Barner, D., A. Athanasopoulou, J. Chu, M. Lewis, E. Marchand, R. Schneider, and M. Frank. “A One-Year Classroom-Randomized Trial of Mental Abacus Instruction for First- and Second-Grade Students”. Journal of Numerical Cognition, Vol. 3, no. 3, Jan. 2018, p. Article e5761, doi:10.5964/jnc.v3i3.106.