Identify outliers with the IQR rule
The IQR rule uses the middle half of a data set to decide whether a value is unusually far from the rest.
1) Find the quartiles
- Put the data in order from least to greatest.
- Find Q1: the median of the lower half.
- Find Q3: the median of the upper half.
2) Compute the IQR
- Use IQR = Q3 - Q1.
- This measures the spread of the middle 50% of the data.
3) Find the outlier fences
- Lower fence: Q1 - 1.5(IQR)
- Upper fence: Q3 + 1.5(IQR)
Any data value below the lower fence or above the upper fence is an outlier.
4) Check each value
- Compare each data point to the two fences.
- Values inside the fences are not outliers.
- Values outside are outliers.
Quick check
After identifying the outliers, make sure your final answer is simplified and clearly states which values are outliers. If no values fall outside the fences, then there are no outliers.