An alternative way of computing the reliability of a sum scale is to divide it in some random manner into two halves. If the sum scale is perfectly reliable, we would expect that the two halves are perfectly correlated (i.e., r = 1.0). Less than perfect reliability will lead to less than perfect correlations. We can estimate the reliability of the sum scale via the Spearman-Brown split half coefficient:

rsb = 2rxy/(1+rxy)

In this formula, rsb is the split-half reliability coefficient, and rxy represents the correlation between the two halves of the scale.