What is the usage and meaning of "contribute unique variance" in this sentence?
For children with typical language, orthographic knowledge and morphological knowledge contributed unique variance to spelling performance.
Source: Werfel, K.L., & Schuele, C.M. (2012, November).
Contribution of linguistic knowledge to spelling performance in elementary school children with and without language impairment. Poster presented at the Annual Convention of the American Speech-Language-Hearing Association, Atlanta, GA.