What are assessments; How often should they be done?

My last post talked about the importance of assessments; however, I feel that I should take a step back and talk about what assessments are and how often they should be done. 

Assessments are defined by Wikipedia as "the process of documenting, usually in measurable terms, knowledge, skills, attitudes and beliefs. ..."

When a teacher begins a new lesson it is important to start with an assessment to have a baseline pre-test.  Depending upon the length of the lesson, I always suggest a pre, mid, and post-test.  If the lesson is ongoing throughout the year, I would suggest assessments at least every month (if not every two weeks).  It is important to assess this often to see if the desired results are achieved, and if not these tests will show the areas of weakness in which the teacher will need to re-cover or re-structure the lesson.

Many packaged programs you can purchase come with assessments of their own; however, most classroom teachers create their own lesson plans they use as well (or in conjunction with).