TEDS Data Dictionary

TEDS Study Samples

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This page summarises and compares the samples for each of the main TEDS studies. Each is a subsample of the initial sample of 16810 twin families, obtained by ONS for TEDS. For a more detailed description of the ONS sample, see the 1st Contact study page.

The size of each study sample is defined here as the number of families contacted and invited to take part. This initial contact was generally made by mail (although in some studies it was made by telephone or email). The sample for each study therefore necessarily excluded families that had withdrawn, or that were address problems (see the glossary for explanations of these terms). Other reasons for excluding families from study samples are compared below.

For more detailed descriptions of how the sample was chosen for each study, and the methods used to contact the families, follow the links at the top left of this page to the main study pages. The data returns in TEDS studies are compared on a separate page.

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. 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.

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 In some cases, the parent or one or both twins independently has withdrawn from TEDS.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.

Study Cohorts included in study Size of ONS sample for included cohorts Size of study sample (families contacted) % of ONS sample contacted (for included cohorts) Number of families NOT contacted (for included cohorts)
1st Contact All 16810 16302 97.0% 508
2 Year 1994 and 1995 only 11350 10646 93.8% 704
3 Year 1994 and 1995 only 11350 9350 82.4% 2000
4 Year All 16810 12528 74.5% 4282
In Home All cohorts (but families were carefully selected within each cohort) 16810 992 5.9% 15818
7 Year All 16810 14581 86.7% 2229
8 Year All 16810 13941 82.9% 2869
9 Year Cohorts 1 and 2 only
(Jan-94 to Aug-95)
9411 7531 80.0% 1880
10 Year Cohorts 1 and 2 only
(Jan-94 to Aug-95)
9411 5944 63.2% 3467
12 Year All 16810 8438 50.2% 8372
14 Year All 16810 11223 66.8% 5587
16 Year All 16810 10868 64.7% 5942
18 Year All 16810 10588 63.0% 6222
TEDS 21 phase 1 All 16810 10571 62.9% 6239
TEDS 21 phase 2 All 16810 8599 51.2% 8211
21 Year g-game and covid phase 1 All 16810 7945 47.3% 8865

Families Excluded from TEDS Studies

This table summarises the numbers of families omitted from TEDS study samples for various reasons. The table includes all major TEDS studies except for the In Home study, which used a relatively small and highly selected sample.

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.

Study Approximate numbers of families excluded because: Comments
Withdrawn Address problems Medical exclusions Inactive No recent data Other reasons TOTAL
1st Contact 450 50 - - - - 500 -
2 Year 550 100 - - - 50 ** 700 ** Reason unknown.
3 Year 650 400 - - 850 * 100 ** 2000 * 1st Contact booklet had not been returned.
** Reason unknown.
4 Year 900 1100 - - 2150 * 150 ** 4300 * 1st Contact booklet had not been returned (1750 families), or 2 and 3 year booklets had not been returned (400 families in the 1994 cohort).
** Reason unknown.
7 Year 900 1200 100 - - - 2200 -
8 Year 1150 1400 350 - - - 2900 -
9 Year 850 750 250 - - 50 ** 1900 ** Overseas families.
10 Year 950 650 100 900 850 * 50 ** 3500 * 9 year data not returned.
** Overseas families.
12 Year 1350 1400 300 2250 3100 * - 8400 * No data returned from recent previous studies (see 12 year study page for details).
14 Year 1550 1500 300 2250 - - 5600 -
16 Year 1600 1850 200 2250 - - 5900 -
18 Year 1750 2000 200 2250 - - 6200 -
TEDS 21 phase 1 2200 # 1800 # - 2250 - - 6250 # After the 18 year study, some uncontactable and unresponsive families were withdrawn, hence the decrease in address problems and the increase in withdrawals.
TEDS 21 phase 2 2200 1800 - 2250 1950 * - 8200 * No participation in phase 1, and no email addresses recorded.
21 Year g-game and covid phase 1 2300 - - 2250 - 4300 ** 8850 ** Families in which neither twin had recorded email addresses.

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. Numbers increase 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.
  • Address problems. Numbers increase steadily over time. More address problems emerge with each 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 a large increase in withdrawn families).
  • Medical exclusions. The number of "medical exclusion" families has remained fairly small (a few hundred) but has varied over time for several reasons. Firstly, there has been 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. Finally, 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. See the exclusion page for further information.
  • Inactive families. Before the start of the 10 year study, a decision was made that inactive families would no longer be included in study samples - if they had not participated in studies up to the age of 9, there was little reason to expect them to participate in any future studies.
  • 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 the samples for some studies.
  • Email problems. From age 18 onwards, twins have been increasingly contacted by email (either in addition to, or instead of, by post). 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. The presence of recorded twin email addresses was a factor in sample selection for TEDS 21 phase 2, and was the main factor in sample selection for the g-game and covid questionnaires.