Lesson 19
Comparing Populations With Friends
19.1: Features of Graphic Representations (5 minutes)
Warmup
In this warmup, students review the useful information that can be gained from different graphical representations of data in preparation for comparing groups based on samples from each.
Launch
For classes that may need help remembering the different representations, consider displaying an example of each type of graphical representation mentioned.
Student Facing
Dot plots, histograms, and box plots are different ways to represent a data set graphically.
Which of those displays would be the easiest to use to find each feature of the data?
 the mean
 the median
 the mean absolute deviation
 the interquartile range
 the symmetry
Student Response
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Activity Synthesis
Poll the class for their answers to each of the problems. Select at least one student to share their reasoning for each question.
19.2: Info Gap: Comparing Populations (30 minutes)
Activity
In this info gap activity, students work together to compare two populations from information about samples from each of the populations. Students must pay attention to the information they need in order to solve the problem and the types of question they could ask to get to the answer.
The info gap structure requires students to make sense of problems by determining what information is necessary, and then to ask for information they need to solve it. This may take several rounds of discussion if their first requests do not yield the information they need (MP1). It also allows them to refine the language they use and ask increasingly more precise questions until they get the information they need (MP6).
Here is the text of the cards for reference and planning:
Launch
Arrange students in groups of 2. In each group, distribute a problem card to one student and a data card to the other student. After you review their work on the first problem, give them the cards for a second problem and instruct them to switch roles.
Tell students they will continue to work with comparing measures of center for populations. Explain the Info Gap structure and consider demonstrating the protocol if students are unfamiliar with it. There are stepbystep instructions in the student task statement.
Supports accessibility for: Memory; Organization
Design Principle(s): Cultivate Conversation
Student Facing
Your teacher will give you either a problem card or a data card. Do not show or read your card to your partner.
If your teacher gives you the problem card:

Silently read your card and think about what information you need to be able to answer the question.

Ask your partner for the specific information that you need.

Explain how you are using the information to solve the problem.
Continue to ask questions until you have enough information to solve the problem.

Share the problem card and solve the problem independently.

Read the data card and discuss your reasoning.
If your teacher gives you the data card:

Silently read your card.

Ask your partner “What specific information do you need?” and wait for them to ask for information.
If your partner asks for information that is not on the card, do not do the calculations for them. Tell them you don’t have that information.

Before sharing the information, ask “Why do you need that information?” Listen to your partner’s reasoning and ask clarifying questions.

Read the problem card and solve the problem independently.

Share the data card and discuss your reasoning.
Pause here so your teacher can review your work. Ask your teacher for a new set of cards and repeat the activity, trading roles with your partner.
Student Response
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Student Facing
Are you ready for more?
Is there a meaningful difference between top sports performance in two different decades? Choose a variable from your favorite sport (for example, home runs in baseball, kills in volleyball, aces in tennis, saves in soccer, etc.) and compare the leaders for each year of two different decades. Is the performance in one decade meaningfully different from the other?
Student Response
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Activity Synthesis
The purpose of the discussion is to help students understand the types of questions they need to answer in order to compare groups.
Select several groups to share their answers and reasoning for each of the problems. In particular help students understand why the information “The distributions are not symmetric” was important for solving the first problem.
Consider asking these discussion questions:
 “What was the most important question you asked for the first problem? For the second problem?”
 “What are some other ways the information could have been given to solve the problems?” (Instead of the characteristics for the first question, a box plot could have been presented. The second question could have had a dot plot or characteristics like the first problem.)
 “If the distributions for the first problem had been symmetric, would the answer have been the same?” (Yes. The difference in means is 34.25 which is only 1.6 MADs apart, so there would not have been enough information to say that the two population means are meaningfully different.)
19.3: Comparing to Known Characteristics (15 minutes)
Optional activity
In this optional activity, students compare two populations using samples again, but this time only one sample is given. For the other sample, the characteristics have either been computed already or are the focus of the question. This type of analysis is useful when comparing two similar populations as in this activity or when comparing a group against a standard.
Launch
Keep students in groups of 2.
Tell students that sometimes it is useful to compare one group to a standard or another group where the important characteristics have already been computed. In these problems, a random sample from one group is given and characteristics of the second group is either given or sought.
Allow students 10 minutes partner work time followed by a wholeclass discussion.
Student Facing

A college graduate is considering two different companies to apply to for a job. Acme Corp lists this sample of salaries on their website:
$45,000
$55,000
$140,000
$70,000
$60,000
$50,000
What typical salary would Summit Systems need to have to be meaningfully different from Acme Corp? Explain your reasoning.
 A factory manager is wondering whether they should upgrade their equipment. The manager keeps track of how many faulty products are created each day for a week.
 6
 7
 8
 6
 7
 5
 7
Student Response
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Activity Synthesis
The purpose of the discussion is to help students understand how to compare groups when one set of characteristics are known and the other group is represented by sample data.
Select some groups to share their answers and reasoning for the two problems.
Consider asking these discussion questions:
 “How did you determine what to use as a typical value for the first problem?” (Since there was one value much greater than the others, the distribution would not be symmetric, so median is a more appropriate measure of center.)
 “How did you determine what measure of center to use for the second problem?” (Since the data were all close, either value could be used, but the new equipment reported the “average” or mean, so man should be used for the sample as well.)
 “The manufacturer for the new equipment guarantees 4 flaws or fewer per day with the new equipment. If the new equipment produces only 3 flaws per day does that change the answer for the second problem?” (No. There is an even greater difference between the current and new equipment, so it is even more meaningful.)
 “What other factors would the college graduate want to consider other than the meaningful difference in median salary between the two companies?” (In addition to the other factors for a job such a benefits, relationship with coworkers, type of work being done at each company, etc., the graduate should consider the salary for the type of job he will get at the company. For example, if his degree is in computer science, he may be looking at a job with computers rather than sales or some other department within the company, so he might be able to get a better comparison of salaries that way.)
 “What other factors would the factory manager want to consider other than the meaningful difference in flaws for the equipment?” (The cost of the frequent flaws as well as the cost of the new equipment will probably factor into her decision to buy new equipment. The age of the current equipment and maintenance for older equipment compared to new equipment may also be important.)
Supports accessibility for: Visualspatial processing; Conceptual processing
Design Principle(s): Support sensemaking
Lesson Synthesis
Lesson Synthesis
Ask students what information is important to collect when attempting to compare large groups and why each of these pieces of information is useful. Ask students if they can think of other situations in which it might be helpful to compare two large groups by generating a sample and collecting information.
19.4: Cooldown  A Different Box Plot (5 minutes)
CoolDown
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Student Lesson Summary
Student Facing
When using samples to comparing two populations, there are a lot of factors to consider.
 Are the samples representative of their populations? If the sample is biased, then it may not have the same center and variability as the population.
 Which characteristic of the populations makes sense to compare—the mean, the median, or a proportion?
 How variable is the data? If the data is very spread out, it can be more difficult to make conclusions with certainty.
Knowing the correct questions to ask when trying to compare groups is important to correctly interpret the results.