Mathieu Ajaka — WritingTechnical notes on building and evaluating production AI systems — RAG, agents, evaluation, and time-series ML.https://www.mathieuajaka.com/enPurged walk-forward vs ordinary cross-validation: preventing leakage in financial time-series MLhttps://www.mathieuajaka.com/writing/purged-walk-forward-vs-cross-validation/https://www.mathieuajaka.com/writing/purged-walk-forward-vs-cross-validation/Why k-fold cross-validation leaks on overlapping-label time series, and the purging, embargo, CPCV, and deflated-Sharpe discipline that make evaluation honest.Wed, 29 Jul 2026 00:00:00 GMTtime-seriesbacktestingevaluationleakageChunking Arabic legal text for RAG (and why BM25 collapses on clitics)https://www.mathieuajaka.com/writing/chunking-arabic-legal-text-for-rag/https://www.mathieuajaka.com/writing/chunking-arabic-legal-text-for-rag/Why Arabic legal retrieval is hard, how naive BM25 silently fails on clitic-heavy queries, and the morphology-aware chunking and hybrid retrieval that fix it.Wed, 15 Jul 2026 00:00:00 GMTragarabic-nlpretrievalevaluation