally by Group Sigma, a commercial company (as mentioned above, they also handled most of the mailing).
The questionnaires were delivered to Group Sigma from the TEDS office in batches. After scanning each batch, Group Sigma returned the data in plain text files (the data files are described in detail in another page).
The questionnaire contains a few free text fields (in the consent form on the front page, and in one question on the final page). However, no attempt was made to take up these fields electronically, other than a data flag to indicate whether anything had been written in the signature box at the bottom of the consent form. Thus, the electronic data from the questionnaire are entirely numeric. There is an annotated version of the questionnaire (pdf). This document shows, in green, the scan positions used for each field in the Group Sigma raw data files. It also shows, in blue, the field names and value codes that have been used in the cleaned raw data.
On the front page of each questionnaire, the FamilyID, parent name, twin names and twin birth orders were printed. Also on the front page, there is a tick box labelled "please cross this box if the birth order is incorrect". The intention here was to identify families where the twin birth order recorded in TEDS (and printed on the questionnaire) was incorrect. However, a surprisingly large number of parents ticked this box in the returned questionnaire. Some of these parents were contacted by telephone for confirmation, and it was found that (in most cases), the parents had misunderstood this instruction. The data from this tick box are therefore of dubious value.
The primary electronic raw data, as returned by Group Sigma, were returned in plain text files, which have been retained for reference. After data cleaning, the 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 8 Year data to be used in analysis. All the raw data files are described in full in 8 Year raw data files.