Stability and change in markers of core numerical competencies.

Dot enumeration (DE) and number comparison (NC) abilities are considered markers of core number competence. Differences in DE/NC reaction time (RT) signatures are thought to distinguish between typical and atypical number development. Whether a child's DE and NC signatures change or remain stable over time, relative to other developmental signatures, is unknown. To investigate these issues, the DE and NC RT signatures of 159 children were assessed 7 times over 6 years. Cluster analyses identified within-task and across-age subgroups. DE signatures comprised 4 parameters: (a) the RT slope within the subitizing range, (b) the RT slope for the counting range, (c) the subitizing range (indicated by the point of slope discontinuity), and (d) the overall average DE RT response. NC RT signatures comprised 2 parameters (NC intercept and slope) derived from RTs comparing numbers 1 to 9. Analyses yielded 3 distinct DE and NC profiles at each age. Within-age subgroup profiles reflected differences in 3 of the 4 DE parameters and only 1 NC parameter. Systematic changes in parameters were observed across ages for both tasks, and both tasks broadly identified the same subgroups. Sixty-nine percent of children were assigned to the same subgroup across age, even though parameters themselves changed across age. Subgroups did not differ in processing speed or nonverbal reasoning, suggesting that DE and NC do not tap general cognitive abilities but reflect individual differences specific to the domain of numbers. Indeed, both DE and NC subgroup membership at 6 years predicted computation ability at 6 years, 9.5 years, and 10 years.