What are outliers?

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Outliers are extreme values as compared to the rest of the data.

  1. The determination of values as “outliers” is subjective. While there are a few benchmarks for determining whether a value is an “outlier”, those benchmarks are arbitrarily chosen, similar to how “p<.05” is also arbitrarily chosen.
  2. One benchmark is to use a BOXPLOT to determine "mild" and "extreme" outliers. Mild outliers are any score more than 1.5*IQR from the rest of the scores, and are indicated by open dots. IQR stands for “Interquartile range”, and is the middle 50% of the scores. Extreme outliers are any score more than 3*IQR from the rest of the scores, and are indicated by stars. Fe40.png - The output below is from SPSS for a variable called "system1". A boxplot is a graphical display of the data that shows: (1) median, which is the middle black line, (2) middle 50% of scores, which is the shaded region, (3) top and bottom 25% of scores, which are the lines extending out of the shaded region, (4) the smallest and largest (non-outlier) scores, which are the horizontal lines at the top/bottom of the boxplot, and (5) outliers. For this variable, there is 1 mild outlier (subject #52) and 1 extreme outlier (subject #18).
System1 boxplot0.png

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