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Descriptive Statistics

Lesson 1: Descriptive Statistics

What are descriptive statistics?

Descriptive statistics summarise the main features of a dataset.

Category

Question

Measures

CentreWhat is a typical value?Mean, median, mode
SpreadHow much do values differ?Range, variance, standard deviation, IQR
ShapeHow are values distributed?Skewness, histograms
PositionWhere does a value sit?Percentiles, quartiles

Use them to understand data before comparing groups or building models.

Mean, median, and mode

Measure

Definition

Strength

Weakness

Best for

MeanSum ÷ countUses every valuePulled by extreme valuesSymmetric numeric data
MedianMiddle value after sortingResistant to outliersIgnores the size of extremesSkewed data (income, prices, delivery time)
ModeMost frequent valueWorks for categoriesMay be several modes, or noneCategories, common sizes

Example

Monthly spending: 100, 110, 120, 130, 900

Measure

Calculation

Result

Mean(100 + 110 + 120 + 130 + 900) ÷ 5 = 1,360 ÷ 5272
MedianMiddle of sorted values120
Range900 − 100800
  • The single value of 900 raises the mean far above what most people spend.
  • Reporting only the mean would mislead readers about typical spending.

Spread

Measure

Meaning

Notes

RangeMax − minSimple, but driven by the two most extreme values
VarianceAverage squared distance from the meanIn squared units, so hard to interpret directly
Standard deviation (SD)Square root of varianceSame units as the data; "typical distance from the mean"
Interquartile range (IQR)Q3 − Q1Spread of the middle 50%; resistant to outliers
  • Sample variance and SD divide by n − 1 rather than n, to avoid underestimating population spread.
  • Two datasets can share the same mean but have very different spreads:

Dataset

Values

Mean

SD (sample)

A48, 50, 52502
B10, 50, 905040

Key takeaways

  • Choose summary measures based on the shape and purpose of the data.
  • For skewed numeric data, report both median and mean to make the effect of extreme values visible.
  • Always pair a measure of centre with a measure of spread.

Try it: Calculate the mean and median for five values, then add one unusually large value and compare how the measures change.


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