Course 2.2Lesson 3 of 5

statistics and datetime

The statistics module describes a list of numbers. The mean is the sum divided by the count, the median is the middle value of the sorted data, or the mean of the two middle values when the count is even, and the mode is the most common value, while multimode gives all the values that tie. pstdev shows how spread out the data is. The datetime module makes a date with date of year, month and day. Subtracting two dates gives a gap, and days is the number of days. strftime writes a date as text, such as the day name, and a timedelta adds days to a date.

about 30 min1 simulation2 practice questions

Lesson 3 of 5Show all lessons

Try the code and the simulations beside the lesson.

What you will be able to do

After this lesson you can

find the mean, the median, the mode and the spread of a list of numbers,
explain how the three averages differ and when each one is the best,
make dates, find the gap between two dates, and add days, and
write a date as text, such as the day of the week.

statistics: describing a list of numbers

The module statistics answers the questions that you ask about a list of data. There are three "averages", and they answer three different questions.

A number line from 2 to 10 with seven dots for the data 4, 8, 6, 5, 3, 8 and 9. Two dots are stacked at 8. A dashed orange line marks the mean, 6.14, the balance point. A green mark under 6 shows the median, 6, the middle value. A red note points at 8 and says the mode is 8, the most common value.

The mean is the usual average: the sum divided by the number of values. It is the balance point of the data.
The median is the middle value when the data is in order. Half of the values are below it, and half above.
The mode is the value that appears most often.

Predict first, then run it. For the data 4, 8, 6, 5, 3, 8, 9:

the mean is 43 divided by 7, which is 6.14,
in order, the data is 3, 4, 5, 6, 8, 8, 9, and the median is the fourth value, 6,
the mode is 8, which appears twice.

More points.

With an even number of values there is no single middle. statistics.median([3, 9, 5, 7]) gives the mean of the two middle values (5 and 7): 6.0.
If two values tie for the mode, mode gives the first one, and multimode gives all of them, as a list.
statistics.pstdev(data) is the standard deviation: a number for how spread out the data is. Data that is close to the mean gives a small spread, and data that is far from it gives a big one.

Which average?

The three can be very different. A village has ten families, and nine of them earn 10 000 a month, and one earns 1 000 000. The mean is about 109 000, which describes nobody. The median is 10 000, which describes a typical family. The mean is pulled by a few very big values, and the median is not. For data with extreme values, such as income or house prices, the median is the better one. The mode is the answer to "which is the most popular?", for example the most common shoe size.

The median of five values is the third one in order. Two values above it and two below it.

datetime: dates

The module datetime works with dates. You make a date with datetime.date(year, month, day). When you subtract two dates, you get the gap.

Predict first: on which day of the week is 11 October 2026? How many days are there until 25 December? Then run it.

(end - start).days is the number of days between two dates: 75. Subtracting dates gives a timedelta, and .days is its length in days.
start.strftime("%A") writes the date as text. The code %A is the full day name (Sunday), %d is the day, %B is the full month name, and %Y is the four-digit year. The name "strftime" means "string from time".
start.weekday() is a number: Monday is 0, and Sunday is 6.
start + datetime.timedelta(days=100) adds 100 days and gives a new date.
Dates can be compared with < and >, because they have an order.
datetime.date.today() is today's date, so that output changes every day.

Common mistakes

Using the mean for data with extreme values. The median is safer.
statistics.mode of an empty list. An error. Check first.
Forgetting that date() takes the year first, then the month, then the day. date(10, 11, 2026) is a ValueError.
A date that does not exist, such as 30 February. A ValueError.
Subtracting and forgetting .days. The result is a timedelta that prints as 75 days, 0:00:00.
Confusing the weekday number with the day name.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

Warm-up. Open the lab Describe the data. You find the mean, the median and the mode, and the checker confirms them.
Build. Open the problem Mean, median and mode and the problem Days to go.
Stretch. Open the problem The day of the week and the problem How spread out is the data?

Remember these

statistics.mean, median and mode describe a list of numbers. pstdev shows the spread.
The mean is the balance point, the median is the middle, and the mode is the most common.
Subtracting two dates gives a gap, and .days is the number of days.
strftime writes a date as text. timedelta adds days.