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