# Lesson 14

Percent Error

### Lesson Narrative

Situations involving percent error can be more difficult than situations involving percent increase or percent decrease because the student has to decide which amount represents the whole. In this lesson students get practice using the language of percent error in various different situations, and identifying the correct amount, which is the whole, and the incorrect amount (MP1). They work with a multi-step problem involving percent error. They also see a common usage of percent error to express a range of possible values by thinking about a scale that claims to be accurate to within 0.5%. Understanding and finding percent error is important for solving real-world problems (MP4).

### Learning Goals

Teacher Facing

• Calculate the percent error, correct amount, or erroneous amount, given the other two of these three quantities, and explain (orally and using other representations) the solution method.
• Compare and contrast (orally) strategies used for solving problems about percent error with strategies used for solving problems about percent increase or decrease.

### Student Facing

Let’s use percentages to describe other situations that involve error.

### Required Preparation

For the Measuring in the Heat activity, student will need access to calculators.

### Student Facing

• I can solve problems that involve percent error.

Building On

Building Towards

### Glossary Entries

• measurement error

Measurement error is the positive difference between a measured amount and the actual amount.

For example, Diego measures a line segment and gets 5.3 cm. The actual length of the segment is really 5.32 cm. The measurement error is 0.02 cm, because $$5.32-5.3=0.02$$.

• percent error

Percent error is a way to describe error, expressed as a percentage of the actual amount.

For example, a box is supposed to have 150 folders in it. Clare counts only 147 folders in the box. This is an error of 3 folders. The percent error is 2%, because 3 is 2% of 150.