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How to Calculate Product Moment Correlation

The product moment correlation (usually known just as the correlation) is a measure of the strength of the linear relationship between two continuous variables, such as height and weight. The correlation ranges from -1 to 1. A perfect inverse relationship (i.e., as one goes up, the other goes down) indicates -1, and a perfect positive relationship (as one goes up, the other goes up) indicates a 1. No linear relationship indicates 0.

Instructions

- 1
  Find the mean and standard deviation of each variable. Many calculators will calculate the mean and standard deviation for you.
- 2
  Subtract the mean of the first variable (for instance, height) from each value (i.e., the height of each person in your sample).
- 3
  Add up these differences.
- 4
  Divide the sum by the standard deviation of the first variable.
- 5
  Repeat Steps 3 and 4 for the second variable (e.g., weight).
- 6
  Multiply the result in Step 4 by the result in Step 5.
- 7
  Divide this product by "n-1," where "n" is the number of subjects in your sample. This is the correlation.

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