---
title: "Uncertainty, Precision and Significant Figures in a Measurement"
description: "Every reading carries an uncertainty set by the instrument and by how the measurement was taken, and quoting a result means quoting that uncertainty with it. Repeating a reading shrinks the random par"
canonical: https://lightmysky.com/learn/science/uncertainty-precision-and-significant-figures-in-a-measurement-mt_HPc3OahYoV
source: https://lightmysky.com/learn/science/uncertainty-precision-and-significant-figures-in-a-measurement-mt_HPc3OahYoV.md
retrieved: 2026-09-12
---

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# Uncertainty, Precision and Significant Figures in a Measurement

Every reading carries an uncertainty set by the instrument and by how the measurement was taken, and quoting a result means quoting that uncertainty with it. Repeating a reading shrinks the random part and leaves a zero error untouched, which is why precise and accurate are not the same word.

Subject: Science · Area: Scientific Inquiry · Ages 16 to 17
Page: https://lightmysky.com/learn/science/uncertainty-precision-and-significant-figures-in-a-measurement-mt_HPc3OahYoV

## Ready when they can

- Quotes a measurement with an absolute and a percentage uncertainty and rounds the value to match
- Separates a random scatter from a systematic offset in a set of repeated readings and says what fixes each
- Chooses an instrument and a method that make the dominant uncertainty smaller, and says which one it was

## Lesson: Measuring with honest doubt

Near truth and near self differ. Rightness means close to the true mark; repeat means reads bunch tight. A tight bunch far from truth is exact yet wrong.

**Example.** Pen 24.6 cm plus or minus 0.2 cm. Round the mark to the same slot as the doubt. The share of doubt is near one in a hundred, so this term rules sums it joins.

Scatter spreads both ways and shrinks with more goes and soft hands. Slant shoves each read one way and lives through means. More goes cure scatter; just tuning or fresh means cure slant.

**Tip.** Count tallies with care. Held zeros count, fore zeros never do, and tail zeros count just with a dot. Times and splits keep the fewest tallies; plus and minus halt at the roughest slot. Strike the chief doubt first.

**Recap.** Quote each mark with its doubt, split scatter from slant, and tune the chief doubt first.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Density and How to Measure It](https://lightmysky.com/learn/science/density-and-how-to-measure-it-mt_HrKI1-UzfI)
- [Scientific Quantities and Units](https://lightmysky.com/learn/science/scientific-quantities-and-units-mt_vdHqdaflog)

## Opens up

- [Combining Uncertainties and Reading a Result off a Graph](https://lightmysky.com/learn/science/combining-uncertainties-and-reading-a-result-off-a-graph-mt_kn8RhTDEOK)