Influence of Outliers on Correlation

This page authored by Roger Woodard, Steve Stanislav, Jennifer Gratton, Pam Arroway, NC State University, based on an applet by David S. Moore, Purdue University.
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This activity has been undergone anonymous peer review.

This activity was anonymously reviewed by educators with appropriate statistics background according to the CAUSE review criteria for its pedagogic collection.

This page first made public: May 9, 2007

This material is replicated on a number of sites as part of the SERC Pedagogic Service Project


This activity begins with an instructor demonstration followed by a student out-of-class assignment. Students will observe their instructor create a scatterplot and observe how the correlation coefficient changes when outlier points are added. Students are then given a follow up assignment, which guides them through the applet. In addition, the assignment provides insight about outliers and their effect on correlation. This activity will show exactly how outliers numerically change the correlation coefficient value and to what degree.

Learning Goals

Context for Use

This visualization activity is used for an introductory college statistics course. Students will be asked to complete an assignment out of class that will help them gain intuition about the influence of outliers and how they affect correlation.

Prerequisites for this activity:

Description and Teaching Materials

This activity is intended to be assigned for out of class use. It can be part of a regular homework assignment or as a laboratory assignment. In this assignment students use the applet to construct specific correlations. They then move points to examine their influence on the correlation. Students summarize the main point of the exercise by answering a multiple choice question. Then the students examine a question about real data that illustrates the concept.

Teaching Notes and Tips

Students should be encouraged to use this applet on their own to gain hands on knowledge on how correlation changes as outliers are moved around and/or removed. A step-by-step process for the students to follow is included in the follow up activity provided.

Use the applet is fairly straightforward and outlined here, or can be read on the webpage.


Students can be assessed with a question on a future quiz which asks, "Which of two correlation coefficients will change the most by removing their respective outliers." Then during an upcoming regression lecture, an example of a regression will be done that has an outlier. During this discussion, the class will take a vote as to how the correlation will change as well as the regression line equation.

References and Resources