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

unit 5 (e.g. 65), unit distances are limited to a maximum of 4 (when excluding multiples of
ten). For such rather small unit differences, the unit-based compatibility effect is weakest and
possibly not significant. Therefore, if one intends to find null unit-effects, standards like 65 or
- for the comparison of two numbers - only small unit distances should be presented.
2. Use of no or only a few within-decade trials
We observed that the compatibility effect is larger when within-decade trials (e.g.,
53_57) are included in the stimulus set (Knops, 2006; Nuerk & Willmes, 2005, for a review).
So, when units are irrelevant in all or the vast majority (e.g. only 15.5 % within-decade trials
in Dehaene et al., 1990) of trials, unit-effects may be smaller as attention may be focussed on
the tens, as these are decisive in all or almost all trials. Consequently, such an attentional bias
towards the tens’ digit(s) leads to a weakening of possible unit-based effects. Therefore, if one
wishes to observe null unit-effects using exclusively or at least a majority of between-decade
trials would be a good advice.
3. Ignore congruity effects in magnitude comparison
We observed automatic within-number comparison of tens and units (e.g. for 62, 6 <
2; Wood et al., 2005) to influence response latencies when this respective number had to be
compared to another number presented 100 ms later (e.g. 62_47). In this example the result of
comparing tens and units within 62 is congruent with the overall decision: the decade digit of
62 is larger than its unit digit (i.e. 6 > 2) and is also larger than the decade digit of 47 (i.e. 6 >
4). Hence, the task-relevant decision (i.e. 62 > 47) is facilitated. Contrarily, respective but
incongruent comparisons as for the number pair 42_57, i.e. the decade digit of 57 is smaller
than its corresponding unit digit (i.e. 5 < 7) but larger than the decade digit of 42 (i.e. 5 > 4),
inhibit the relevant decision (i.e. 42 < 57).
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