FAIR PERSONALIZATION & EXTERNAL VALIDITY AT THE LIMIT Calibration Privilege, Exclusion Risk, Subgroup Uncertainty, Cross-Site Transfer, Device Heterogeneity, and Equitable Adaptive BCI

Abstract

FAIR PERSONALIZATION & EXTERNAL VALIDITY AT THE LIMITCalibration Privilege, Exclusion Risk, Subgroup Uncertainty, Cross-Site Transfer, Device Heterogeneity, and Equitable Adaptive BCI Feng Cheng-en (33) x Starli Does personalization help people differently - or does it simply make access easier for the people who were already easiest to personalize? FAIR PERSONALIZATION & EXTERNAL VALIDITY AT THE LIMIT is a large-scale research monograph exploring adaptive brain-computer interfaces as a problem of access, burden, heterogeneity, transfer, and external evidence rather than a race for higher personalized performance. Its flagship Three-Coupling is: Fair Access x Policy Robustness x External Validity The Fair Personalization Index (FPI) is introduced as a book-defined research heuristic linking fair access, policy robustness, external validity, and transferability while explicitly accounting for calibration debt, exclusion risk, and subgroup overfitting. FPI is not a natural law, clinical score, legal fairness standard, diagnostic threshold, or treatment rule. Across 82 chapters, the volume develops research programs for fair personalization, calibration privilege, exclusion risk, calibration burden and debt, few-session calibration, sensory and cognitive access, language and cultural access, caregiver-support inequality, hardware affordability, connectivity constraints, rural home use, subgroup calibration, small-subgroup uncertainty, intersectional risk, subgroup overfitting, performance and burden disparity, dropout and missingness disparity, domain shift, population and context shift, temporal drift, device heterogeneity, therapist effects, policy portability, cross-device and cross-site transfer, external validation, multicenter replication, shared state and outcome definitions, benchmark heterogeneity, minimal reporting sets, fair adaptation rules, participant veto, human override, de-personalization tests, privacy-preserving personalization, federated adaptation, auditability, public standards, negative-result registries, failure atlases, and successor handoff. Every chapter begins with a Three-Coupling System, a named Core Relation, and a geometric research model. The geometric research grammar includes the Fairness-Burden-Benefit Surface, Subgroup Uncertainty Polytope, Cross-Site Transfer Lattice, Calibration Privilege Gradient, Access Constraint Map, External Validity Prism, Heterogeneity Manifold, Burden Disparity Basin, Policy Portability Bridge, Equity Profile Radar, Multicenter Evidence Pyramid, and Fairness-Audit Feedback Torus. The volume contains 200 Research Gates and 33 Answer Embryos. The Research Gates are designed as falsifiable research spaces for experiments, doctoral projects, fairness research, calibration-equity studies, subgroup analysis, transfer science, multicenter validation, standards development, and governance research. The Answer Embryos are deliberately provisional. Each states a possible answer, the evidence that would strengthen or weaken it, a design consequence, and a question for the next research hand. The central principle is simple: Personalization should reduce mismatch, not manufacture a privileged subgroup. A mature adaptive BCI should explain who benefits, who does not, who never reaches calibration, what burden and support are required, how the policy behaves across devices and sites, and whether the claimed benefit survives outside the environment that produced it.