# Precision vs. Accuracy

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## Objective

Students will be able to differentiate between precision and accuracy and explain how the terms relate to chemistry class.

#### Big Idea

Precision describes the repeatability of measurements, while accuracy refers to how closely the measurement mirrors the actual value that is being measured.

## Introduction

In this lesson students read about precision and accuracy. They then  play a target practice game and they construct a cubic centimeter from paper, and these activities are used to help students apply these two terms in different contexts. This lesson relates to the NGSS Science Practice 4: Analyzing and interpreting data because it gets at  important issues about data. Namely, it is important to get as accurate data as possible while considering the constraints imposed by available technology and measurement tools. Precision of data suggests that the measurements are repeatable and consistent, and can be trusted to some extent, depending on how accurate the results are. Accuracy is easiest to measure when there is a known value that can be compared with the measured data.

This lesson also relates to the NGSS Science Practice 5: Engaging in argument from evidence because students will be asked to explain how precise and accurate their target practice is using the definitions and their targets.

Students will not need any prior knowledge to complete this lesson.

To do the penny toss game I use a large piece of paper, markers, and pennies. However, any materials that would allow students to aim for a target would be appropriate.

To make cubic centimeters I use paper, scissors, metric rulers, and tape.

## Do Now/Activator

10 minutes

Do Now: Students read an article such as this one and then using their own words they try to record in their notebook the meaning of the words precision and accuracy in science.

By having students do this, I get them thinking about today's objective. During this time I take attendance and make sure my materials are accessible.

Activator: I then invite students to share their answers at their table. Students have assigned seats at tables with 4 students per table. Their conversation is important because learning happens best when students have time to discuss ideas. While students are talking about the ideas I walk around the room and listen to the conversations. Because students have clear pictures that differentiate precision and accuracy, they show that they understand the terms. However, changing the context of these terms from a bull's-eye to data is more challenging for students. However, I will have all year to reinforce these ideas in a data context.

## Mini-lesson and Guided Practice

15 minutes

Mini-lesson:

Note: Students will be instructed in 2 different parts of class: Part 1 is taught right after the Do Now and Activator, while Part II is taught after students have completed following the instructions from Part 1. Part 2 is contextualized by Part 1. Part 1 helps students get comfortable with the terms of precision and accuracy, but Part 2 better relates precision and accuracy to chemistry because it allows students the opportunity to make and measure something, much like they will be doing when they have to conduct chemical reactions.

Part 1: During this time I refrain from talking too much about precision and accuracy. I believe that by the end of the class students will have a clear understanding of these terms. Instead, I review with the class how the penny toss game works and focus on the game component of class. Essentially, one student at a time tosses 5 pennies at a target. A partner marks with a marker the spot that the penny initially lands. After everyone in the group has gone students identify which members of their group showed precision and accuracy in their tosses.

This sample student target is a sample of what students should produce during this part of the lesson.

Part 2: After we have debriefed the penny toss game, I explain that now we will see how the terms relate to chemistry. I show them this slide with a pattern that they will use to create a cubic centimeter. I tell them to make very careful measurements, cuts and folds. Each square in the pattern is 1 cm, and each line is a fold. They can use scotch tape to close their cubic centimeter.

## Application

25 minutes

Student Activity:

In Part 1 students take turns tossing their pennies. Each person who tosses will need another student to mark the target with a unique color or symbol identifier based on where their partner's pennies initially land. It is in steps 4 & 5 of the Penny Toss that students begin to make meaning about the words precision and accuracy.

In Part 2 students make a cubic centimeter following the pattern. It is hard to measure and cut exactly, especially with scissors, and it is challenging to keep the box together with tape, which adds measurement error for students who use more than one layer of tape.

Catch and Release Opportunities:

Part 1: Throughout the activity I circulate and lightheartedly comment on students aim or lack thereof, but I am especially eager to listen to conversations about which student, based on the target, was most precise and/or accurate. An example of how I participate can be seen in this video that models how I process with students

Part 2:  During the construction I am supportive of students' challenges, which are technical in nature. After students have constructed their cubic centimeters I tell them that I doubt anyone has precisely and accurately constructed a cubic centimeter. Some students might be quick to claim the contrary, and so I challenge them to explain what an accurate and precise cubic centimeter looks like. They explain that it has to be 1 cm on every side. When they measure, they often find that it is not. However, students who are careful with their construction find that they are often precise, meaning that the amount they are off from 1 cm, which is around 1 mm, is consistent, which makes their work precise.

## Debrief

10 minutes

During this time I ask students to turn and talk at their tables. The topic is "How does precision and accuracy relate to what we will be doing in Chemistry class this year?" After about 2-3 minutes I ask the different groups to share with the whole class. Some student responses reflect the following ideas:

Precision is a challenge in chemistry because of human error—we need to pay close attention to how we measure.

Even when we do everything correctly, accuracy is a challenge because we can only be as accurate as our tools allow.

Throughout the lesson I check for understanding by listening to and participating in conversations. By the end of class students vary in their understanding of the two ideas, but the seed has been planted. By regularly returning to these two concepts during subsequent data analysis, students will come to fee comfortable with their understanding of precision and accuracy.