Ginting, H., and E. Surya. “Development Learning Device Based for Measuring Contextual Critical Thinking Skills Students SD Class VI Mathematical”. International Journal of Sciences: Basic and Applied Research (IJSBAR), vol. 33, no. 3, June 2017, pp. 301-10, https://www.gssrr.org/index.php/JournalOfBasicAndApplied/article/view/7544.