What's a Correlation?

Correlation measures the relationship between data points. The closer to 1 or -1, the stronger the correlation. (e.g. A correlation of 0.8 is strong. 0.2 is weak.)

If the line-of-best-fit trends upward, the correlation is positive. One that trends downward is negative.

A straight line-of-best-fit indicates a linear correlation. A curved date set is non-linear and is neither strong nor weak.