Table 1.

Critical predictors (necessary for testing our hypotheses) and controlled predictors (motivated by the literature on mental addition and included to substantiate the results) included in the stepwise multiple regression analysis.

Critical predictors
Break 5
Carry
Problem size
Unit sum
Break 5 × carry
Problem size × carry
Problem size × break 5 × carry
Controlled predictors	Citation
Size of the second (smaller) summand	Groen and Parkman (1972)
Logarithmic sum	Butterworth et al. (2001)
Logarithmic unit sum	Dehaene (2007), Deschuyteneer et al. (2005)
Square of the sum	Ashcraft and Battaglia (1978)
Product of the summands	Widaman et al. (1989)
Sum of the square of the addends (SSA)	Widaman et al. (1989)
Parity of the two summands	Campbell et al. (2004)
	Lemaire and Siegler (1995)
	Vandorpe et al. (2005)

For the latter, the respective citation (referencing one or more studies indicating the impact of this predictor) is provided.