1. Aaten, A. B., Van den Heuvel-Panhuizen, M., & Elia, I. (2011). Kindergartners’ perspective taking abilities. In M. Pytlak, T. Rowland, & E. Swoboda (Eds.), Proceedings of the Seventh Congress of the European Society for Research in Mathematics Education (pp. 1812–1821). University of Rzeszów, Poland: European Society for Research in Mathematics.
2. Bates, D., Maechler, M., Bolker, B., & Walker, S. (2014). lme4: Linear mixed-effects models using Eigen and S4. R package version 1.1-5. http://CRAN.R-project.org/package=lme4 . Accessed 20 January 2015.
3. Berry, J. W. (1971). Ecological and cultural factors in spatial perceptual development. Canadian Journal of Behavioural Science, 3(4), 324–336.
4. Bishop, A. J. (1973). The use of structural apparatus and spatial ability—A possible relationship. Research in Education, 9, 43–49.
5. Bishop, A. J. (1980). Spatial abilities and mathematics education—A review. Educational Studies in Mathematics, 11, 257–269.