I took some time last night to look through 3rd grade units from all over the map. I was amazed to see that about 95% of them suggested teaching perimeter first. Why is that? The two most common definitions 3rd graders give for area and perimeter are:
- Perimeter: you add up all the sides
- Area: it’s the space inside
The first issue that concerns me is that neither definition makes any connection to measurement. The second being that students are explaining how to solve for perimeter which is not the question I asked.
Perimeter is the distance around a plane figure and area is the amount of two-dimensional surface that is contained within that shape (6th grade teachers would love us if we defined area this way). Area is really difficult for students to conceptually digest…and I get that! As mentioned in my last post, students can easily solve for area by using the formula (length x width) but they have no conceptual understanding of square units (inch, foot, etc).
The Measurement Progressions document start in 3rd grade with the definition of Perimeter and I don’t know if I personally agree with this. Many teachers and textbook companies (I think I just threw up) will take this for meaning that perimeter and its formal definition should be taught before area!!! This can be extremely misleading and detrimental to a fundamental understanding.
What I’m suggesting is this…don’t mention the term perimeter until after students have had ample time exploring area and “covering” a surface!!! If you already do this then you have reaped the benefits!
This year we started with the 3-Act task Piles of Tiles. We worked for 2 full weeks on area and culminated this idea with the Flooring Your House task. By delaying the introduction of the term perimeter it was much easier for students to conceptually see the length (in units) around different figures because of their understanding with area.
The last reason to delay: area and arrays go hand in hand which could lead to students deriving the formula for area themselves…Not the teacher telling students.
So from my experience I know that the egg, the chicken and area all came first!