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Laurie Rogers: In Defense of Using the Number Line to Teach Negatives
by Laurie H. Rogers It’s simple to teach mathematical positives and negatives to a child. It’s been done successfully with the number line around the world, in private schools, homes, tutoring businesses and online. Unfortunately, many schools in America no longer teach the number line, don’t teach it to mastery, or they cloud any fledgling [...]
by Laurie H. Rogers
It’s simple to teach mathematical positives and negatives to a child. It’s been done successfully with the number line around the world, in private schools, homes, tutoring businesses and online. Unfortunately, many schools in America no longer teach the number line, don’t teach it to mastery, or they cloud any fledgling understanding of it by emphasizing other, less-effective methods.
First, I’ll explain the number line. Then I’ll show you what’s being emphasized in its place.
Traditional Math Method Used to Teach Negatives
The Number Line
A number line is a straight line with a series of real numbers listed at intervals. Typically, “zero” is a point in the middle, negative numbers are listed to the left of zero, and positive numbers are listed to the right of zero. Arrowheads are placed at each end to show that the line and numbers continue indefinitely. Each point is assumed to correspond to a real number, and each real number corresponds to a point. Like this:
(Comment: A number line is flexible and comprehensive. It can be drawn with integers, fractions, decimals, irrational numbers or a combination thereof. It can illustrate zero, positive and negative numbers, and small or large numbers. It provides an effective visual for adding and subtracting positive and negative numbers. It can be used to articulate changes in measurements, temperature, money, time or quantities. It’s clear, efficient and functional. It’s easy to draw, explain and understand.)
Naturally, people who love reform math prefer to not use the number line. Various alternate models are used that are not efficient and not comprehensive. Here are a few.
Reform Math Methods Used to Teach … Uh …
These models (with myriad variations) are common to reform curricula. The examples illustrated below are quoted (with permission) from materials originating from two public entities: Henrico County Public Schools and the Georgia Department of Education. The wording is theirs. Please notice that the Georgia materials are said to be based on the Common Core State Standards.
In this model, children are given a story that supposedly illustrates a mathematical calculation or equation.
Example 1: “If you have spent money you don’t have (-5) and paid off part of it (+3), you still have a negative balance (-2) as a debt, or (-5) + 3 = (-2).”
Example 2: “Getting rid of a negative is a positive. For example: Johnny used to cheat, fight and swear. Then he stopped cheating and fighting. Now he only has 1 negative trait so (3 negative traits) – (2 negative traits) = (1 negative trait) or (-3) – (-2) = (-1)”
(Comment: Stories and problems do help to apply math skills that have already been learned, but – as a teaching method – the Story Model provides no real mathematical understanding. The story in Example 2 also attributes adverse characteristics to negatives. In mathematics, a negative is simply a lessening of a quantity, not an adverse characteristic.)
Charged Particles Model
In this model, imaginary items are added to replace items that are already there: “When using charged particles to subtract, 3 – (-4) for example, you begin with a picture of 3 positive particles.”
“Since there are no negative values to ‘take away,’ you must use the Identity Property of Addition to rename positive 3 as 3 + 0. This is represented by 4 pairs of positive and negative particles that are equivalent to 4 zeros.”
“Now that there are negative particles, you can ‘take away’ 4 negative particles. The modeled problem shows that the result of subtracting 4 negative particles is actually like adding 4 positive particles. The result is 7 positive particles. This is a great way to show why 3 – (-4) = 3 + 4 = 7.”
(Comment: How does a child make the leap from 3 particles to 11 particles? Where is the explanation of what’s actually happening? As I was typing in the Charged Particles Method, my brain felt like it was melting. Brain cells began to give up and die. My daughter had to rescue me with chocolate.)
The Stack Model and The Row Model
In the Stack Model, students draw boxes on top of each other in “stacks” and then count them. In the Row Model, students draw boxes in “rows” and then count them. Subtraction for the Stack and Row methods means creating pairs, as in the Charged Particles Method, then “adding zeroes,” crossing out items, and redrawing boxes over and over. This figure illustrates the solution to 3 – (-4) = ?
(Comment: The children will draw a lot of boxes, but they will not come to understand negatives.)
The Postman Model
In this model, a story is provided that supposedly leads students through understanding negatives. Children are to act out parts using manipulatives and props. The concept can be done with other scenarios, but this example from Henrico County School District is based on a mail carrier:
“A postman only brings financial mail. Sometimes she brings bad news, e.g., a bill for $5 = –5. Sometimes she brings good news – a check for $5 = +5. If she brings both you get two pieces of paper but zero dollars. You always start a zero with a cash drawer full of matching checks and bills that equal zero dollars. So if she brings me two checks for $5, no sweat, she helps me by $10, answer = +10. Similarly if she brings me 2 checks for $5 the result is 2 • –5 = –10. Now here is the tricky part: –2 • + 5 = ? Well the – sign means takes away from me. But if we start at zero how can she take anything away? This is where the cash drawer of matching checks and bills saves us. We just take away 2 checks and are left with 2 bills to pay. –2 • + 5 = –10. Similarly, if she takes away our bills, she helps us and the money we would have used to pay the bill can now be spent on bubblegum. –2 • + 5 = +10.”
I think the second “2 checks for $5” is actually supposed to be two bills for $5. Please also remember that young students are the intended audience. The story suggests using a bag filled with Monopoly money and paperclips. Students pretend to deliver mail, cash checks at a bank and pay bills. At some point, however, the mail carrier makes a mistake, and the story goes on to say:
“If it was a check, that would be subtracting a positive. To get the check out of the bank, you would have to pay the bank (which would make you lose money). If it was a bill (taking away a negative), you can keep the money attached to the bill and give the bill back to the mail carrier. This would show that taking away a negative would give you more money.”
(Comment: There is so much wrong with this example, the explanation and the reasoning behind it, it’s hard to know where to begin. The process is complicated; the financial philosophy is suspect. And what does that second paragraph even mean?)
The Balloon Model
In the Balloon Model, things are moved up (added) and down (subtracted). This concept can be done with anything that moves up and down (airplane, elevator, swimmer, etc.), but in the Balloon Model, sand bags represent negatives, and air bags represent positives. This illustration shows -3 + 4 = 1.
(Comment: The Balloon Model is a vertical number line, but it labors to be more relevant by using a balloon (or elevator or airplane or the sea). Thus, the concepts don’t match up with what happens in “real life.” The biggest problem: “Up” and “down” is not equivalent to “more” and “less.”
Balloon Model: In real life, subtracting a sandbag lessens weight, while adding a sandbag adds weight. In the model, however, subtracting a sandbag somehow makes a quantity larger, while adding a sandbag makes it smaller. In the Building and Airplane models, nothing is added or subtracted to make a quantity larger or smaller; the movement up and down is mechanically driven. In the Sea Model, you could say that blowing air out of one’s lungs (subtracting air) makes a person sink, although the change in weight would be negligible. But how does one add air under water?
Where in these models is infinity? Where are fractions and decimals? Where is zero located? At ground level? If so, are negatives below the ground? Since when have balloons and airplanes flown, elevators descended, and swimmers paddled below the ground?)
The (Hey, let’s call them) “Net Changes” model
In reform curriculum Investigations in Number, Data, and Space (aka TERC), students are taught to think of “net changes,” rather than addition and subtraction. Students use manipulatives to act out the “changes.” They count things and keep tallies.
If you search the term “negatives” on the TERC Web site, you’ll have a difficult time finding actual negatives, but games about thinking about negatives are explored. Actually, take a few minutes and skim through the 1st Edition units for TERC. This is the approach to K-6 math that has helped to kill off math proficiency in the United States.
(Comment: Looking through the TERC Web site, you can see the end of mathematics … and possibly all of life as we know it. The school district in my city still uses this curriculum, despite my best efforts to change that.)
Each of the reform math models illustrated here is missing one or more critical concepts, such as zero, negatives, fractions, decimals, large numbers, and/or infinity. They don’t adequately explain negatives, or show that the larger the negative number, the smaller the quantity. They don’t handle all scenarios. Perhaps some are “fun” for a very short while; others are probably no fun at all. Time spent on these models wastes time, builds frustration and creates misunderstandings. Instead of clearly cementing concepts about addition, subtraction and negatives, the children are filling paper bags or talking about “negative” traits.
Without an understanding of the principles behind the number line, students can’t add and subtract negative numbers with proficiency. Without that capability, algebra, geometry and calculus will be beyond them.
I don’t know why so many people in public education seem not to respect the need for efficiency, effectiveness, sufficiency or student proficiency. Math is a tool, used in the “real world” to get a job done. Time is important; efficiency is vital; correct answers are critical. Those who can use math properly will be hired over those who can’t. There is no time in the “real world” to discover methods, struggle with basic math, or constantly ask a “partner” for help. But math reformers seem to think they have all the time in the world. To them, math isn’t about efficiency and correct answers; it’s about struggle, failing, striving and playing games. Many reformers truly believe that if the teaching was efficient, the lesson failed. Thus, they’re motivated to not just prefer the inefficient models, but to actually eliminate efficient models, to mock them and to label them as counterproductive.
Help your children gain a solid grounding in math by teaching them the traditional methods, such as long division, vertical multiplication and the number line. Traditional methods were developed and honed over thousands of years by very clever adults so that they would be efficient. More-efficient models will be developed, no doubt, but the reform models I’ve described are not better models.
Don’t let anyone convince you that efficiency and effectiveness in math are unnecessary or counterproductive. People who actually use math (outside of a K-12 classroom) don’t believe that.
Laurie H. Rogers has a bachelor’s degree in mass communication and a master’s in interpersonal communication, emphasizing the evaluation of argumentation and logic. In 2001, she founded Safer Child, Inc., a nonprofit child advocacy information resource. In 2007, she narrowed her advocacy to public education, and in 2010, she founded Focus on the Square™, a nonprofit organization dedicated to improving American K-12 education.
Laurie is the author of the blog “Betrayed,” located at http://betrayed-whyeducationisfailing.blogspot.com/. Her book Betrayed: How the Education Establishment Has Betrayed America and What You Can Do about It (Rowman & Littlefield Education, 2011) is now available from Amazon and Barnes & Noble.
Besides serving on the executive committee for Where’s the Math?, Laurie has a background in finance, journalism and child advocacy. She has volunteered in schools – tutoring children in literacy and math, and teaching chess, argumentation and knitting. She lives in Spokane with her husband, daughter and two cats.
Contact Laurie Rogers at firstname.lastname@example.org.
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