AQA A Level Maths: Statistics复习笔记2.3.3 Skewness


The distribution of a data set is either symmetrical or it has skewness.

What is skewness?

  • Skewness describes the way in which data in a non – symmetrical distribution is leaning
    • A distribution that has its tail on the right side has positive skew
    • A distribution that has its tail on the left side has negative skew


  • If the distribution is shown on a box plot looking at the difference between the quartiles can help decide how it is skewed
    • If the median is closer to the lower quartile then the distribution has positive skew
      • Q3 -  Q2 > Q2- Q1
    • If the median is closer to the upper quartile then the distribution has negative skew
      • Q3 -  Q2 < Q2 - Q1


  • Looking at the values of the statistics can help you decide whether distribution is positively skewed or negatively skewed
    • In a positively skewed distribution
      • Mode < median < mean
    • In a negatively skewed distribution
      • Mean < median < mode


Exam Tip

  • You only need to be able to recognise the different types of skewness. It can also help to comment on skewness when asked to compare distributions.