# Covariance

## Covariance :

Covariance is a measure of the joint variability of two random variables. It indicates how the two variables are related and whether they change together. If the two variables change in the same direction, the covariance is positive. If they change in opposite directions, the covariance is negative. And if there is no linear relationship between the two variables, the covariance is zero.
The formula for covariance is:
Cov(X,Y) = ∑ (x-E(x)) * (y-E(y)) / n
where X and Y are the two random variables, E(x) and E(y) are the expected values of X and Y, and n is the number of observations.
To better understand covariance, let’s look at an example. Suppose we have a dataset containing the heights and weights of a group of people. We can use the formula for covariance to calculate the covariance of the height and weight variables.
First, we need to calculate the expected values for the height and weight variables. The expected value of a variable is the average value of the variable. To find the expected value for the height variable, we add up all the heights and divide by the number of observations. We do the same for the weight variable.
Next, we need to subtract the expected value from each observation for both the height and weight variables. This gives us the deviation of each observation from the expected value.
Finally, we multiply the deviations for the height and weight variables for each observation and sum them up. This gives us the covariance of the height and weight variables.
The covariance can tell us whether the height and weight variables are positively or negatively related. If the covariance is positive, it means that as the height increases, so does the weight. If the covariance is negative, it means that as the height increases, the weight decreases.
In addition to indicating the direction of the relationship between two variables, the covariance can also tell us the strength of the relationship. A larger covariance indicates a stronger relationship, while a smaller covariance indicates a weaker relationship.
However, it’s important to note that the covariance alone does not tell us the exact strength of the relationship. This is because the covariance is not normalized, meaning that it is not standardized to have a specific range. To standardize the covariance and compare the strength of the relationship between different variables, we can use the correlation coefficient.
The correlation coefficient is calculated by dividing the covariance by the product of the standard deviations of the two variables. The standard deviation is a measure of the spread of the data. By dividing the covariance by the product of the standard deviations, we standardize the covariance and make it comparable across different variables.
The correlation coefficient can range from -1 to 1. A value of -1 indicates a perfect negative relationship, where as one variable increases, the other decreases by the same amount. A value of 1 indicates a perfect positive relationship, where as one variable increases, the other also increases by the same amount. And a value of 0 indicates no linear relationship between the two variables.
In summary, covariance is a measure of the joint variability of two random variables. It indicates the direction and strength of the relationship between the two variables, but it is not standardized and cannot be directly compared across different variables. The correlation coefficient, on the other hand, is a standardized version of the covariance and can be directly compared across different variables. Understanding covariance and correlation is important in many fields, such as statistics and machine learning, where analyzing the relationships between variables is crucial.