--- title: "Uncertainty in a Measurement and Where It Comes From" description: "Every measured value carries a range, and quoting the value without it says nothing about whether two results agree. Some of that range is found by repeating the measurement and the rest is read off t" canonical: https://lightmysky.com/learn/science/uncertainty-in-a-measurement-and-where-it-comes-from-mt_JXw4D9DReI source: https://lightmysky.com/learn/science/uncertainty-in-a-measurement-and-where-it-comes-from-mt_JXw4D9DReI.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Uncertainty in a Measurement and Where It Comes From Every measured value carries a range, and quoting the value without it says nothing about whether two results agree. Some of that range is found by repeating the measurement and the rest is read off the equipment. Subject: Science · Area: Chemistry · Ages 18 to 19 Page: https://lightmysky.com/learn/science/uncertainty-in-a-measurement-and-where-it-comes-from-mt_JXw4D9DReI ## Ready when they can - Estimates an uncertainty from repeated readings and states the number of readings used - Estimates an uncertainty from a manufacturer's tolerance when repeating is not possible - Distinguishes a random contribution from a systematic one and names an experiment that would reveal each - Quotes a result with a matched number of significant figures in the value and its uncertainty ## Lesson: Every measurement comes with a range You never get to report a bare number. Every measurement carries a small range, and you must quote it, or nobody can tell whether two results agree. Precision means your repeats agree with each other. Accuracy means they sit near the truth. **Example.** You measure a table five times and get 120.1, 120.3, 120.2, 120.3 and 120.1 cm. Order them, then subtract the lowest from the highest: 120.3 minus 120.1 is 0.2 cm. Report half of that range, plus or minus 0.1 cm, and always state that it came from 5 readings. Some things you can measure only once, like a flask you may fill a single time. Then your uncertainty comes from the tolerance the maker printed on the glassware. Learn the two kinds of error: scatter around the middle is random and repeats reveal it, while a steady push one way is systematic, like a scale that always reads 2 kg heavy, and only a known standard reveals that. **Tip.** Quote the value and its uncertainty so their digits stop together, as in 120.2 cm plus or minus 0.1 cm. Extra digits claim knowledge your readings never gave you. **Recap.** Quote every result with its range, and let repeats, tolerances and a known standard each play their part. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Standard Deviation and Variance](https://lightmysky.com/learn/mathematics/standard-deviation-and-variance-mt_eNpow5tIX2) - [Enthalpy Change, Standard Conditions and Calorimetry](https://lightmysky.com/learn/science/enthalpy-change-standard-conditions-and-calorimetry-mt_GPgwgyqyvN) ## Opens up - [Propagating Uncertainty Through a Calculation](https://lightmysky.com/learn/science/propagating-uncertainty-through-a-calculation-mt_xssxg6NqQM)