Machine Learning (ML)
One model scored a perfect 1.0000 on the rows it was trained on and 0.3474 on new ones. Another scored 0.9842 and 0.9822.
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Target Variable
Defining churn as leaving within 7 days versus 365 moved the positive rate from 0.2318 to 0.9879 and AUROC from 0.7452 to 0.9619 on identical customers.
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Classification Models
Training on the number and cutting afterward matched the classifier’s AUROC of 0.9967 — and also answered the numeric question at R-squared 0.9900.
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Neural Network Forward Propagation
A beginner-friendly explanation of neural-network structure, notation, hidden layers, and forward propagation with a verified NumPy example.
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Machine Learning Fundamentals
A function fitted to a sample fits that sample better than it fits anything else. Everything difficult follows from that.