nunique() for cardinality in Data Analysis Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The nunique() function counts the number of unique values in a data column or series. It helps find how many different items exist.

beginner

Why is cardinality important in data analysis?

Cardinality tells us how many unique values a feature has. It helps understand data variety and decide how to handle or encode data.

beginner

How do you use nunique() on a pandas DataFrame column?

Use it like this: df['column_name'].nunique(). It returns the count of unique values in that column.

intermediate

What is the difference between count() and nunique()?

count() counts all non-missing values, while nunique() counts only unique values.

intermediate

Can nunique() handle missing values by default?

Yes, by default nunique() ignores missing values (NaN) when counting unique values.

What does df['age'].nunique() return?

A The number of unique ages in the 'age' column

B The total number of rows in the DataFrame

C The number of missing values in 'age'

D The sum of all ages

nunique() counts unique values, so it returns how many different ages appear.

If a column has values [1, 2, 2, 3, NaN], what does nunique() return?

A 4

B 2

C 3

D 5

nunique() counts unique non-missing values: 1, 2, 3 → 3 unique values.

Which method counts all non-missing values, not just unique ones?

A count()

B nunique()

C sum()

D mean()

count() counts all non-NaN values in a column.

Why might you check cardinality with nunique() before encoding categorical data?

A To remove missing data

B To calculate the average value

C To sort the data

D To know how many unique categories exist

Knowing unique categories helps choose the right encoding method.

Does nunique() include NaN values in its count by default?

A Yes, always

B No, it ignores NaN by default

C Only if specified

D It depends on the data type

By default, nunique() skips NaN values when counting unique entries.

Explain how the nunique() function helps understand data cardinality.

Think about how many different items are in a list or column.

Describe a real-life example where checking cardinality with nunique() is useful.

Consider a survey with different answer choices.