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Cramer, K. & Post, T. (1993, May). Connecting Research To Teaching Proportional Reasoning. Mathematics Teacher, 86(5), 404-407.

 

Connecting Research to Teaching
Proportional Reasoning

Kathleen Cramer and Thomas Post

 

The attainment of proportional reasoning is considered a milestone in students' cognitive development. According to the NCTM's Curriculum and Evaluation Standards (1989), this ability is "of such great importance that it merits whatever time and effort must be expended to assure its careful development (p. 82), As teachers and researchers know, students' understanding of proportionality develops slowly over a number of years. This article reports research findings regarding the learning and teaching of proportional reasoning that have potential for making contributions to classroom practice.

PROPORTIONAL REASONING

Consider the following proportional situation: Three meters equal 300 centimeters, How many centimeters equal 4.5 meters? The relationship between meters and centimeters is multiplicative and can be expressed in either of two ways,

# of cm = 100 (# of m)

or

# of m = (1/100)(# of cm).

The critical component in proportional situations is the multiplicative relationship that exists among the quantities that represent the situation (Cramer, Post, and Carrier, in press). Because of this relationship, all proportional situations can be expressed through an algebraic rule of the form y = mx.

PROPORTIONAL REASONING TASKS: TYPES AND DIFFICULTY

Assessing understanding of this multiplicative relationship has been done in various ways. Learning tasks devised in research studies can be a rich source of creative problem sets for classroom instruction and assessment. Research reports not only suggest varieties of tasks but give information about the relative difficulty of the tasks and factors (e.g., context, numerical complexity) that influence difficulty.

The Rational Number Project developed three different types of tasks to assess proportionality: (1) missing value, (2) numerical comparison, and (3) qualitative prediction and comparison (Post, Behr, and Lesh 1988; Heller et al. 1990). Each problem type was posed to students in four different real world contexts: speed, scaling, mixture, and density (see examples in fig. 1).

Missing-value problems. In missing-value problems three pieces of numerical information are given and one piece is unknown. Karplus's tall-man-short-man problem is representative of this type of problem (Karplus, Karplus, and Wollman 1974). For the tall-man-short-man task, students are given a chain of six paper clips and told that this chain represents Mr. Shores height in paper clips. The students are also told that Mr. Short measures four large buttons tall. They are then told (not shown) that Mr. Tall is similar to Mr. Short but is six large buttons tall. Students are asked to find the height of Mr. Tall in paper clips and to explain their answers. The information in a missing value problem can be represented as rates. In the tall-man-short-man task, 6 paper clips/4 buttons is a complete rate and __ paper clips/6 buttons is an incomplete rate.

Numerical comparison problems. In these problems, two complete rates are given. A numerical answer is not required, however the rates are to be compared. Noelting's (1990) orange-juice task is an example of this type of problem (see fig. 2). Students are told that the shaded glasses represent orange-juice mix and that the unshaded glasses represent water. They are asked to imagine that the orange-juice mix and water are pitcher, Students determine which pitcher has the' strongest-tasting orange juice or if the mixtures would taste the same.

In studying the results of these tasks, researchers found that students were more successful when one quantity in a complete rate was an integral multiple of the corresponding given quantity of the other rate, as, for example, two parts of orange-juice mix and five parts of water to six parts of orange-juice mix and eleven parts of water. When multiples were nonintegral, students often reverted to additive strategies. For example, in solving problem I of figure 1 (speed example), students would often conclude that the answer was eight minutes, reasoning that six miles is two more than four miles so the answer must be two more than six minutes. This typical error in proportional reasoning is discussed at length by a number of researchers (Hart 1981; Noelting 1980).

Qualitative prediction and comparison problems. These types of problems (see problems 3 and 4 in fig. 1) require comparisons not dependent on specific numerical values. Such thinking is a part of proportional thinking and, at least for seventh and eighth graders, is not the same as solving numerical comparison and missing-value problems (Heller et a. 1990). On the one hand, students may use a memorized skill to solve numerical-comparison and missing-value problems. Qualitative-prediction and comparison problems, on the other hand, require students to understand the meaning of proportions. Thinking qualitatively allows students to check the feasibility of answers and to establish appropriate parameters for problem situations. Since this type of thinking is often necessary before actual calculations, the inclusion of such problems in teaching encourages students to use such approaches and improves their calculations and problem solving.

 

Problem 1: Missing-value speed

Lisa and Rachel drove equally fast along a country road. It took Lisa 6 minutes to drive 4 miles. How long did it take Rachel to drive six miles?

Problem 2: Numerical comparison scaling

Anne and Linda are using different road maps of the city. On Anne's map a road 3 inches long is really 15 miles long. On Linda's map a road 9 inches long is really 45 miles long. Who is using the larger city map? a) Anne b) Linda c) Their maps are the same d) Not enough information to tell

Problem 3: Qualitative prediction mixture

If Nick mixed less lemonade mix with more water than he did yesterday, his lemonade drink would taste a) Stronger b) Weaker c) Exactly the same or) Not enough information to tell

Problem 4: Qualitative comparison density

Two friends hammered a line of nails into different boards. Bill hammered more nails than Greg. Bill's board was shorter than Greg's. On which board are the nails hammered closer together? a) Bill's board b) Greg's board c) Their nails are spaced the same d) Not enough information to tell

Fig. 1
Rational Number Project problem types


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Importance of context. Researchers found that problem context, as well as the nature of the numerical relationships, influenced problem difficulty. Of the four contexts studied in the Rational Number Project, scaling was significantly more difficult for middle-grades students than the other contexts. This finding was true even though the numbers in the problems were held constant. However, a proportional reasoner should not be radically affected by the awkward numerical relationships or the context in which the problem is posed. Therefore, in both instruction and testing, teachers should vary the numerical relationships and the context of proportional-reasoning problems,

Besides furnishing teachers with creative assessment items, research tasks can also be a rich source of instructional activities. The task presented in figure 3 can help students develop proportional reasoning and understand the multiplicative relationship inherent in proportional situations. The activity comes from the Rational Number Project and was one of twelve such experiments used by students to highlight the multiplicative relationship in proportional situations. In this activity, students collect data, form a table, and determine an algebraic rule that represents the data. They then transfer the data to a coordinate axis. The rule highlights the multiplicative nature of proportional situations; the graph models the characteristic that in any proportional situation all the rate pairs fall on a straight line crossing the origin. Additional examples for instruction can be found in Cramer, Post, and Behr (1989) and Cramer, Post, and Currier (in press).

 

Problem: You are using the overhead projector in your classroom. You place a red Cuisenaire rod on the projector. You measure its image to be 10 cm long. You wonder what will happen to the images of the other rods. Continue to collect data by placing a purple, a dark green, and then a brown rod in the center of the projector. Measure their images to the nearest cm. What patterns do you see? Can you predict the image length for a line segment 21 cm long?

Solution plan: Measure lengths of rods in cm and record their image lengths. Build a table to show the results.

Rod Color
Rod Length
Image Length
Red
Purple
Dark green
Brown
Questions:

1. Is it possible to determine the length of the image if you know the length of the rod? If the rod is 10 cm long, how long will its image be?

2. Write a formula that can be used to determine the length of the image given the length of the rod. Use R to stand for the length of the rod and / for the image length.

3. Test your formula using the table's number pairs. Use your formula to determine the image length of a 21 -cm line segment. - 4. Graph the data on a pair of coordinate axes and connect the points. 5. Describe the graph.

Note to the teacher: Place the projector so the image length is five times the rod length.

Fig. 3
Activity from the Rational Number Project teaching experiment

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SOLUTION STRATEGIES FOR PROPORTIONAL REASONING TASKS

When seventh and eighth graders attempted missing-value and numerical-comparison problems, success rate was low. Analysis of students' correct responses showed four distinct solutions strategies: unit rate, factor of change, fraction, and cross product. Table 1 shows the percent of problems correct by grade level and by solution strategy. Each strategy will be illustrated by using it to solve the following problem: Steve and Mark were driving equally fast along a country road. It took Steve 20 minutes to drive 4 miles. How long did it take Mark to drive 12 miles?

 
Table 1
Percent of Correct Solutions for Missing-Value (MV)
and Numerical-Comparison (NC) Word Problems
 
Seventh Grade
N = 421
Eighth Grade
N = 492
Method
MV
 
 
NC
MV
 
 
NC
Unit-rate strategy
28*
(15)**
48
(26)
14
(6)
30
(18)
Factor-of-change
strategy
17
(5)
12
(5)
7
(3)
8
(2)
Fraction strategy
2
(1)
8
(10)
12
(2)
26
(25)
Cross-product
algorithm
3
(4)
1
(1)
33
(45)
10
(15)
Incorrect
50
(75)
31
(57)
35
(44)
32
(40)

*The first entry is an average of three problems; in each problem all numerical values were integral multiples of one another.

**The entry in parenthesis is an average for a single problem whose numerical values were act integral multiples of one another.

 
The unit-rate strategy. As the name implies this is a "how many for one?" strategy. It always involves two rates corresponding to a given pair of quantities: 20 minutes/4 miles and its reciprocal, 4 miles/20 minutes. The first pair can be represented by the unit rate: 5 minutes per 1 mile; the second by the unit rate: 1/5 mile per 1 minute. Since the question about Mark asks for the amount of time to drive twelve miles, the unit rate describing the length of time for one mile is used ([5 minutes/ 1 mile] x 12 miles = 60 minutes). If the question was, How many miles did Mark drive in 60 minutes? The other unit rate would be used ([l/5 mile/ 1 minute] x 60 minutes = 12 miles).

The factor-of-change strategy. This is a "times as many" strategy. A student using this method would reason as follows: "It takes twenty minutes to drive four miles. Since Mark is driving three times as far, it should take him three times as long. So the answer is twenty minutes times three, or sixty minutes." The ease in using this method is related to the numerical aspects of the problem. Students would be less apt to use this method if the factor to be used was not an integer for example, if the factor was 2/7, as it would be if the problem had been, It takes Steve 20 minutes to drive 7 miles, how long will it take Mark to drive 2 miles? (2/7 of 20 is 40/7, or 5 and 5/7 minutes).

The fraction strategy. When using rates, the labels for each quantity are usually kept in the expression. This strategy is used when using the unit-rate strategy. If students dropped the labels and used ideas of equivalence, it is a fraction strategy. Students treated the rates as fractions, applying the fraction rule for equivalent fractions (multiply the numerator and denominator by 3) to calculate the answer of 60.

The cross-product algorithm. As with many standard algorithms, this is an extremely efficient but mechanical process devoid of meaning in the real world. To solve the problem about Steve and Mark, a student sets up a proportion, forms a cross product, and solves the resulting equation by division:

20 minutes
4 miles
=
? minutes
12 miles
20 minutes x 12 miles
=
? minutes x 4 miles
20 minutes x 12 miles
4 miles
=
? minutes
60
=
?
 

The unit-rate strategy was used most by seventh graders in the Rational Number Project study. The unit rate seemed to be an intuitive approach building on students' real-life experiences, whereas the cross-product algorithm used most often by eighth graders seemed more contrived. (What meaning does 20 minutes x 12 miles have?) The more intuitive unit-rate and factor-of change strategies related more meaningfully to the situation. On the one nonproportional problem included on the assessment, eighth graders were less successful (close to 30 percent inappropriately applied the cross-product algorithm to this task). Seventh graders, who had not learned this algorithm, were more successful and solved the nonproportional problem using other problem-solving strategies. Knowing how to do a procedure does not mean that a student knows when it can be applied, and value is found in intuitive methods.

Since students not instructed in the cross-multiplication algorithm used the unit-rate and factor-of-change strategies to solve proportion problems, teachers may capitalize on these natural thought patterns and begin instruction focusing on these strategies. A sample lesson can be found in a Mathematics Teacher article by Cramer, Bezuk, and Behr (1989).

CONCLUSION

Research can have a positive effect on classroom practice. This article suggests that research tasks can and should be used in classrooms. They can function as instructional activities as well as assessment tools. The types of problems generated by research inform teachers of the different ways in which understanding can be assessed. Being a proportional reasoner means more than applying the cross-product algorithm. Students should be able to solve multiple-problem types, including missing value, numerical comparison, qualitative comparison, and qualitative prediction. Just as researchers analyze the effect of different variables on performance, teachers can consider these same variables as they teach and assess. Two important variables include context and the presence of integral multiples. Instruction should start with familiar contexts and extend to less familiar ones. With the availability of calculators, proportional problems should include "nasty" numbers so that students can encounter nonintegral relationships early on. Research suggests teaching multiple strategies, including unit rate, factor of change, fractions, and the cross-product algorithm. Teachers should begin instruction with more intuitive strategies, such as the unit rate and factor of change. These teaching suggestions emphasize learning concepts over learning procedures, As textbooks often focus on procedural knowledge, teachers will have to go beyond the content of textbooks to offer meaningful instruction for this important domain.


Edited by Anna 0. Graeber
University of Maryland
College Park, MD 20742

Kathleen Cramer and Tom Post teach at the University of Wisconsin, River Falls, WI 54022. Their research interests involve studying children's learning of, and teachers' conceptions of, rational number and proportionality concepts.


The preparation of this article was supported in part by the National Science Foundation, NSF DPE 84-70077, Any opinions, findings, conclusions, and recommendations. are those of the authors and do not necessarily reflect the views of the National Science Foundation.


REFERENCES

Cramer, Kathleen, Nadine Bezuk, and Merlyn Behr. "Proportional Relationships and Unit Rates." Mathematics Teacher 82 (October 1989):537-44.

Cramer, Kathleen, Thomas Post, and Merlyn Behr, "Interpreting Proportional Relationships." Mathematics, Teacher 82 (September 1989):445-53.

Cramer, Kathleen, Thomas Post, and Sara Currier. "Learning and Teaching Ratio and Proportion: Research Implications." In Research Ideas for the Classroom: Middle Grades Mathematics, edited by Douglas Owens, Reston, Va.: National Council of Teachers of Mathematics and Macmillan. In press.

Hart, Kathleen. "Ratio and Proportion." In Children's Understanding of Mathematics 11 16, edited by Kathleen Hart, 88-101. London: John Murray, 1981.

Heller, Patricia, Thomas Post, Merlyn Behr, and Richard Lesh. "Qualitative and Numerical Reasoning about Fractions and Rates by Seventh- and Eighth-Grade Students." Journal for Research in Mathematics Education 21 (November 1990): 388-402.

Karplus, Elizabeth, Robert Karplus, and Warren Wollman. "Intellectual Development Beyond Elementary School IV: Ratio, the Influence of Cognitive Style." School Science and Mathematics 74 (October 1974): 476-82.

National Council of Teachers of Mathematics. Curriculum and Evaluation Standards for School Mathematics, Reston, Va.: The Council, 1989.

Noelting, Gerald. "The Development of Proportional Reasoning and the Ratio Concept: Part 1-the Differentiation of Stages." Educational Studies in Mathematics 11 (May 1980):217-53.

Post, Thomas, Merlyn Behr, and Richard Lesh. "Proportionality and the Development of Prealgebra Understandings." In The Ideas of Algebra, K-12, 1988 Yearbook of the National Council of Teachers of Mathematics, edited by Arthur Coxford and Albert Shulte, 78-90. Reston, Va.: The Council, 1988.