Figure 8.8a: Bivariate scatterplot for
data in Table 8.2
To obtain a quantitative index of the strength of a relationship, the Pearson r statistic
would be calculated. The correlation between two variables, X and Y is denoted rXY. An
equation for computing r which is an analogue of the correlation formula for Spearman’s
correlation shown in Chapter 7 is,
where X and Y are two random quantitative variables. This formula is equivalent to the
cross product sums of squares (SSXY) divided by the square root of the sums of squares
for X (SSXX) multiplied by the sums of squares for Y (SSYY). This can be written as,
Inferences involving continuous data 285