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

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Regression<br />

<strong>The</strong> stepwise regression analysis on <strong>the</strong> modelled RT data <strong>of</strong> <strong>the</strong> strictly decomposed<br />

model resulted in a final model including <strong>the</strong> predictors difference between <strong>the</strong> logarithms <strong>of</strong><br />

<strong>the</strong> to-be-compared <strong>number</strong>s and unit distance [adj. R 2 = .91, R = .95, F(3, 237) = 1199.20, p<br />

< .001, see Table 2]. Closer inspection <strong>of</strong> <strong>the</strong> beta weights indicated that reaction times<br />

increased as <strong>the</strong> difference between <strong>the</strong> logarithms <strong>of</strong> <strong>the</strong> two <strong>number</strong>s decreased and, most<br />

importantly, response latencies also increased as <strong>the</strong> unit distance decreased. As unit distance<br />

is negative for incompatible <strong>number</strong> pairs but positive for compatible pairs this clearly<br />

indexed an <strong>influence</strong> <strong>of</strong> unit-decade compatibility on item RTs estimated by <strong>the</strong> strictly<br />

decomposed model.<br />

Table 2: Predictors included in <strong>the</strong> final regression model for <strong>the</strong> decomposed data<br />

Predictor B b t sign.<br />

Change in<br />

R 2<br />

Raw<br />

correlation<br />

Partial<br />

correlation<br />

Constant 834.62 - 354.23 < .001 - - -<br />

Difference <strong>of</strong> logarithms -303.40 - .94 48.20 < .001 .89 - .94 - .95<br />

Unit distance - 1.84 - .13 6.90 < .001 .02 - .17 - .41<br />

Again, a multiple regression incorporating <strong>the</strong> four significant predictors <strong>of</strong> item RTs<br />

as observed by Nuerk et al. (2001) accounted for a considerable amount <strong>of</strong> variance [adj. R 2 =<br />

.85, R = .92, F(5, 235) = 325.56, p < .001] without a substantial loss <strong>of</strong> predictive power.<br />

Moreover, inspecting <strong>the</strong> beta weights revealed that apart from logarithmic overall distance<br />

[constant = 938.76; B = -168.35; b = - .87, t = 34.18, p < .001] and problem size [B = 0.86; b<br />

= .20, t = 7.93, p < .001], unit distance [B = -2.02; b = - .15, t = 5.78, p < .001] reliably<br />

predicted item RT in <strong>the</strong> expected direction. Item RT increased as logarithmic overall distance<br />

decreased, problem size increased and, most importantly, as unit distance increased; <strong>the</strong>reby,<br />

indicating a significant <strong>influence</strong> <strong>of</strong> unit-decade compatibility on Item RT. On <strong>the</strong> o<strong>the</strong>r hand,<br />

absolute unit distance was not a reliable predictor <strong>of</strong> item RTs [B = 1.27; b = .04, t = 1.73, p =<br />

255

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