Bias risks in ILSA related to non-participation: evidence from a longitudinal large-scale survey in Germany (PISA Plus)

This study uses evidence from a longitudinal survey (PISA Plus, Germany) to examine the potential of bias in international large-scale assessments (ILSAs). In PISA Plus, participation was mandatory at the first measurement point, but voluntary at the second measurement point. The study provides evidence for relevant selection bias regarding student competencies and background variables when participation is voluntary. Sample dropout at the second measurement point was related to characteristics such as family background, achievement in mathematics, reading and science, and other student and school demographic variables at both the student and school levels, with lower performing students and those with less favorable background characteristics having higher dropout frequencies, from which higher dropout probabilities of such students can be inferred. We further contrast the possibilities for addressing non-response through weight adjustments in longitudinal surveys with those in cross-sectional surveys. Considering our results, we evaluate and confirm the validity and appropriateness of strict participation rate requirements in ILSAs. Likely magnitudes of bias in cross-sectional studies in varying scenarios are illustrated. Accordingly, if combined participation rates drop below 70%, a difference of at least one-fifth of a standard deviation in an achievement score between non-respondents and participants leads to relevant bias. When participation drops below 50%, even a very small difference (one-tenth of a standard deviation) will cause non-negligible bias. Finally, we conclude that the stringent participation rate requirements established in most ILSAs are fully valid, reasonable, and important since they ensure a relatively low risk of biased results.