To read a box plot (also called a box and whisker plot), follow these key steps and understand its components:
Components of a Box Plot
- Median (Q2): The vertical line inside the box represents the median, which is the middle value of the data set. Half the data points lie above this value and half below it
- Interquartile Range (IQR): The box itself shows the interquartile range, which contains the middle 50% of the data. The left edge of the box is the first quartile (Q1, 25th percentile), and the right edge is the third quartile (Q3, 75th percentile)
- Whiskers: Lines extending from the box to the minimum and maximum values within 1.5 times the IQR. They represent the range of the bulk of the data outside the middle 50%
- Minimum and Maximum: The ends of the whiskers mark the lowest and highest data points excluding outliers
- Outliers: Individual points plotted beyond the whiskers indicate outliers, or extreme values that fall outside the typical range
How to Interpret a Box Plot
- The length of the box shows the spread of the middle 50% of data. A short box means the middle values are close together, indicating less variability; a tall box means more variability
- The position of the median line inside the box shows skewness. If the median is closer to the bottom (Q1), the data is skewed higher; if closer to the top (Q3), it is skewed lower
- The length of whiskers can indicate the spread of the lower and upper 25% of data. Unequal whiskers suggest skewness or variability in the tails
- Comparing box plots for different groups can reveal differences in central tendency, spread, and skewness between those groups
Summary of Steps to Read a Box Plot
- Identify the minimum (end of left whisker).
- Find the first quartile (Q1) (left edge of the box).
- Locate the median (line inside the box).
- Find the third quartile (Q3) (right edge of the box).
- Identify the maximum (end of right whisker).
- Look for any outliers (points beyond whiskers).
Box plots provide a quick visual summary of data distribution, central tendency, spread, and potential outliers
. This explanation covers the essentials of reading and interpreting box plots based on the provided sources
