The following types of checks are carried out by the TwinLife team or the survey institute:

1. Longitudinal consistency
    • variable pid, sex identical to prior waves
    • body height and weight
    • variable coding longitudinally consistent

2. Identity checks
    • match of sex, first name, date of birth, relationship to twins between preload and survey
    • checks for duplicates (persons included several times in the sample?)
    • change of surveyed sibling identified and corrected
    • household composition

3. Logical relations
    • age of twins vs. cohort
    • age vs. ptyp
    • age and sex of (biological) parents
    • twins’ age and sex identical
    • body height and weight

4. Completeness
    • starting sample vs. realized sample (on personal, household, and family level)
    • instrument/questionnaires vs. delivered variables

5. Instrument
    • completeness and correctness of preload variables and generated filter variables
    • filter conditions

6. Variables
    • correctness of variable labels and value labels
    • correctness of missing values