Multi-level validation of the German physical activity self-efficacy scale in a sample of female sixth-graders.

The validation speaks for the use of the abridged 6-item scale, which allows for a unidimensional assessment of PA self-efficacy. Generally, the results support the relevance of a multi-level approach, which not only differentiates between self-efficacy on the individual level and on the class level but also between the respective implications regarding reliability and criterion validity on both levels. Thereby, this study offers a rigorously validated scale and further illustrates possible consequences of the usual neglect of group-level variance in scale validation.