Lost 2 Year Data
The table below shows discrepancies between the number of families that returned the booklets, and the number of families with data in the dataset.
In the case of the 1994 cohort, the discrepancy is relatively small (around 40 families), and this is typical of TEDS studies in general. Small discrepancies of this sort arise because of logging errors, unusable booklets, and loss of data.
In the case of the 1995 cohort, the discrepancy is large: the data are missing for nearly 700 families. This is because NOP Numbers, the organisation hired for the data-entry, failed to return one or more files of entered data; these data were subsequently found to be irretrievably lost.
Returns
The figures shown in the table below are numbers of families (twin pairs). Note that families in the 1996 cohort were not contacted for this study.
| Cohort (birth year of twins): | 1994 | 1995 | TOTAL |
|---|---|---|---|
| Original ONS sample | 5640 | 5710 | 11350 |
| 2 Year booklets sent | 5241 | 5405 | 10646 |
| 2 Year booklets returned | 3568 | 3440 | 7008 |
| Percentage of booklets returned | 68.1% | 63.6% | 65.8% |
| Data in the dataset | 3526 | 2760 | 6286 |