Using corrwith() Function in Pandas: Analyzing Pairwise CorrelationData analysis and manipulation have become imperative across various industries. Pandas, a library built upon the NumPy package, is widely used for data analysis in Python.
One of the key features of Pandas is its ability to calculate correlation between variables. While the corr() function calculates the pairwise correlation among all numerical columns in a DataFrame, the corrwith() function calculates the pairwise correlation between two series or columns.
In this article, we will explore the syntax, differences between the two functions, and provide an example of using the corrwith() function to calculate pairwise correlation.
Syntax of the corrwith() function
The syntax of the corrwith() function in Pandas is similar to that of the corr() function. The basic syntax for the corrwith() function is as follows:
df1.corrwith(df2, axis=0, drop=False, method='pearson')
Where df1 and df2 are two Pandas Series or DataFrames, axis refers to the axis along which to calculate the correlation, drop specifies whether to drop missing values, and method refers to the correlation method to use.
Difference between corr() and corrwith() functions
While both functions calculate the pairwise correlation, they differ in the input parameters and output. The corr() function calculates the correlation between all the numerical columns in a DataFrame, while the corrwith() function calculates the correlation between two series or columns.
Additionally, the corr() function returns a correlation matrix, while the corrwith() function returns a series with the pairwise correlation values.
Example of using the corrwith() function in Pandas
Let’s consider an example of how to use the corrwith() function in Pandas. First, we create two DataFrames with the same name but different contents:
import pandas as pd
import numpy as np
df1 = pd.DataFrame(np.random.randn(5, 2), columns=['a', 'b'])
df2 = pd.DataFrame(np.random.randn(5, 2), columns=['a', 'b'])
The above code creates two DataFrames named df1 and df2 with two columns each. Next, we can calculate the pairwise correlation between the ‘a’ columns in the two DataFrames:
print(df1['a'].corrwith(df2['a'], method='pearson'))
The output will be a single value representing the pairwise correlation between the ‘a’ columns in the two DataFrames.
Conclusion
The corrwith() function in Pandas is a useful tool for calculating pairwise correlation between two series or columns. It can be used to gain insight into the relationships between variables in a dataset and to identify any potential correlations.
By understanding the syntax and differences between the corr() and corrwith() functions, data analysts can better utilize the various tools available to them for efficient data analysis and manipulation. Additional Resources: Kendall and
Spearman Correlation Coefficients in PandasCorrelation is a measure of the relationship between variables.
Pandas offers two correlation coefficients that can be used to measure non-linear relationships between variables: the
Kendall Correlation Coefficient and the
Spearman Correlation Coefficient. In this section, we will explore the differences between Pearson, Kendall, and
Spearman Correlation Coefficients, and how to compute these coefficients in Pandas.
Kendall Correlation Coefficient
The
Kendall Correlation Coefficient (KCC), named after Maurice Kendall, is a rank-based correlation coefficient that measures the ordinal association between two variables. It is a nonparametric test that does not rely on the assumptions of normality, linearity, and homoscedasticity.
The KCC ranges from -1 to 1, where -1 indicates a perfect negative correlation, +1 indicates a perfect positive correlation, and 0 indicates no correlation.
To compute the KCC in Pandas, we can use the kendall() function that is similar to the corr() function used to compute Pearson’s correlation coefficient.
Here’s an example:
import pandas as pd
import numpy as np
df = pd.DataFrame(np.array([[1, 2, 3], [4, 5, 6], [3, 2, 1], [6, 5, 4]]), columns=['A', 'B', 'C'])
print(df['A'].corr(df['B'], method='kendall'))
The output will be the
Kendall Correlation Coefficient between columns A and B in DataFrame df.
Spearman Correlation Coefficient
The
Spearman Correlation Coefficient (SCC), named after Charles Spearman, is another rank-based correlation coefficient that measures the monotonic association between two variables. Like the KCC, the SCC is also a nonparametric test that does not depend on the assumptions of normality, linearity, and homoscedasticity.
The SCC ranges from -1 to 1, where -1 indicates a perfect negative correlation, +1 indicates a perfect positive correlation, and 0 indicates no correlation. To compute the SCC in Pandas, we can use the spearmanr() function from the SciPy library.
The spearmanr() function returns two outputs: the
Spearman Correlation Coefficient and the corresponding p-value. Here’s an example:
import pandas as pd
from scipy.stats import spearmanr
df = pd.DataFrame(np.array([[1, 2, 3], [4, 5, 6], [3, 2, 1], [6, 5, 4]]), columns=['A', 'B', 'C'])
print(spearmanr(df['A'], df['B']))
The output will be the
Spearman Correlation Coefficient between columns A and B in DataFrame df. Differences between Pearson, Kendall, and
Spearman Correlation Coefficients
Pearson’s correlation coefficient measures the linear association between two variables and assumes that the variables are normally distributed.
The KCC and SCC, on the other hand, are nonparametric and do not make any assumptions about the distribution of the variables. The KCC measures the ordinal association between two variables, while the SCC measures the monotonic association between two variables.
The KCC and SCC are more appropriate than the Pearson correlation coefficient when dealing with non-parametric data or data with outliers. It is also a better measure of association where the relationship is non-linear.
For example, if we have two variables with a perfect stepwise pattern, the KCC and SCC coefficient will be +1, while Pearson correlation coefficient will be 0.
Conclusion
In conclusion, Pandas provides a range of tools for calculating correlation coefficients, including the Pearson Correlation Coefficient, the
Kendall Correlation Coefficient, and the
Spearman Correlation Coefficient. The choice of correlation coefficient depends on the nature of the data and the type of association between the variables.
By understanding the nuances of each coefficient and how to calculate them in Pandas, data analysts can better utilize the various tools available to them for efficient data analysis and manipulation. In conclusion, correlation coefficients are essential for measuring the relationships between variables.
Pandas provides a range of correlation functions, including the Pearson Correlation Coefficient, the
Kendall Correlation Coefficient, and the
Spearman Correlation Coefficient. The choice of correlation method depends on the nature of data and the type of association between the variables.
By understanding each coefficient and its application, data analysts can better use the available tools for efficient data analysis and manipulation. Remember, while Pearson’s correlation coefficient is widely used, it may not provide an accurate representation of the association between variables in non-linear data.
Hence, the use of KCC and SCC is necessary when analyzing non-parametric data or data with outliers.