#### Module #3, The impact of an outlier

Download PDF Copy Go to Regression Applet

This problem is a little different than the others. Again, if you don’t have a copy of this page you should print it. If you have a copy of the page, go directly to the applet.

For this problem, first distribute the data points so that you have a correlation of about +.90.

**A. **Imagine that you added an extreme point to the upper left-hand corner (don’t actually add the point yet). How do you think this would change your correlation?

**B. **Would you expect the correlation to become larger or smaller? ______________

**C. **If you had to guess, what do you think your correlation value would be after the addition of this single point? ___________

**D. **Now add a point to the upper left-hand corner (to add a point, double click the spot where you want to add the point). This new point will represent a single point that deviates from your strong pattern of correlations.

**E. **What is your new correlation? ___________________

**F. **Does this differ from your expectation? How? Comment on the impact of the addition of an extreme score. (It may be interesting to look at the squares for errors – Check “Show SS error” and “Show Error as squares.”)

**G. **Imagine that you had a set of scores where one value was incorrectly entered (e.g., X = 100 entered when x was actually 10). How might this type of error impact your results?

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