5 Best Ways to Find the Standard Deviation of Specific Columns in a Pandas DataFrame

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πŸ’‘ Problem Formulation: When working with data in Python, it’s often necessary to compute statistical metrics to understand the variability or dispersion within your dataset. For data analysis tasks, you may need to find the standard deviation for specific columns within a Pandas DataFrame. The standard deviation is a measure that quantifies the amount of variation or dispersion of a set of values. This article will describe how to determine the standard deviation for selected columns in a Pandas DataFrame, providing input in the form of a DataFrame and aiming for an output of standard deviation values for the specified columns.

Method 1: Use std() Function on DataFrame

The std() function in Pandas computes the standard deviation of a DataFrame or specific columns within it. For a Series containing numeric data, the std() function calculates the standard deviation of the elements. When applied to a DataFrame, you can specify the columns in which you are interested.

Here’s an example:

import pandas as pd

# Sample DataFrame
df = pd.DataFrame({'A': [1, 2, 3, 4], 'B': [4, 5, 6, 7], 'C': [7, 8, 9, 10]})
# Calculating standard deviation for specific columns
std_deviation = df[['A', 'C']].std()

print(std_deviation)

Output:

A    1.290994
C    1.290994
dtype: float64

This code snippet creates a simple DataFrame with three columns ‘A,’ ‘B,’ and ‘C.’ We use the std() function to calculate the standard deviation of the specific columns ‘A’ and ‘C.’ The output shows the standard deviation for each of these columns.

Method 2: Subset DataFrame Before Using std()

Another approach is to subset your DataFrame to include only the columns of interest and then apply the std() function to the resulting DataFrame. This avoids accidentally including extra columns in your calculation.

Here’s an example:

import pandas as pd

# Sample DataFrame
df = pd.DataFrame({'A': [10, 11, 12, 13], 'B': [13, 14, 15, 16], 'C': [16, 17, 18, 19]})
# Subset DataFrame and calculate standard deviation
selected_columns = df[['A', 'B']]
std_deviation = selected_columns.std()

print(std_deviation)

Output:

A    1.290994
B    1.290994
dtype: float64

By first creating a subset DataFrame that contains only columns ‘A’ and ‘B’, and then applying the std() function, we get the standard deviations for just those columns without affecting or using data from column ‘C’.

Method 3: Using agg() Method to Compute Multiple Statistics

The agg() method in Pandas allows you to apply one or more operations over the specified axis. For standard deviation, you can use agg() combined with a dictionary to compute the standard deviation of specific columns.

Here’s an example:

import pandas as pd

# Sample DataFrame
df = pd.DataFrame({'X': [20, 21, 19, 18], 'Y': [22, 23, 21, 20], 'Z': [25, 26, 24, 23]})
# Calculate standard deviation using the agg() method
std_deviation = df.agg({'X': 'std', 'Z': 'std'})

print(std_deviation)

Output:

X    1.290994
Z    1.290994
dtype: float64

This code uses the agg() method to define a dictionary that specifies the computation of standard deviation for columns ‘X’ and ‘Z’. The result is a Series with the standard deviation values for the selected columns.

Method 4: Using describe() Method for Descriptive Statistics

The describe() method in Pandas provides descriptive statistics that summarize the central tendency, dispersion, and shape of a dataset’s distribution. While it returns multiple statistics, you can extract standard deviation from the result.

Here’s an example:

import pandas as pd

# Sample DataFrame
df = pd.DataFrame({'Column1': [100, 110, 120, 130], 'Column2': [130, 140, 150, 160]})
# Using describe() to obtain descriptive statistics
description = df.describe()
# Extracting standard deviation
std_deviation = description.loc['std', ['Column1', 'Column2']]

print(std_deviation)

Output:

Column1    12.909944
Column2    12.909944
Name: std, dtype: float64

After getting the descriptive statistics with describe(), we extract the ‘std’ row, which contains the standard deviation for the specified columns, ‘Column1’ and ‘Column2’.

Bonus One-Liner Method 5: Use List Comprehension and std()

For a quick and concise calculation, use a list comprehension to apply the std() function to a list of specified columns within the DataFrame.

Here’s an example:

import pandas as pd

# Sample DataFrame
df = pd.DataFrame({'First': [2, 4, 6, 8], 'Second': [3, 6, 9, 12], 'Third': [4, 8, 12, 16]})
# Calculating standard deviation using list comprehension
std_deviation = {column: df[column].std() for column in ['First', 'Third']}

print(std_deviation)

Output:

{'First': 2.581988897471611, 'Third': 5.163977794943222}

The list comprehension iterates through the list of column names and calculates the standard deviation for each, creating a dictionary with the results.

Summary/Discussion

  • Method 1: Direct Application of std(). Great for simplicity and direct use cases. Limited to basic usage scenarios.
  • Method 2: Subset before std(). Offers more control over selected data. Requires an extra step of subsetting.
  • Method 3: Use of agg(). Flexible for computing multiple statistics. Might be overkill for a single operation.
  • Method 4: describe() Method. Provides a full overview of statistics. Inefficient if only standard deviation is needed.
  • Method 5: List Comprehension. Quick and concise, ideal for one-liners. May become cumbersome with a large number of columns.