How it is calculated
The pairs of numbers must be in the same order. If one series never changes, the coefficient is undefined.
r = Σ (x − x̄)(y − ȳ) / √(Σ (x − x̄)² Σ (y − ȳ)²)
Where the numbers come from
- Series X
- Series Y
Example
The rows 1, 2, 3 and 2, 4, 6 give a coefficient of 1: the second is exactly twice the first.
Questions
Does correlation prove a cause?
No. It measures only movement together along a line.
Does row order matter?
Yes. The first number on the left is paired with the first on the right.
Does an outlier change it a lot?
Yes, the Pearson coefficient is sensitive to extreme points.