Many internet-panels consist of self-selected respondents and hence cover a relatively small part of the population. Estimates based on Internet-panels therefore may suffer from non-coverage and self-selection bias. One way to correct for these biases is to use adjustment weighting (Lee 2006). However, when Internet-panel respondents are intrinsically different from the general population, previous studies showed that weighting may result in an increase in bias (for example, see (Loosveldt and Sonck 2008)).

Classification accuracy of single exams is well studied in the educational measurement literature. However, when making important decisions, such as certification decisions, one usually uses several exams: an exam set. This chapter elaborates on classification accuracy of exam sets. This is influenced by the shape of the ability distribution, the height of the standards, and the possibility for compensation. This is studied using an example from vocational education and training (VET). The classification accuracy for an exam set is computed using item response theory (IRT) simulation. Classification accuracy is high when all exams from an exam set have equal and standardized ability distributions. Furthermore, exams where few or no students pass or fail increase classification accuracy. Finally, allowing compensation increases classification accuracy.

The ISO/IEC 17024:2012 describes best-practices regarding competence assessment. The standard emphasizes validity and sound processes to create high-quality competence assessments. The question is how to comply with the ISO/IEC standard when creating large-scale, worldwide assessments. In addition, the competence framework describes competences that require years of experience as part of the competence. We determine to what extent of mastery candidates need to master the competence to start working. We assess this by testing the requisite knowledge with a multiple-choice exam and the required minimum level of mastery of the competence with a Practical Assignment. This assignment is assessed by trainers, which creates the need for supervision and accreditation of the trainers. This paper shows an example of a certification scheme to explain how we comply with the ISO/IEC standard. The creation of the certification scheme and the accreditation of the trainers are described. The compliance with the ISO/IEC standard is explained.