Abstract: A robust left digit effect (LDE) arises in number line estimation such that adults’ estimates for numerals with different leftmost digits but nearly identical magnitudes (e.g., 298/302) are placed in systematically different locations from one another. Number line estimation tasks are important because they are used for measuring and training magnitude estimation skills and are predictive of many measures of numerical competency, such as children’s future math achievement and adults’ math performance on standardized achievement tests. Given the importance of understanding quantitative estimation skills, a question that emerges is whether it is possible to reduce or eliminate the left digit effect in number line estimation. Past efforts to motivate individuals to perform more accurately on the number line estimation task using trial-by-trial and summary feedback did not lead to a reduction in the LDE. In the current study, we instructed individuals about the LDE to see if the bias is reduced or eliminated when one is explicitly taught about it. All participants (N = 135) completed two blocks (60 trials per block) of a self-paced 0-1000 number line estimation task and were randomly assigned to either an instructional or a control condition. Between the first and second block, participants in the instructional condition were educated about the LDE and answered questions to confirm their understanding. However, participants in the control condition were not educated about the LDE. Preregistered analyses revealed a large LDE in each block in each condition. Further, the intervention did not lead to a reduction in the LDE in the instructional condition relative to the control condition. These findings illustrate the robustness of the LDE in number line estimation and demonstrate that the effect cannot be easily corrected, even when one is made aware of the bias.
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