The influence of the place-value structure of the Arabic number ...

or the decade digits was found (393 ms vs. 507 ms, respectively; t(18) = 1.46, p = .16).
Finally, the two-way interaction of carry and digit identity was significant [F(1, 18) = 4.85, p
< .05] indicating that the requirement of a carry led to a reliably stronger increase of total
reading times on the unit digits than on the decade digits of the problem (+ 111 ms vs. + 65
ms). Interestingly, this two-way interaction was further specified by the reliable three-way
interaction of carry, problem element and digit identity [F(2, 36) = 12.40, p < .001]. Breaking
down this interaction in its constituting two-way interactions revealed a significant interaction
of carry and digit identity for the first summand [F(1, 18) = 13.58, p < .01] and the result
[F(1, 18) = 24.35, p < .001] while it was only marginally significant for the second summand
[F(1, 18) = 4.46, p = .06]. However, a closer inspection of the marginal means showed that
for the first summand as well as for the second summand a required carry operation resulted
in a more pronounced increase of total reading time on the unit digits compared to the decade
digits (first summand: + 119 ms vs. ± 0 ms; second summand: + 205 ms vs. + 101 ms, see
Figure 2, Panel B). Contrarily, for the result the carry effect on total reading time was stronger
on the decade digit than on the unit digit (+ 80 ms vs. + 2 ms fixations, respectively, see
Figure 2, Panel B).
Origin of regressions: The ANOVA evaluating the number of regressions from the
second summand back to the first summand showed that carry addition problems were
associated with a reliably higher number of regressions [0.87 vs. 0.61 regressions; F(1, 18) =
27.58, p < .001]. Moreover, the significant interaction of carry and digit identity [F(1, 18) =
4.26, p = .05] indicated that, in particular, the number of regressions from the unit digit of the
second summand back to the first summand (as compared to regressions from the decade digit
of the second summand) was increased in carry addition problems (+ 0.37 vs. + 0.15
regressions). The main effect of digit identity was not significant [F(1, 18) = 2.37, p = .14].
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