Nadia Behizadeh said:

If a child does not perform well on [one timed large-scale assessment essay], there will be a single story told about this student: he/she has below basic skills in writing, or maybe even far below basic skills. Yet this same student may be a brilliant poet or have a hundred pages of a first novel carefully stowed in his/her backpack. However, when a single story of deficiency is repeated again and again to a student, that student develops low writing self-efficacy and a poor self-concept of himself/herself as a writer. . . . [T]he danger of the single story is the negative effect on students when one piece of writing on a decontextualized prompt is used to represent writing ability. (pp. 125-126)

via http://edr.sagepub.com/content/43/3/125