Which statement is best supported by data showing fewer errors during treatment?

The statement that the subject made fewer errors compared to baseline is best supported by data showing fewer errors during treatment. This assessment directly correlates the measurement of errors observed in the subject with their performance before and during the treatment process. A reduction in errors indicates that the treatment may be having a positive effect on the subject's abilities, and it provides clear evidence of improvement as opposed to merely anecdotal observations or subjective impressions.

In contrast, the other options do not directly relate to the measurement of errors. While the statement about significant improvement after the initial treatment could suggest effectiveness, it does not specify error rates or provide a direct comparison to the baseline. The option stating no carryover effects does not address error rates, and the statement regarding ineffective treatment contradicts the implication of reduced errors, which suggests that the treatment has had some positive influence. Thus, the choice that focuses on fewer errors is the most direct and evidence-based conclusion drawn from the data.