Calibration-derived decoder discriminability is associated with online P300-speller accuracy, but the fitted mapping does not transport across cohorts

Objective A calibration-derived score has been related to online P300-speller accuracy in the same session, always within the cohort measured. We evaluated whether a fitted mapping from that score to expected accuracy transports to withheld cohorts. Approach Retrospective secondary analysis of BigP3BCI 1.0.0, the largest published evaluation of this relationship to date: 18 of 20 source studies yielded eligible online outcomes, contributing 271 participants, 739 session-condition records, and 19,611 character selections. Four cohorts document an amyotrophic lateral sclerosis (ALS) population. The predictor was calibration-derived decoder discriminability: cross-validated discriminability of a classifier fitted only to calibration epochs. One source study at a time was withheld from development. Between-cohort variation was summarised by the random-effects standard deviation tau, with participant-clustered standard errors. Main Results The association was positive in all 18 cohorts but varied widely in magnitude and precision (Pearson r 0.190 to 0.928; participant-level r = 0.714, p < 0.001). Pooled estimation error was 0.098 (95% CI 0.091 to 0.107) against a benchmark of 0.146. Calibration did not transport: the intercept had tau 0.87, with a 95% interval for an unrepresented cohort of -1.97 to 1.85 on the log-odds scale, and the slope varied more than tenfold across cohorts (tau 0.43, unrepresented-cohort interval 0.111 to 2.005). A protocol proxy for the stopping rule, median time per selection, reduced the between-cohort slope variance by 66%. The four ALS cohorts alone gave an uncorrected between-cohort slope spread of 0.223, against 0.560 overall. Significance The score carries a reproducible signal about accuracy in the corresponding session, but the mapping between them is cohort-specific: usable for ranking sessions within a setting, not for reporting expected accuracy elsewhere without local recalibration. Few-cohort evaluation, as the ALS subgroup illustrates, understates how much performance varies elsewhere.