Vijayakumar P — Notes & PostsPlain-language posts on HR, analytics, teaching, and research from Vijayakumar P.https://vijaymbaphd.github.io/en-inDegrees of Freedom Walks Into a Tea Stallhttps://vijaymbaphd.github.io/blog/degrees-of-freedom/https://vijaymbaphd.github.io/blog/degrees-of-freedom/Vijay's calculator keeps using n − 1 instead of n. He doesn't know why. Two strangers — one with a fan of blank slips, one with a brass padlock — explain why every parameter you estimate costs you one of your data points.Fri, 08 May 2026 00:00:00 GMTAnalyticsstatisticsdata-sciencehypothesis-testingfundamentalsexplainerVijayakumar PNominal, Ordinal, Interval, Ratio Walk Into a Tea Stallhttps://vijaymbaphd.github.io/blog/levels-of-measurement/https://vijaymbaphd.github.io/blog/levels-of-measurement/Vijay's notebook has four columns. He has tried to take the average of each. Three of them gave him an answer; only one of those answers was meaningful. Four siblings turn up, climb a ladder, and explain why.Thu, 07 May 2026 00:00:00 GMTAnalyticsstatisticsdata-sciencefundamentalsexplainermeasurementVijayakumar PNull and Alternative Hypothesis Walk Into a Tea Stallhttps://vijaymbaphd.github.io/blog/null-vs-alternative-hypothesis/https://vijaymbaphd.github.io/blog/null-vs-alternative-hypothesis/Vijay started serving fresh ginger biscuits this month. His daily mean rose from 87 to 91. Real effect or noise? Two strangers explain why every test is a courtroom — and which side is presumed innocent.Thu, 07 May 2026 00:00:00 GMTAnalyticsstatisticsdata-sciencehypothesis-testingfundamentalsexplainerVijayakumar POne-tailed and Two-tailed Tests Walk Into a Tea Stallhttps://vijaymbaphd.github.io/blog/one-tailed-vs-two-tailed-tests/https://vijaymbaphd.github.io/blog/one-tailed-vs-two-tailed-tests/Vijay added cardamom to his tea. Mean rating rose from 3.8 to 4.0. He's about to run the t-test, and the software asks him a question he's never thought about — one-sided or two? Two strangers, one with a single arrow and one with two, explain why it matters.Thu, 07 May 2026 00:00:00 GMTAnalyticsstatisticsdata-sciencehypothesis-testingfundamentalsexplainerVijayakumar Pt-test and z-test Walk Into a Tea Stallhttps://vijaymbaphd.github.io/blog/t-test-vs-z-test/https://vijaymbaphd.github.io/blog/t-test-vs-z-test/Vijay's cousin keeps claiming he should average 100 customers a day. The notebook says 87. Two strangers turn up to test it — one needs to know sigma, the other doesn't.Thu, 07 May 2026 00:00:00 GMTAnalyticsstatisticsdata-sciencehypothesis-testingfundamentalsexplainerVijayakumar PType I and Type II Errors Walk Into a Tea Stallhttps://vijaymbaphd.github.io/blog/type-i-vs-type-ii-errors/https://vijaymbaphd.github.io/blog/type-i-vs-type-ii-errors/Vijay's last 7 days dropped from his usual 87 to 75. The t-test rejects. Two strangers turn up — one rings a bell at every shadow, the other sleeps through real wolves — and explain what could go wrong with that verdict.Thu, 07 May 2026 00:00:00 GMTAnalyticsstatisticsdata-sciencehypothesis-testingfundamentalsexplainerVijayakumar PCorrelation and Regression Walk Into the Same Tea Stallhttps://vijaymbaphd.github.io/blog/correlation-vs-regression/https://vijaymbaphd.github.io/blog/correlation-vs-regression/Vijay the tea stall owner needs to know how many samosas to fry tomorrow. Correlation can describe his data, Regression can predict from it — and the difference is the whole story.Tue, 28 Apr 2026 00:00:00 GMTAnalyticsstatisticsdata-sciencefundamentalsexplainerVijayakumar PCovariance and Correlation Walk Into a Tea Stallhttps://vijaymbaphd.github.io/blog/correlation-vs-covariance/https://vijaymbaphd.github.io/blog/correlation-vs-covariance/Vijay the tea stall owner has a question. Two old friends arrive with two different answers — same idea wearing different clothes.Sat, 25 Apr 2026 00:00:00 GMTAnalyticsstatisticsdata-sciencefundamentalsexplainerVijayakumar P