Assessing difficulties in written language is difficult since proficiency in this area requires simultaneous competence in several skills - handwriting, spelling and written expression. These skills are intertwined and problems with one skill significantly affect competence in the others and vice versa.
Poor spellers are often poor readers and poor readers are often poor writers.
To compound the problem, screening for writing disabilities is difficult because many students other than those with learning disabilities write poorly. In addition, there seems to be much controversy concerning effective ways to intervene once assessment is made.