The mathematical purpose of the lesson is for students to recognize outliers, to investigate their source, to make decisions about excluding them from the data set, and to understand how the presence of outliers impacts measures of center and measures of variability. This lesson relies on work in previous lessons in which students found measures of center and variability. This lesson connects to upcoming work because students will investigate outliers when dealing with bivariate data in another unit. Students encounter the term outlier which is a data value that is unusual in that it differs quite a bit from the other values in the data set.
When students have to analyze data in the context of a problem to determine whether or not to exclude an outlier, they are reasoning abstractly and quantitatively (MP2). This reasoning process is also an aspect of mathematical modeling (MP4).
Lesson overview
 14.1 Warmup: Health Care Spending (10 minutes)
 14.2 Activity: Investigating Outliers (15 minutes)

14.3 Activity: Origins of Outliers (10 minutes)
 Includes "Are you Ready for More?" extension problem
 Lesson Synthesis
 14.4 Cooldown: Expecting Outliers (5 minutes)
Learning goals:
 Describe (orally and in writing) how outliers impact measure of center and measures of variability.
 Determine (in writing) when values are considered outliers, investigate their source, and determine if they should be excluded from the data.
Learning goals (student facing):
 Let’s investigate outliers and how to deal with them.
Learning targets (student facing):
 I can find values that are outliers, investigate their source, and figure out what to do with them.
 I can tell how an outlier will impact mean, median, IQR, or standard deviation.
Required materials:
 Statistical technology
Required preparation:
 Acquire devices that can run GeoGebra (recommended) or other spreadsheet and statistical technology.
 It is ideal if each student has their own device. (A GeoGebra Spreadsheet is available under Math Tools.)
Glossary:
 outlier  A data value that is unusual in that it differs quite a bit from the other values in the data set. In the box plot shown, the minimum, 0, and the maximum, 44, are both outliers.
 Access the complete Algebra 1 glossary.
Standards:
 This lesson builds on the standard: CCSS.HSSID.A.1MS.SID.1MO.A1.DS.A.1
 This lesson builds towards the standard: CCSS.HSSID.A.3MS.SID.3MO.A1.DS.A.3
IM Algebra 1, Geometry, Algebra 2 is copyright 2019 Illustrative Mathematics and licensed under the Creative Commons Attribution 4.0 International License (CC BY 4.0).
The Illustrative Mathematics name and logo are not subject to the Creative Commons license and may not be used without the prior and express written consent of Illustrative Mathematics.