UNIT B.7 — MATHEMATICS

Probability, statistics, and measurement

BRIDGE · EE Foundations Bridge · ~10 min

Community content — not verified by a licensed electrician. Lessons and questions here are written for study practice and pinned to NEC 2023. Code editions and local amendments vary by jurisdiction. Always confirm against your adopted code book and your licensing board's published exam outline before test day.

Measure the same receptacle five times and you'll get five slightly different readings — 119.8, 120.1, 119.9, 120.3, 120.0. Statistics turns that scatter into knowledge: the mean (average, here 120.02 V) is your best estimate; the standard deviation measures the spread, i.e. how much any single reading can be trusted.

Your meter's spec sheet speaks this language: an accuracy of ±(0.5% + 2 digits) is a probabilistic promise about where the true value lies. Understanding it separates a real deviation from instrument noise — chasing noise is how afternoons get wasted.

Probability also underlies why demand factors exist (Unit 2.3): not all loads run at once, and the 35% discount on lighting load beyond 3 000 VA is a statistical statement about diversity, baked into the code.

Go deeper: MIT's Fundamentals of Statistics and the free Grinstead & Snell probability text — Semester 2 of the Go Deeper pathway.

Mean

Best estimate

Std deviation

Spread / trust

Demand factors

Applied probability

Knowledge check

Quiz — 3 questions

1. Readings 10, 12, 14 have a mean of:

2. A larger standard deviation in repeated measurements means:

3. NEC demand factors (like 35% beyond 3 000 VA) exist because:

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