2. The method of claim 1 wherein the text variation measures include syntactic complexity, semantic difficulty, degree of academic style, and text cohesion.
2. The method of claim 1 wherein the text variation measures include syntactic complexity, semantic difficulty, degree of academic style, and text cohesion. 4. The method of claim 2 wherein semantic difficulty is determined from word length, word familiarity, word frequency, and rare word frequency.