Effectiveness and use of optional support: A comparison of incremental scaffolds, prompts, and worked-out examples in an intervention on phylogenetic trees Information In an experimental study using a pretest-posttest design, it was investigated whether incremental scaffolds, incremental prompts and incremental worked-out examples differ in their effectiveness regarding learning performance, self-efficacy expectations and intrinsic motivation. The use of this support was optional. A total of N =133 secondary school students took part in the study. Paper-based questionnaires were used to collect data that are based on the following instruments: • Prior knowledge on the topic of phylogenetic family trees: five self-developed items based on Jenkins et al. (2022). • Knowledge on the topic of phylogenetic family trees in the posttest: eight items from the questionnaire by Jenkins et al. (2022) as well as an additional self-developed item to determine the probability of family trees. • Self-efficacy expectations regarding phylogenetic tree reading skills in the pre- and posttest: five self-developed items. • Biology-related self-efficacy expectations in the pre-test: five items from the shortened scale by Jerusalem and Satow (1999). • Biology-related intrinsic motivation in the pre-test: three items from the instrument by Thomas and Müller (2016). • Intrinsic motivation during the processing of the intervention material in the posttest: twelve items from the instrument by Wilde et al. (2009). The following information is provided in the attached documents: • The document Fragebogen.pdf contains an overview of the instruments used, the assignment of variable names and information on the coding of the data. • The documents SPSS-Datenfile.sav and SPSS-Datenfile.csv contain the data set with the raw data and calculated scale values. Further information about the project can be found at: Gutowski, R., Großmann, N., & Großschedl, J. (accepted). Effectiveness and use of optional scaffolds: an intervention study in biology lessons on phylogenetic trees. International Journal of Science Education.