Estimation Strategies

Choose · estimate · compare · justify

Mode

Estimation scene

Situation

Strategy

Rounding unit10

Your estimate140

Animation speed1×

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Equations and comparison

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Voice and language

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Ready · 10 visual chapters.

Chapters

Learning objectives

Select a strategy that matches the numbers and operation.
Use rounding, front-end, compatible-number, clustering and benchmark methods.
Calculate absolute and percent error.
Use bounds, magnitude and context to judge reasonableness.

Key ideas

Absolute error = |estimate − exact|. Percent error = absolute error ÷ |exact| × 100. Rounding to the nearest unit p changes one number by at most p ÷ 2.

Estimates should be reported with appropriate precision. A useful estimate is close enough for its purpose and simple enough to calculate mentally.

Try these experiments

Compare all five strategies on the same grocery total.
Change the rounding unit from 1 to 10 to 100.
Use compatible numbers for fuel efficiency and warehouse boxes.
Use benchmarks for recipe decimals.
Generate a challenge and justify your choice.

Questions

When is a range better than one value? Why can rounding both factors upward overestimate a product? When must a practical quotient round up?

Seven-question check

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Ten independent pages. Works with touch, stylus, mouse, Android devices and interactive smart boards.