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A box plot (box-and-whisker plot) visualises the distribution of data using five statistics: minimum, Q1, median, Q3, and maximum. Outliers are shown as individual points beyond 1.5×IQR from the quartiles.
Formule
Q1 = 25th percentile, Q2 (median) = 50th, Q3 = 75th; IQR = Q3 − Q1
- Min
- minimum value
- Q1
- first quartile (25th percentile)
- Q2
- median (50th percentile)
- Q3
- third quartile (75th percentile)
- Max
- maximum value
- IQR
- interquartile range — Q3 − Q1
Guide étape par étape
- 1Q2 (median): middle value of sorted data
- 2Q1: median of lower half
- 3Q3: median of upper half
- 4IQR = Q3 − Q1; outlier threshold = 1.5×IQR
Exemples résolus
Entrée
Data: 2,4,5,7,8,9,11,13
Résultat
Min=2, Q1=4.5, Med=7.5, Q3=10, Max=13
Entrée
IQR = 5.5, outlier threshold = 8.25 from quartiles
Résultat
Any value < −3.75 or > 18.25 is an outlier
Questions fréquentes
What is a boxplot used for?
Boxplots visualize the distribution, spread, and outliers of data at a glance.
How are outliers determined in a boxplot?
Points beyond 1.5×IQR from Q1 or Q3 are typically marked as outliers.
Is the median always at the center of the box?
No, the line inside the box shows the median's position relative to Q1 and Q3, reflecting data skewness.
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