Lære Covariance, Correlation, and Decorrelation

Understanding how features relate to each other is crucial when preparing data for machine learning. Two fundamental concepts in this context are covariance and correlation. Both measure how two variables change together, but they do so in different ways.

The covariance between two random variables, $X$ and $Y$ , is mathematically defined as:

\text{Cov}(X, Y) = \frac{1}{n-1} \sum_{i=1}^n (x_i - \bar{x})(y_i - \bar{y})

where $x_i$ and $y_i$ are the individual sample values, and $\bar{x}$ and $\bar{y}$ are the sample means. Covariance indicates the direction of the linear relationship between variables. A positive value means that as one variable increases, the other tends to increase as well; a negative value means the opposite. However, the magnitude of covariance is not standardized, making it hard to compare across different variable pairs.

The correlation coefficient, often called Pearson's correlation, standardizes this measure:

\text{Corr}(X, Y) = \frac{\text{Cov}(X, Y)}{\sigma_X \sigma_Y}

where $\sigma_X$ and $\sigma_Y$ are the standard deviations of $X$ and $Y$ . Correlation values always range from -1 to 1, making them easier to interpret. A value of 1 means perfect positive linear relationship, -1 means perfect negative linear relationship, and 0 means no linear relationship.

Suppose you have two features, height and weight, measured for a group of people. If taller individuals tend to be heavier, the covariance and correlation between height and weight will both be positive. However, if you compare height and a completely unrelated feature, such as shoe size in a dataset where all shoes are the same size, the covariance and correlation would be close to zero.

Note

When two features are highly correlated, they contain overlapping information. This redundancy can confuse machine learning models, especially those that assume features are independent. Decorrelation aims to transform features so that they are statistically independent or at least uncorrelated. This often improves model performance, reduces overfitting, and speeds up learning.


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import numpy as np

# Small dataset: 3 samples, 2 features
data = np.array([
    [2.0, 8.0],
    [4.0, 10.0],
    [6.0, 14.0]
])

# Compute covariance matrix (features in columns)
cov_matrix = np.cov(data, rowvar=False)
print("Covariance matrix:")
print(cov_matrix)

# Compute correlation matrix
corr_matrix = np.corrcoef(data, rowvar=False)
print("\nCorrelation matrix:")
print(corr_matrix)

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Can you explain why the correlation matrix values are all 1 in this example?

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