What is Subitizing and Why Teach It?

My last blog was titled What is Number Sense? How Can I Get It For My Students? 

One of the concepts mentioned in that blog post was subitizing.  This was a new term for me a few years ago but I found out in was first coined in 1949 by E.L. Kaufman and "refers to the rapid, accurate, and confident judgments of number performed for small numbers of items. The term is derived from the Latin adjective subitus (meaning "sudden") and captures a feeling of immediately knowing how many items lie within the visual scene, when the number of items present falls within the subitizing range." Wikipedia

Subitizing is often referred to as a "magnitude of a group".  It is one's ability to look at a group of objects (usually two, three, four, and five) and know which group has more even without counting.  It is also the ability to see four objects and know "perceptually" that it is four without counting.  In other words, it is a visual recognition of number.  Research has found that babies only four months old and many animals (such as birds) have this mathematical skill.   Students coming into kindergarten should hold the skill of subitizing up to five objects.  As I have been diagnostically assessing students I often find that most kindergarteners can subitize three objects but many struggle with four and five objects.  It is the goal that by the end of kindergarten all students can subitize up to five objects. 

Why Teach Subitizing?

1. Provides students with landmarks.  We don't want students to keep counting every individual finger on their hand.   We want them to be able to look at five objects and say "five".
2. "Subitizing is a fundamental skill in the development of students' understanding of number" and the ability to "count on". (Baroody 1987, 115).
3. Subitizing is useful when learning addition and subtraction specifically if using a tens frame.