“…This finding complements previous results on strategy flexibility in single-digit addition and subtraction, evidencing more flexible strategy choices with increasing experience in the domain (Siegler, 1996(Siegler, , 2000. Moreover, it significantly adds to our understanding of the structures and mechanisms underlying children's strategy choices on multi-digit subtraction problems using an extended definition of strategy flexibility as fitting strategies not only to the numerical characteristics of items (Blöte et al, 2000(Blöte et al, , 2001De Smedt et al, 2010;Heinze et al, 2009;Peters et al, 2012Peters et al, , 2013Selter, 2001;Torbeyns, De Smedt, et al, 2009a, 2009b, but also to individual strategy efficiency competencies (Verschaffel, Luwel, Torbeyns, & Van Dooren, 2009).…”