why pre assess?
pre assess before teaching
Pre assess ensures we can teach at the sweet spot - neither too easy, nor too hard - that is we can teaching "something new" on "firm learning foundations".
Once learners have finished the finding and firming learning foundations phase and moved on to the teaching on firm learning foundations phase there is no need to do further pre assess.
The sooner the teacher pre assesses ahead of teaching the more accurate 
...
accurate pre assess
A trial and improvement pre assess plan is more efficient at collecting accurate data and less stressful for the learners than a pre assess everything approach.
We've found one question asked and answered to give a false positive approximately 20% of the time, so learner should be asked 2, ideally 3 questions on suitable layers from a topic before the teacher teaches the topic. The easiest way to do this is for the teacher of "new to timely practice learners" to use a pre assess scheme in conjunction with the scheme of learning.
... and the more useful the data is.
using pre assess data improves attainment
When the teacher uses pre assess data to plan their teaching they will find
- the easier and more effective the teaching and learning in the lesson will be,
- and the more speedily and easily the learning of the lesson will become firm foundations for future learning.
How to create a pre assess assignment
The two example mp4s (in the link above) are examples of creating pre assess assignments, following a pre-assess plan which marries with a scheme of learning.
Follow your scheme of learning to know when to create a pre assess assignment.
Follow you pre-assess plan to know what to pre assess.
How to assess a pre assess assignment
The mp4 in the link above shows how to do this.