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

(ii) In the magnitude comparison task children had to evaluate 120 two-digit number
pairs and indicate which one of the two numbers presented above each other on a computer
screen was larger by pressing the corresponding response key. The two to-be-compared
numbers were presented in Arial font (size: 60) until one of the two response buttons was
pressed. The stimulus set comprised 80 between-decade trials for which decade distance (i.e.,
small vs. large; e.g., 23_47 vs. 21_85) and unit-decade compatibility (i.e., compatible vs.
incompatible; e.g., 42_57 vs. 47_62) was manipulated in a 2 x 2 within participant design.
Additionally, the stimulus set involved 40 within-decade trials (e.g., 32_37). The split-half
reliability of the magnitude comparison task was r tt =.45.
(iii) The WM tasks were designed to tap the three WM components suggested by
Baddeley (1986): verbal, visuo-spatial WM and central executive: a) verbal WM was assessed
by a letter repetition task. This task involves the presentation of spoken sequences of letters
for immediate serial recall. After a practice trial, a maximum of three lists were presented to
the children at each length, starting with a two-item sequence. Only when two of three lists of
a particular length were recalled correctly, list length was increased by one otherwise testing
was stopped. Letters were presented at a rate of approximately one per second. The maximum
sequence length at which two lists were correctly recalled was then used a measure of verbal
WM span. b) Visuo-spatial WM was measured by the Corsi block tapping task (Corsi, 1972).
The location of a certain number of cubes on a board had to be remembered and reproduced in
correct order. After a practice trial subsequent procedure and classification of results was
similar to that for the letter repetition task. c) For central executive (CE) assessment each
child had to recall sequences of spoken letters and Corsi blocks in reversed order (for a
similar consideration of backward spans as a central executive task see e.g., Gathercole &
Pickering, 2000). Practice trials ensured that the concept of “reverse” was correctly
understood by each child. Subsequent procedure and scoring were identical to that described
above for forward span tasks.
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