Calculating and Interpreting the Linear Correlation Coefficient

The linear correlation coefficient, r, is a numerical measure developed by Karl Pearson that quantifies the strength and direction of the linear association between two variables. It indicates whether variables move together in a predictable straight-line pattern. Values range from -1 to +1, where values closer to -1 or +1 indicate stronger linear relationships, while values near 0 suggest weak or no linear association.

Compare the absolute value of the calculated correlation coefficient to the critical value from a significance table at your chosen significance level. If the absolute value of r exceeds the critical value, the correlation is statistically significant. For example, at a 0.05 significance level with 7 data points, a critical value of 0.754 means any |r| greater than 0.754 indicates significant linear correlation between variables.

The coefficient of determination, r², is the square of the correlation coefficient expressed as a percentage. It represents the percent of variation in the dependent variable that can be explained by the independent variable using the regression line. For instance, an r² of 76.2% means 76.2% of temperature variation is explained by carbon dioxide levels, while 23.8% remains unexplained.

To calculate r, you must first determine x², y², and the product of x and y for each data point, then sum these values. The correlation coefficient formula uses these sums along with the number of data points, n. These intermediate calculations provide the necessary components to quantify the linear relationship strength between your two variables.

The coefficient is named after Karl Pearson, who developed it in the early 1900s. The term 'product-moment' refers to the calculation method, which involves computing products of paired values and their deviations from the mean. This naming convention distinguishes it from other correlation measures and honors its mathematical foundation.

When r falls between the positive and negative critical values from the significance table, the correlation coefficient is not statistically significant. This means there is insufficient evidence to conclude a true linear relationship exists between the variables. In such cases, using the regression line for prediction is not recommended.

The unexplained variation, calculated as 1 – r² and expressed as a percentage, represents the portion of dependent variable variation not explained by the regression line. This unexplained variation appears as the scattering of observed data points about the best-fit line, which can be examined using residual plots. Greater scatter indicates more unexplained variation and a weaker linear relationship.