How do you know if an outlier is the result of a data glitch, or a real data point -- indeed maybe not an outlier. Difficult question to answer, but the chart below shows that in some cases, the outlier is not an error.
This example illustrates that an outlier can be a valid and very interesting point, but also has a dramatic impact on model fitting and R-Square if you use non-robust metrics.
Read more and see chart at http://goo.gl/x4RiNR
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