Time is often short in a class and often the next two phases of the WHAT IF model are lost in the race to the bell. The Investigate phase is the very important step of evaluating how successful the learning activity was.
The main purpose of Investigate is to gather data that can be used to inform you as the teacher of the lesson's success and help you to plan for the next lesson. Have students "got it" and therefore you can move on or do they need more time, more practice, further explanation, etc. The Investigate component also acquires data that can be used to inform students of their progress during the learning process and not just at the end of a topic.
Investigate involves the collection of formative feedback from the students. There are many simple and efficient ways of doing this during the course of the lesson.