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Tuesday, August 5, 2014

How to Evaluate a Math Textbook

Regardless of Common Core, everybody knows that practically speaking, the textbook IS the curriculum. Therefore, it behooves textbook adoption committees to choose carefully. First, ignore the beautiful graphics. The beauty may truly be only skin deep. Reject books that teach tricks, procedures and shortcuts. Choose books that teach the profound understanding of fundamental mathematics. You do not have to read the entire book. Look especially for how the book handles the following topics:

Place value---Place value is arguably the most essential foundation stone of all future math understanding. Yet most textbooks provide only a rudimentary presentation of place value. Students are expected to do no more than name the place of a given digit or write a certain digit in a given place. The understanding of place value actually begins with counting. Make sure children name what they are counting and start with zero, “0 frogs, 1 frog, 2 frogs, 3 frogs...there are 7 frogs altogether.” Remember, place value depends on fully knowing the name of what is being counted, and not simply as part of a memorized pattern. 203 means you have counted 2 hundreds, 0 tens, 3 ones. 203 can also mean you have counted 20 tens, 3 ones. Which version is more useful depends on the context of the real life math. An early emphasis on place value helps students with later concepts such as fractions (203 thousandths), volume and area (203 cubes vs 203 squares), or the difference between like and unlike terms (3a + 2b). There are many more math concepts that depend first on naming what is being counted and understanding the significance of the name to place value.

The equal sign—An equal sign means everything around the equal sign is equal to everything else. Therefore an expression like 2 + 3 = 5 x 4 = 20 is not allowed because 2 + 3 does not equal 5 x 4. However the separator bar within the vertical format is allowed, because the separator bar does not mean equal; it is a separator bar.

Long Division---Although the idea that division is nothing but repeated subtraction is a bit oversimplified, the long division algorithm exactly depends on repeated subtraction because when you multiply within the algorithm, you are multiplying negative numbers. That is why you subtract the result of the multiplication. Look for a text that presents long division as more than memorizing the steps of the algorithm.

Multiplication and Division of Fractions---½ x 2/3 means one-half of two thirds. This example highlights the value of word problems. Word problems put math where it belongs and from where it arises, that is, math is the solving of real life problems. All math problems have a story. A page of naked problems has simply lost the stories. Suppose I have a ribbon 60 cm long. 2/3 of the ribbon is 40 cm, and half of that is 20 cm. 20 cm is 1/3 of 60 cm. Through examples like this, students can see that ½ x 2/3 = 2/6 = 1/3.

Division works the same way. Say I need to measure ¾ cup sugar and all I have is a 1/8-cup measuring cup. How many times do I need to fill my measuring cup to get ¾ cup sugar? ¾ cup divided by 1/8 cup therefore equals 6 times. (Notice again usefulness of knowing what you are counting. In this example, the answer is counting “times,” not “cups”). Texts should require kids to solve math problems by drawing pictures. When the student can reliably use a diagram to solve a problem, they are ready for the algorithm. Only at the end of the learning process should we teach the shortcuts. Math first, then shortcuts. Pictures are also the first step to proofs.

Absolute Value---Make sure absolute value is presented as distance from zero, NOT as simply a negative number turning into a positive number. A football analogy may help. If the quarterback is sacked, the ball may be 5 yards from the scrimmage line, but from the quarterback's point of view, it is still a negative 5.

Canceling---I loathe this word. Students are not “canceling.” They are simplifying a fraction. Simplifying a fraction means finding “1.” It does NOT mean crossing off numbers. Canceling leads students to lose track of the difference between “0” and “1.”

Multiplying and Dividing Decimals---Multiplying and dividing decimals has nothing to do with moving decimal points. It has everything to do with multiplying or dividing by powers of ten. 12 x 1.4 means 12 times 14 tenths. 14 tenths means 14 divided by ten, so 12 x 1.4 means [(12 times 14) divided by 10], which means 168 divided by 10, which equals 16.8. Students can tell where the decimal point goes, not by counting decimals places but by realizing the answer must be a number close to the product of the whole numbers. 12 x 1 = 12, so the answer must be close to 12. 1.68 is too small. 168 is too big. Therefore the answer is 16.8.

It is easy to confuse students by changing the problem slightly to 12 x 1.40. They will likely say they need to count 2 decimal places so the answer is 1.68. Giving them a new rule about ignoring zeroes does NOT build math understanding. Shortcuts are just that: shortcuts---and should be taught only when the student knows the actual road, not to replace the actual road.

Ignore the glitzy graphics and choose textbooks that handle all these topics well.

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