TEDS Data Dictionary

Sample Sizes for TEDS Studies

This table summarises the sample size, in terms of number of families initially contacted, for each of the main TEDS studies. It does not include TEDS studies that used smaller subsets of the cohorts (e.g. 4 Year In Home, post-18 spatial web studies, post-21 g-game and covid studies). Nor does it include secondary data collections from the main studies (e.g. LEAP-2 at age 16, TEDS21 phase 2).

The figures are based on records kept in the TEDS admin database. Some attempts at initial contact inevitably fail (generally because some families change address and/or phone number without notifying TEDS), so the quoted figures are likely to be slight overestimates of the actual number of contacts made.

The table is divided into two parts. Up to and including the 9 Year Study, within the cohorts contacted for any given study, invitations were sent to all contactable families from the initial ONS sample: 16810 families. From the 10 Year Study onwards, "inactive" families who had never participated in TEDS were removed, and invitations were only sent to contactable families in the TEDS sample: 13945 families.

In the later studies, starting with the 18 Year and increasingly thereafter, there were families in which only one or two of the three family members (parent + two twins) could be contacted. In some cases, the parent or one or both twins independently has withdrawn from TEDS. In other cases, twins moved away from the parental home then either parent or twin became uncontactable. The figures in the table below include all families in which at least one family member was contacted.

For more information about families not contacted (last column), see the following section about families excluded from TEDS studies.

Families Excluded from TEDS Studies

This table summarises the numbers of families omitted from TEDS study samples for various reasons. As above, for the sake of simplicity, the table includes all major TEDS studies but not TEDS studies that included smaller subsets of families, nor secondary data collections from the main studies.

Based on the surviving records, especially for the earlier studies, it is not always possible to recreate the exact reasons why particular families were not contacted. For example, the dates when some families withdrew are unclear, as are the dates when some families became address problems. Furthermore, the admin records are likely to contain occasional errors in the logging of items sent to and received from families. The numbers in this table are therefore approximate, and have been rounded to the nearest 50 families.

As above, the table is divided into two parts. Up to and including the 9 Year Study, numbers reflect subsets of the ONS sample of 16810 families. From the 10 Year Study onwards, numbers reflect subsets of the TEDS sample of 13945 families. This change meant that roughly 2250 "inactive" families, who had never participated, were not invited to take part in any study after age 9.

The table above illustrates the following trends, contributing to overall declines in sample sizes over time (see the glossary for further explanations of terms):

• Withdrawn families. The number of withdrawn families increases steadily over time. With each study, more families tell us that they wish to withdraw, for a variety of reasons including lack of time, and twins losing interest as they grow older. The "inactive" families, removed from the TEDS sample between the 9 Year and 10 Year studies, included several hundred withdrawn families, causing an apparent drop in the number of withdrawn families at this time.
• Address/contact problems. The number of address problems has also increased steadily over time. More address problems emerge with each postal mailing, and only a minority can be traced. The apparent decrease in address problems from 18 year to 21 year was caused by the deliberate withdrawal of a number of long-term uncontactable and unresponsive families (hence also an increase in withdrawn families). A further decrease from 21 year to 26 year is linked to the use of twins' own contact details (email or postal addresses), with less reliance on parent postal addresses.
• Medical exclusions. The number of "medical exclusion" families has remained fairly small (a few hundred) but has varied over time for several reasons. Firstly, during twin childhood and adolescence, there was a general increase as families informed TEDS of severe medical conditions that were previously not recorded for the twins. Secondly, the definition of what types of medical condition should be excluded has been modified from one study to the next. The redefinition of medical exclusions has varied to some extent according to the demands of a particular study, and judgments by TEDS staff as to which twins might find activities difficult (or in some cases which twin pairs might be excluded from analysis even if they provided data). During the 18 year study, a final attempt was made to involve many medical exclusion families who had been very unresponsive in recent studies; those who did not respond were withdrawn, hence from the 21 year study onwards there was less reason to remove medical exclusions from samples. Then, after the completion of the TEDS21 study, in which twins themselves were asked about their medical conditions, the entire categorisation of medical exclusions was overhauled and was defined per-twin instead of per-twin-pair; this resulted in a significant reduction in the number of twins treated as medical exclusions. See the exclusion page for further information.
• No recent data. Some families become passively withdrawn from TEDS by not responding when asked for data; families that do this for several studies in succession are unlikely to respond when asked again for data in the next study. The table shows that there have been attempts to eliminate such families from some study samples (e.g. 4 Year and 12 Year), with the aim of increasing participation rates.
• Email addresses. From age 18 onwards, twins have been increasingly contacted by email (either in addition to, or instead of, by post). Until age 18, initial invitations were generally made by post, but since age 21 initial invitations have been sent by email before following up with postal invitations. The existence of twin email addresses is linked to past twin activity, because email addresses have been collected as part of twin studies from age 16 onwards.