###
**Variance and Covariance**

When we measure the spread of the distribution of some random variable X, we calculate variance and standard deviation, as:

The variance between X and Y is called covariance. To find covariance of X and Y, we use the same approach as above:

So, the covariance of X and Y could be negative or positive. Because covariance is not normalized, it only describes a trend between two variables.

###
**Correlation and Collinearity**

To measure the strength of the trend, we need to normalize the covariance. So, covariance normalized by the standard deviations of X and Y is a correlation coefficient (or Pearson's correlation coefficient), which is defined below:

Thus, correlation coefficient values are between -1 and +1.

To classify the strength of the correlation, the following ranges are commonly used:

Positive and negative signs indicate the trend of the correlation.

When two variables are strongly correlated with each other, they are collinear. If there are strong correlations with multiple variables, it is multicollinearity. Depending on the goal of the analysis, one can consider dropping strongly correlated features. To work with collinear features, we also can use variance inflation factors(VIF) and Principal Component Analysis (PCA).

###
**Application**

Here I will use a London bike sharing dataset to play with covariance and correlation.

```
# https://www.kaggle.com/hmavrodiev/london-bike-sharing-dataset?select=london_merged.csv
df = pd.read_csv('data/london_merged.csv')
data = df.iloc[:,2:].copy()
data.head()
```

1) Let's check covariance of features:

```
# covariance
data.cov()
```

2) Correlation of features:

```
# correlation
data.corr()
```

```
# correlation
abs(data.corr()) > 0.70
```

Not surprisingly, we can see that temperatures t1 and t2 are strongly and positively correlated.

3) When applying PCA, we can see the number of principal components vs. Explained Variance:

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