groupby() basics in Data Analysis Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the groupby() function do in data analysis?

It splits data into groups based on one or more columns, so you can perform calculations on each group separately.

beginner

How do you use groupby() to find the average of a column for each group?

First, use groupby() on the column to group by, then call .mean() on the grouped object to get averages.

beginner

What type of object does groupby() return before applying an aggregation?

It returns a GroupBy object, which is like a special container holding the groups but no calculations done yet.

intermediate

Can you group by multiple columns using groupby()? How?

Yes, by passing a list of column names to groupby(), like df.groupby(['col1', 'col2']).

intermediate

What is the difference between groupby() and filtering data before grouping?

groupby() organizes data into groups for aggregation, while filtering removes rows before grouping.

What does df.groupby('Category').sum() do?

A Sorts the dataframe by 'Category'

B Counts the number of rows in 'Category'

C Adds up values in each group of 'Category'

D Filters rows where 'Category' is sum

The groupby() groups rows by 'Category', and sum() adds values in each group.

Which method is used to get the average value of groups after groupby()?

A sort()

B count()

C max()

D mean()

mean() calculates the average of each group.

What type of object is returned immediately after calling groupby()?

A DataFrame

B GroupBy object

C Series

D List

A GroupBy object is returned, which holds grouped data but no calculations yet.

How do you group data by two columns 'A' and 'B'?

A df.groupby(['A', 'B'])

B df.group(['A', 'B'])

C df.groupby('A', 'B')

D df.groupby('A' & 'B')

Pass a list of columns to groupby() like ['A', 'B'].

Which of these is NOT a typical aggregation function used after groupby()?

A filter()

B sum()

C mean()

D count()

filter() is used to filter groups, not aggregate values.

Explain how groupby() works and why it is useful in data analysis.

Think about sorting your data into buckets to analyze each bucket separately.

Describe how to group data by multiple columns and get the average of another column.

Use square brackets to group by more than one column.