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

of the probes unit and/or decade digit but may have tried to calculate the correct result before
pressing the corresponding response button. As this reflects the more difficult and thus more
error prone strategy these children may have committed more errors which in turn would
account for the negative correlation of WM measures with addition performance.
Basic numerical competencies as precursors of future performance
First evidence indicating the relevance of basic numerical competencies as precursors
of later arithmetic achievement comes from cross-sectional evaluation of typically developing
children (Holloway & Ansari, 2009; see also De Smedt et al., 2009b; Landerl & Kölle, 2009)
as well as from dyscalculia intervention (Kaufmann et al., 2003). The current results fit nicely
with both lines of evidence identifying basic numerical concepts such as symbolic/nonsymbolic
magnitude representation or counting principles to serve as fundamentals for later
arithmetic achievement. However, as previous studies have only used single digit stimuli in
basic numerical tasks, they have not been able to address the issue of place-value integration.
In this way, the current study extends the findings of Holloway and Ansari (2009) in
two important aspects: (i) Here, it was observed that when evaluating the predictive power of
performance in a magnitude comparison task effective magnitude representation (as reflected
by the distance effect) is only part of the story. Rather, it was found that successful
identification and comparison of the single digit magnitudes of tens and units complying with
the place-value structure of Arabic numbers (as indexed by the unit-decade compatibility
effect) is of crucial importance for the further development of arithmetic competencies as
well. Please note that in line with this argument Landerl and Kölle (2009) observed the
compatibility effect to be reliably more pronounced in children with dyscalculia than in a
typically developing sample of control children. This suggests that poor integration of tens
and units into the place-value structure of the Arabic number system may be a key symptom
of developmental dyscalculia. (ii) The present study was the first to evaluate the
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