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 In some cases, the parent or one or both twins independently has withdrawn from TEDS.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)|
|2 Year||1994 and 1995 only||11350||10646||93.8%||704|
|3 Year||1994 and 1995 only||11350||9350||82.4%||2000|
|In Home||All cohorts (but families were carefully selected within each cohort)||16810||992||5.9%||15818|
|9 Year||Cohorts 1 and 2 only
(Jan-94 to Aug-95)
|10 Year||Cohorts 1 and 2 only
(Jan-94 to Aug-95)
|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|
|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
** Reason unknown.
|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).|
|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.