Why do you need to use the Spearman-Brown formula for split-half reliability? The Spearman-Brown formula determines the standard error of measurement (SEM) in both halves of a test. An estimate of reliability based on two half-length tests would be underestimated, so the Spearman-Brown formula corrects the correlation between the two parts. The Spearman-Brown formula estimates how long a test must be to achieve a desired level of reliability. The Spearman-Brown formula compares two equivalent forms of a test that measure the same attribute