Describing Data: Mean, Median & Mode

12 min 30 XP

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Lesson 1 of 5 · View course roadmap

Step 1

Learn the idea

Before charts, before machine learning, analysis starts with three questions, and Python's built-in statistics module answers all of them:

mean(data): the average. Honest for balanced data, easily dragged around by one extreme value
median(data): the middle value. Robust: one billionaire in the room changes the mean salary wildly, the median barely
mode(data): the most common value. The only one that also works on words and categories

Knowing which to reach for is the analysis skill: report medians for incomes and house prices, means for balanced measurements, modes for "what's most popular?".

Where you'll use this

'Median household income', 'average response time', 'most common error': every metrics dashboard and news statistic is these three functions. Choosing mean vs median honestly is half of data literacy.

Common mistakes

Reporting the mean of skewed data (salaries, house prices, response times): one outlier misleads everyone downstream.
Calling mode() on data with no repeats: in older Pythons it raised; modern statistics.mode returns the first value, which may surprise you.
Doing sum(x)/len(x) on an empty list → ZeroDivisionError. Guard empty datasets before describing them.

Pro tip

statistics.quantiles(data, n=4) gives quartiles: the p25/p50/p75 shape of your data. Engineers report p95 latency, not the average, for exactly the outlier reasons above.

Step 2

Try it yourself

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PYexample.py

⌘ + Enter to run

Output appears here…

Step 3

Pass the challenge +30 XP

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For scores = [72, 88, 95, 64, 88, 79] print three lines: the mean shown with 1 decimal place, the median, and the mode.

Target output

81.0
83.5
88

PYchallenge.py

import statistics

scores = [72, 88, 95, 64, 88, 79]
print(f"{statistics.mean(scores):.1f}")
print(statistics.median(scores))
print(statistics.mode(scores))

Run your code to check it…

Step 4

Check your understanding

1. House prices in a street: eight around €300k, one at €5M. Which average should you report? Mean: it uses all the data Median: it resists the outlier Mode Sum

2. Which statistic works on non-numeric data like ["red", "blue", "red"]? mean median mode None of them

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