Measures used for Comparisons
The following measures all originate from the 1st Contact booklet study, although twin sexes have subsequently been maintained and updated in the TEDS admin database. Further details of the variables can be found in the 1st Contact pages.
|aethnic||Ethnic origin of twins||1=white, 0=other||% white|
|sex1/2||Twin sex||0=female, 1=male||% female (% of individuals, not pairs)|
|ases||Composite variable measuring family SES, derived from 5 separate measures. See ases derivation for full details.||standardised with mean=0 and standard deviation=1||mean value|
|amojob||Did the mother of the twins have a job, at the time when the 1st Contact booklet was completed. See amojob derivation for full details.||0=no (not employed), 1=yes (employed), 2=full-time parent||% employed|
The comparison figures shown in the table below were obtained using all participants having data in the respective datasets and having 1st Contact data. No exclusions were made for the purposes of these comparisons. Smaller-scale studies with selected samples (e.g. the 4 year in home study, and the 16 year Leap-2 booklets) are not included in the table.
The N column shows the number of individual twins for whom we have data in each study. The N for each representativeness comparison is generally slighly lower, due to missing data, but for brevity detailed Ns are not shown here.
Note that, due to the large Ns in all the major TEDS studies, differences in the comparison figures are generally significant even where the magnitude of the differences are small. For brevity, analysis of significance using p-values is omitted.
Parent and twin booklets were returned as a complete set by most families, even at age 16. Entries shown as "parent/twin booklets" include all twins for whom the parent and/or the twin booklet were returned.
|Study||Description of sample (data in dataset)||N (twins)||% white||% female||mean ases||% mothers employed|
|1st Contact||Parent booklet||27444||91.7%||50.1%||0.000||43.1%|
|2 Year||Parent/twin booklets||12572||93.3%||50.3%||0.027||41.3%|
|3 Year||Parent/twin booklets||12118||93.2%||50.7%||0.063||43.4%|
|4 Year||Parent/twin booklets||16302||93.3%||51.1%||0.131||44.3%|
|7 Year||Parent booklet||15668||93.7%||51.3%||0.128||46.4%|
|Twin phone tests||10887||93.6%||51.3%||0.077||45.6%|
|8 Year||Parent questionnaire||13524||93.7%||50.8%||0.191||47.0%|
|9 Year||Parent/twin booklets||6879||93.6%||52.7%||0.128||46.4%|
|10 Year||Twin web tests||6171||93.7%||54.1%||0.166||46.8%|
|12 Year||Parent/twin booklets||11829||93.5%||52.7%||0.193||47.0%|
|Twin web/phone tests||12179||93.7%||53.3%||0.182||47.7%|
|14 Year||Parent/twin booklets||7240||93.9%||54.3%||0.240||47.0%|
|Twin web tests||6642||93.5%||58.3%||0.247||46.7%|
|16 Year||Parent/twin booklets||10320||93.5%||55.3%||0.229||47.2%|
|Twin GCSE results||13506||93.2%||52.6%||0.225||48.1%|
|Twin web tests||5762||92.2%||57.6%||0.174||46.0%|
|18 Year||Twin A-level results||14321||93.0%||52.9%||0.200||47.4%|
|21 Year||TEDS21 phase 1 parent/twin qnrs||12661||93.1%||56.9%||0.234||47.3%|
|TEDS21 phase 2 twin questionnaire||8718||93.5%||63.4%||0.251||47.0%|
|G-game web tests||4644||94.4%||68.0%||0.300||46.3%|
|Covid phase 1 twin questionnaire||5062||94.0%||68.0%||0.317||46.9%|
The results in the table above show the following patterns among respondents in the TEDS studies.
- Ethic origin
The % white increased from just under 92% at 1st Contact to just over 93% in the 2 Year study. However, since then it has remained more or less constant at 93-94%.
- Twin sex
The % female increased gradually from 1st Contact (50%) up to age 12 (53%), and has increased more rapidly from age 12 upwards. It has risen to over 60% in recent studies where twins have participated independently of parents.
Both variables (ases, amojob) show a gradual increase in SES with twin age. The mean of 'ases' has increased from 0 at 1st Contact to roughly 0.2 at age 12 then up to 0.3 at later ages. The % mothers employed has increased from 43% at 1st Contact to 46-48% from age 7 onwards.
Factors that may have affected the observed patterns are as follows:
- Cohort effects
It has previously been observed that mean SES (measured using variables like ases) is higher for the 1996 cohort than for the 1994 and 1995 cohorts. The studies at ages 2, 3, 7 (phone tests), 9 and 10 did not include the 1996 cohort, hence their participants might be expected to show a lower mean SES than those in other studies that did include the 1996 cohort at roughly the same age. This might help to explain, for example, why the mean SES is lower at age 7 for the phone tests (which excluded the 1996 cohort) than for the parent and teacher studies (which included the 1996 cohort).
- Study sample selection
In some studies, for example at ages 7, 8, 16 and 21, the sample included essentially the entire set of contactable TEDS families. In other studies, for example at ages 4, 10 and 12, the sample was selected so as to include only those families that had participated in previous recent studies. (See the study samples page for details.) Given that families who have remained active over time have shown an increase in mean SES, excluding the less active families may have exaggerated the trend towards higher mean SES in such studies.
- Phoning and reminders
Some studies, like the 16 year GCSE results, 18 year A-level results and the 7 year phone testing study, have made use of extensive phone calling to families in order to maximise returns. A possible effect of this approach is to make the participants in such studies more representative of the wider population than studies that involved little or no phoning, and fewer reminders. This may help to explain, for example, the relatively high mean SES for participants in the 14 Year study, which involved very little phoning and few reminders.
- Twin independence
The increasing % female with age seems to imply that male twins have been more reluctant to participate than female twins as they have grown older. This may also reflect a greater degree of twin independence, and a decline in parental influence, with increasing twin age. This seems most likely to have been a factor in web studies, where web tests may have been completed by twins with minimal parental supervision in many families. In booklet studies, in contrast, families were generally invited to return the entire set of booklets in a single postage-paid envelope, so it is possible that it was less easy for twins to act independently in these circumstances. This may help to explain why the % female is higher for web studies than for booklet studies at ages 12, 14 and 16. The % female has remained lowest in teacher studies (ages 10 and 12), where parents provided teacher contact details without necessarily involving the twins, and in parent-only studies (8 Year, 14 Year Parent SLQ). The % female is also relatively low in the 16 Year GCSE and 18 Year A-level studies where, particularly at the phone reminder stage, data were often gathered directly from parents. These factors may help to explain variations in % female between different samples at a given age, but it is less clear whether they explain variations in SES.