Thinking ratios in symbolic and analogical format: Sensitivity to the interference of natural numbers/counting knowledge

This study investigates the reason of the interference of natural numbers with the processing of fractions in adults. Mix et al. (1999) hypothesized that the Arabic digits making up a fraction can activate the natural number knowledge in children, as they are frequently used to express natural numbers. If this hypothesis is right in adults, natural number knowledge should interfere with the processing of probabilities with the same number of favourable cases (for a given number of favourable cases, higher is the number of possible cases lower is the probability to have a favourable case), only when they are expressed by a fraction. Adults had to compare two probabilities, either with the same number of possible cases either with the same number of favourable cases. Probabilities were presented symbolically (fraction) and analogically (subset of a set of items). In our sample, comparing probabilities with the same number of favourable cases is more complex than comparing probabilities with the same number of possible cases, but whatever the way of representation. However, the difference between the two types of comparison is higher in the symbolic way than in the analogical ways. These results suggest that natural number/counting knowledge interferes with the processing of probabilities expressed symbolically and analogically. Consequently, the interference of natural numbers with the processing of fractions cannot only be explained by the presence of Arabic digits in the fraction symbol