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90 questions · full solutions

Machine Learning

The quant-researcher round: bias and variance, leakage-proof validation, regularization, and the finance-specific failure modes most ML courses never mention.

  1. 01Bias, Variance, and the Noise FloorMedium · Private
  2. 02Why Your Backtest LiesMedium · Private
  3. 03Ridge vs Lasso, and Why Quants Love RidgeMedium · Private
  4. 04Log-Loss from First PrinciplesMedium · Private
  5. 05Two Roads to the Same LineMedium · Private
  6. 06Neighbors Who Live Far AwayHard · Private
  7. 07Margins and HingesMedium · Private
  8. 08The Kernel Trick in One ParagraphMedium · Private
  9. 09Naive Bayes Counts TwiceEasy
  10. 10Why One Tree Is a GambleEasy
  11. 11The Forest's Variance FloorMedium · Private
  12. 12Boosting Is Gradient Descent in Function SpaceHard · Private
  13. 13What the λ and γ Are DoingHard · Private
  14. 14Importance Is Not ImportanceMedium · Private
  15. 15AUC by EnumerationMedium · Private
  16. 16The 99% Accurate Crash DetectorMedium · Private
  17. 17Right Order, Wrong ProbabilitiesHard · Private
  18. 18The Cost of Saying NeverMedium · Private
  19. 19Accuracy TheaterEasy
  20. 20When Gradient Descent ExplodesMedium · Private
  21. 21The Useful Noise in SGDMedium · Private
  22. 22Which Objectives Are Convex?Easy
  23. 23One Step of NewtonMedium · Private
  24. 24Why Deep Sigmoids Stop LearningEasy
  25. 25Universal Approximation, Limited ComfortHard · Private
  26. 26Dropout: An Ensemble in DisguiseMedium · Private
  27. 27Batch Norm, and the Leak Nobody MentionsMedium · Private
  28. 28Past the Interpolation ThresholdHard · Private
  29. 29Two Eigenvalues, One DirectionMedium · Private
  30. 30What k-Means Actually OptimizesEasy
  31. 31PCA Meets the Cross-SectionMedium · Private
  32. 32The Label Is the StrategyHard · Private
  33. 33A Thousand Rows, Two Hundred FactsHard · Private
  34. 34Trained on a World That EndedHard · Private
  35. 35The Best of a Hundred NothingsHard · Private
  36. 36Gross Genius, Net RealityMedium · Private
  37. 37One Model for Direction, Another for ConvictionHard · Private
  38. 38Models That AgeMedium · Private
  39. 39Count the WeightsEasy
  40. 40One Number from Four CellsEasy
  41. 41The Dummy Variable TrapEasy
  42. 42Ridge, One Eigenvalue at a TimeMedium · Private
  43. 43The Prior That PullsMedium · Private
  44. 44When the Noise Isn't FlatMedium · Private
  45. 45One Split, By HandMedium · Private
  46. 46The Word That Never AppearedMedium · Private
  47. 47Lasso's Knife, Ridge's SqueezeMedium · Private
  48. 48Ten Flavors of MomentumMedium · Private
  49. 49Fifty Times the VarianceMedium · Private
  50. 50Only the Tails Get TradedMedium · Private
  51. 51Confidence, AuditedMedium · Private
  52. 52Anatomy of One HeadMedium · Private
  53. 53The Unit That Stopped AnsweringMedium · Private
  54. 54Bias and Variance, With Actual NumbersHard · Private
  55. 55Deliberately Wrong on PurposeHard · Private
  56. 56Two Copies, One CoefficientHard · Private
  57. 57Every Penalty Is a PriorHard · Private
  58. 58The Best Possible ShrinkageHard · Private
  59. 59The Likelihood That Never Stops ClimbingHard · Private
  60. 60Trained on a Balanced LieHard · Private
  61. 61Manufacturing Skill from NothingHard · Private
  62. 62Purge, Then EmbargoHard · Private
  63. 63The Score You Tuned Into ExistenceHard · Private
  64. 64Can Your Validation Set Even Tell?Hard · Private
  65. 65Twenty Tickets in the Sharpe LotteryHard · Private
  66. 66Your Labels Remember Each OtherHard · Private
  67. 67How Much Data Do You Actually Have?Hard · Private
  68. 68Two Regimes, Zero SignalHard · Private
  69. 69Left Out of the BagEasy
  70. 70The Floor No Model BeatsMedium · Private
  71. 71The Gradient Is the MissMedium · Private
  72. 72Coverage Without Believing the ModelMedium · Private
  73. 73Prediction as Weighted SimilarityMedium · Private
  74. 74Why 0.5 Is Almost Never the ThresholdMedium · Private
  75. 75Two Rounds of AdaBoost, By HandHard · Private
  76. 76One Honest Step of EMHard · Private
  77. 77One Attention Lookup, By HandHard · Private
  78. 78The Chain Rule, AuditedHard · Private
  79. 79Ten Layers Later, the Signal Is GoneHard · Private
  80. 80Betting on Experts with Multiplicative WeightsHard · Private
  81. 81Taking the Brier Score ApartHard · Private
  82. 82The Importance That Halved ItselfHard · Private
  83. 83The R-Squared You Get for FreeHard · Private
  84. 84Epochs Are a Regularization DialHard · Private
  85. 85The Feature That Contains Its Own AnswerHard · Private
  86. 86Training Error Owes You MoneyHard · Private
  87. 87What a Tiny R-Squared Is WorthHard · Private
  88. 88The Loss That Forecasts a QuantileHard · Private
  89. 89Find the Three LeaksHard · Private
  90. 90The Category Trick That CheatsHard · Private