Does assessment hinder learning?

There are some excellent insights in this talk by Professor Dylan Wiliam, one of the original authors of 'Inside the Black Box', on the need for the effective use of assessment to support and encourage learning. Wiliam argues:
If we are to design assessment systems that help rather than hinder learning, we must go beyond looking at the assessments themselves and look at deeper issues about how the assessments help learners and their teachers know where the learners are in their learning, where they are going, and how to get there.

Here are some quotes:
One of the great traps of teaching is the belief that teachers create learning. This is particularly important when teachers are under pressure to improve student results, because studies have shown that when teachers are told they are responsible for making sure that their students do well, the quality of their teaching deteriorates, as does their students’ learning (Deci et al. 1982), hence the old joke about schools being places where children go to watch teachers work.

As a result of their experiences, some students come to believe that ability is fixed. The reason that this is so injurious to future learning is that every time a student with this belief is faced with a challenging task, her or his first reaction is to engage in a calculation about whether they are likely to succeed or not. If they feel confident that they will succeed, or if they feel confident that the task is so hard that many others will fail, they will attempt the task. However, if they feel that there is a danger that they will fail while others will succeed, they will disengage in order to protect their sense of self. Put simply, they are deciding that they would rather be thought lazy than stupid. Given the stark choice between these two, it is the same choice that most adults would make.

Wiliam asks us to imagine:
what would happen if an airline pilot navigated the way that most teachers teach. The pilot would set a course from the starting point (say London) to the destination (say New York). The pilot would then fly on this heading for the calculated time of travel, and then, when that time had elapsed, would land the ’plane at the nearest airport, and upon landing ask “Is this New York?” Worse, even if the ’plane had actually landed in Boston, the pilot would require all the passengers to leave, because he had to get on to his next job.
This would be absurd, and yet, this is how most teachers teach. They teach a topic for two or three weeks, and at the end of that teaching, they assess their students. And whatever the result of that assessment, the teacher is then on to the next topic, because she “has a syllabus to cover”. If we are to ‘keep learning on track’ assessment cannot wait until the end of the topic. Instead, like the pilot, the teacher plans a course but then takes frequent readings along the way, adjusting the course as conditions dictate.