Peculiarities concerning families or persons are coded in the variable pec in the master data set. It records cases which in some way deviate from the sampling design, and which should, therefore, be considered when analyzing the data. For instance, this is the case when the twins are in fact part of triplets. Other examples are that the twins are orphans or, in very rare cases, the twins do not belong to the target birth cohorts defined within the sample design.