I think that the relationship between formative and summative assesment looks like the following.
According to Wikipedia:
Formative assessment is a self-reflective process that Intends to promote student attainment. Cowie and Bell define it as the bidirectional process between teacher and student to enhance, recognize and respond to the learning. Black and Wiliam consider an assessment ‘formative’ when the feedback from learning activities is actually used to adapt the teaching to meet the learner’s needs. Nicol and Macfarlane-Dick have re-interpreted research on formative assessment and feedback and shown how these processes can help students take control of their own learning (self-regulated learning).
Summative assessment (or Summative evaluation) refers to the assessment of the learning and summarizes the development of learners at a particular time.
Summative assessment is characterized as assessment of learning and is contrasted with formative assessment, which is assessment for learning.
Is my graph appropriate? Is it possible to have an assessment which is both used to guide the learning process and help the learner is therefore formative and also be a snap-shot of what the learner knows? Or are formative and summative assessment mutually exclusive?