Mean Value in Each Group in Pandas Groupby

You can use Pandas groupby to group the underlying data on one or more columns and estimate useful statistics like count, sum, mean, median, minmax etc. In this tutorial, we will look at how to get the mean for each group in pandas groupby with the help of some examples.

Mean of each group in pandas groupby

To get the average (or mean) value of in each group, you can directly apply the pandas mean() function to the selected columns from the result of pandas groupby. The following is a step-by-step guide of what you need to do.

  1. Group the dataframe on the column(s) you want.
  2. Select the field(s) for which you want to estimate the mean.
  3. Apply the pandas mean() function directly or pass ‘mean’ to the agg() function.

The following is the syntax –

# groupby columns Col1 and estimate the mean of column Col2
df.groupby([Col1])[Col2].mean()
# alternatively, you can pass 'mean' to the agg() function
df.groupby([Col1])[Col2].agg('mean')

Let’s now look at some examples of using the above syntax to get the average values for each group. First, we will create a sample dataframe that we will be using throughout this tutorial.

import pandas as pd

# create a dataframe of car models by two companies
df = pd.DataFrame({
    'Name': ['Rob', 'Rob', 'Rob', 'Emma', 'Emma', 'Emma', 'Hasan', 'Hasan', 'Hasan'],
    'Subject': ['English', 'Science', 'Maths', 'English', 'Science', 'Maths', 'English', 'Science', 'Maths'],
    'Marks': [67, 81, 59, 91, 80, 82, 73, 76, 54],
    'Projects': [0, 1, 0, 1, 1, 0, 0, 2, 0]
})
# display the dataframe
df

Output:

Dataframe containing the marks and project submission data of three students in different subjects.

Here we created a dataframe storing the marks obtained by some students in a high school in different subjects. The “Projects” field tells the number of projects submitted by the student in that subject.

Let’s get the average marks obtained by each student across all the subjects in the above dataframe. For this, we need to group the dataframe on “Name” and then estimate the mean of the “Marks” column.

# average marks for each student
df.groupby('Name')['Marks'].mean()

Output:

Name
Emma     84.333333
Hasan    67.666667
Rob      69.000000
Name: Marks, dtype: float64

You can see that we get the average marks for each student considering all the subjects. “Emma” has the highest average among the three students.

Alternatively, you can also use the agg() function with “mean” as the argument to get the mean of each group in pandas groupby.

# average marks for each student
df.groupby('Name')['Marks'].agg('mean')

Output:

Name
Emma     84.333333
Hasan    67.666667
Rob      69.000000
Name: Marks, dtype: float64

We get the same result as above.

You can also get the mean of multiple columns at a time for each group. For example, let’s get the average number of projects submissions for each student along with their average marks.

# mean marks and project submissions for each student
df.groupby('Name')[['Marks', 'Projects']].mean()

Output:

Mean value of "Marks" and "Projects" for each student.

Here, we grouped the data on the “Name” column and then calculated the mean of values in the columns “Marks” and “Projects” for each group.

Alternatively, we can use the agg() function with “mean” as the argument.

# mean marks and project submissions for each student
df.groupby('Name')[['Marks', 'Projects']].agg('mean')

Output:

Mean value of "Marks" and "Projects" for each student.

We get the same result as above.

With this, we come to the end of this tutorial. The code examples and results presented in this tutorial have been implemented in a Jupyter Notebook with a python (version 3.8.3) kernel having pandas version 1.0.5


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