There is a coefficient for measuring the strength of fit of observed data to a straight line which is called the “product moment correlation coefficient” or (pmcc) for short.
Here I show you how to use a calculator to do this.

Pearson's Product Moment Correlation Coefficient on the Casio fx-991EX ClassWiz Calculator - youtube Video

The formula for working out Pearson’s Product Moment Correlation Coefficient

If your calculator does not support finding PMCC r, then you can use the formula as shown in the video below

Pearson's Product Moment Correlation Coefficient - The formula | ExamSolutions - youtube Video