# What are outliers?

### From PsychWiki - A Collaborative Psychology Wiki

(Difference between revisions)

Line 1: | Line 1: | ||

*'''What are outliers?''' | *'''What are outliers?''' | ||

- | Outliers are extreme values as compared to the rest of the data. | + | :Outliers are extreme values as compared to the rest of the data. |

*'''What does "extreme" mean?''' | *'''What does "extreme" mean?''' | ||

#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<u><</u>.05” is also arbitrarily chosen. | #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<u><</u>.05” is also arbitrarily chosen. | ||

- | #One benchmark is to use a | + | #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. [[Image: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). |

+ | <center>[[Image:System1_boxplot0.png]]</center> | ||

+ | |||

+ | |||

+ | |||

+ | |||

+ | |||

- | |||

## Revision as of 04:14, 16 February 2008

**What are outliers?**

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

**What does "extreme" mean?**

- 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. - 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. - 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).

◄ Back to Research Tools mainpage