Assessments are formative when they are used to make adjustments to teaching after diagnosing student thinking and uncovering misconceptions.
"Misconceptions are a problem for two reasons. First, they interfere with learning when students use them to interprete new experiences. Second, students are emotionally and intellectually attached to their misconceptions because they have actively constructed them. Hence, students give up their misconceptions with great reluctance." -Jose Mestre in Uncovering Student Thinking in Mathematics 25 Formative Assessments
My Three YouTube Tutorials for Formative Assessment
Why Formative Assessment is Essential
Resources for Concept Cartoons
Make your own Concept Cartoons:
Examples of Formative Assessments Made with Forms & Sites
Books on Formative Assessments & Constructing Explanations