Identifying distinct sources of whole number interference in children's decimal comparison: the role of numerical magnitude and inhibitory control

When learning about decimals, whole number knowledge can be a detriment, leading children to incorrectly report, for example, that 0.26 is larger than 0.8. Two potential sources of whole number interference could lead to such errors. Digit length interference arises from the whole number rule that more digits = larger number, while whole referent magnitude interference arises from ignoring the decimal point and comparing the whole referents (26 > 8). The independent effects of each interference type have been measured in adults, but we still do not know how these effects play out in early decimal learning. Further, inhibitory control has been linked to overcoming whole number interference, but it is unclear which specific type of interference is inhibited. Here, we used carefully designed decimal stimuli to examine these two interference effects in middle school students who are more susceptible to whole number interference than adults. Students in 2 U.S. school districts (grades 6-8, n = 178) completed computerized decimal comparison and inhibitory control tasks. We implemented cluster analysis to account for heterogeneous strategy use. The two most prominent groups, Whole Number Biased and High Performing, differed in their extent of digit length interference, but were equally susceptible to whole referent magnitude interference. Crucially, inhibitory control only related to overcoming digit length interference, not whole referent magnitude interference, in both groups. Taken together, whole referent magnitudes are a pervasive source of interference in decimal comparison, independent of overall task performance or individual differences in inhibitory control.