--- title: "Choosing a Model and Naming What It Ignores" description: "Set up a mathematical model for a real situation, state the assumptions it rests on, and say which of them would break the answer if they were wrong." canonical: https://lightmysky.com/learn/mathematics/choosing-a-model-and-naming-what-it-ignores-mt_UtK9aauJ6e source: https://lightmysky.com/learn/mathematics/choosing-a-model-and-naming-what-it-ignores-mt_UtK9aauJ6e.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`. # Choosing a Model and Naming What It Ignores Set up a mathematical model for a real situation, state the assumptions it rests on, and say which of them would break the answer if they were wrong. Subject: Mathematics · Area: Mathematical Thinking · Ages 14 to 15 Page: https://lightmysky.com/learn/mathematics/choosing-a-model-and-naming-what-it-ignores-mt_UtK9aauJ6e ## Ready when they can - Turn a described situation into a formula or graph and list the assumptions made - Identify which assumption the answer is most sensitive to - Judge whether a model's answer is usable, and say what would have to change to trust it further ## Lesson: Models answer, assumptions decide A model turns a wordy situation into maths you can compute with. A gym charges a 20 pound joining fee plus 15 pounds per month, so total cost C for m months is C equals 20 plus 15m. Pick variables, write the equation, solve. **Example.** Snow depth follows D equals 3 plus 2h, where D is cm and h is hours. The 3 is the starting depth and the 2 is the hourly rate. It works while snow falls steadily all night. Every model rests on assumptions: the things you ignore or hold fixed. Write them beside the formula. The sensitive one is where a small nudge moves the answer most, like the growth rate in a population forecast. **Tip.** Judge a model before you trust it. A sunshine model failed on football-final weekend because crowds, not sun, ruled that day. Fitting last year perfectly never guarantees next year. **Recap.** Write the formula, name what it ignores, find the assumption that moves the answer most, and judge the verdict before you use it. ## Practice 14 questions on this page, each with its working shown. ## Needs first - [Simple formulae](https://lightmysky.com/learn/mathematics/simple-formulae-mt_3qrCtdoVAU) - [Linear Function Graphs](https://lightmysky.com/learn/mathematics/linear-function-graphs-mt_WBdHkc2HTf) - [Real-World Mathematical Modelling](https://lightmysky.com/learn/mathematics/real-world-mathematical-modelling-mt_yqZiX6vS7D) ## Opens up - [Modelling Motion in a Straight Line](https://lightmysky.com/learn/mathematics/modelling-motion-in-a-straight-line-mt_no78_6w1rS)