Diagnostic assessment is a crucial tool in a teacher's toolkit to help understand the specific areas of strength and weakness in learning, especially before teaching of a particular topic or unit begins. Take fluency and comprehension in reading for example
Reading is a complex skill, but it is made up of two interacting dimensions: visual word recognition and language comprehension. Proficient readers are skilled in both of these dimensions, whilst weaker readers may struggle with one or both of them.  Prioritising one area ensures that effort is spent on the best next step, and not wasted rehearsing skills or content that a child already knows well.
The goal is fluency and comprehension in reading but in the short-term it is critical to identify need and teach accordingly. Pupils may not make good progress for a number of reasons, for example, activities may be too hard or easy, they may be disengaged, or have poor prior knowledge or poor working memory. It is important to take account of the diagnostic assessment data and change the teaching approach. This may mean using a focused intervention or using an approach more suited to the pupil's needs. Where activities are found to be too hard or easy then scaffolding provides a useful analogy.
It may seem counter-intuitive to assess children on something we haven’t yet taught them, but there is good evidence to support this as a strategy to maximise the effectiveness of diagnostic testing. If, as teachers, we acknowledge that children can learn from a range of different stimuli and inputs – including teaching – the usefulness of diagnostic assessment to find out where children ‘are’ before we begin teaching grows. Knowing what children know and which gaps exist in their learning can be an informative exercise; diagnostic assessments and tools such as concept maps can help provide this rich picture.
1. Further details on diagnosing literacy challenges will be available in October 2016 in the forthcoming Guidance Report on Improving Literacy in KS1.