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Survivorship Bias in Operational Data: When the Failures Are Missing from the Table
A reliability study follows every machine in service today for two years and estimates a median life of 7.7 years. The true median is 4.0. The machines that ...
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Measurement Error in Predictors: Regression Dilution and the Field Deployment Gap
The fitted effect of temperature comes out at half what the physics says. The model built on lab measurements loses two thirds of its accuracy on field senso...
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Leaky Gut: Real Physiology, Weak Diagnosis
The intestinal barrier is real and becomes more permeable in defined diseases and under physiological stress. The mistake is turning that observation into a ...
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Preconditioning Changes the Problem Your Iterative Solver Sees
Large sparse linear systems are often limited less by arithmetic than by conditioning and spectral geometry. A preconditioner changes the system seen by the ...
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Permutation Importance with Correlated Features: When the Ranking Lies
A near-duplicate sensor with no effect of its own outranks a feature that genuinely drives the outcome. Permutation importance is working exactly as designed...
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Unlabelled Data Does Not Identify the Decision Boundary
Unlabelled observations can estimate where the data live, how dense different regions are and which points are close. They do not, by themselves, identify wh...
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Post-Stratification: Weighting a Survey That Answered Unevenly
The survey says 7.36 and the population says 7.10. Nobody lied: older customers answered twice as often as younger ones and they score higher. Weighting to t...
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When the Bootstrap Fails: Dependent Data, Small Samples, and Extremes
Resample the data, recompute the statistic, read off the spread. On an autocorrelated series of 200 points the interval that comes out is typically 2.4 times...
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DPO Changes Preferences, Not Knowledge
Supervised fine-tuning teaches a model what a good answer looks like. Preference optimization teaches it which of two answers should be preferred. Those are ...
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Interference in Experiments: When Treated Users Take What Control Users Would Have Bought
The ranking change raises purchase intent from 10 to 12 percent, and the A/B test reports a 20 percent lift in sales. Rolled out to everyone, it delivers 3 p...
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