Stephanie’s Development of Reasoning by Cases to Solve Tower Tasks: Part 3 of 3 (Grade 4)

Author Victoria Krupnik (Rutgers University - graduate) Overall Description This analytic is the third of three analytics that showcase Stephanie’s development of an argument by cases to solve a counting task over two school years. The three analytics focus on the reasoning, argumentation, and mathematical representations constructed by Stephanie in a variety of settings, over time, in third (Part 1) and fourth (Parts 1, 2, and 3) grades. The first analytic begins with Stephanie working with a partner, in a whole class setting, building towers with plastic cubes available in two colors that are 4-tall, in third grade, and 5-tall, in fourth grade. This is followed by events from a first one-on-one interview with a researcher. The second analytic follows with events from a second and third one-on-one interview with a researcher. The third analytic follows with events when Stephanie participated in a small group formative assessment interview and then with a partner on a summative assessment. In this analytic (the third of three), Stephanie’s learning progression is shown as she builds a justification by cases for her solution to Tower Tasks. In fourth grade, Stephanie, solves the 3-tall Tower Task by cases in an attempt to convince a small group of students of her solution and modifies her method in a partner assessment. Her problem solving is presented in a series of events that serve to focus, in detail, on the explanations, reasoning, and argumentation she offers during her problem solving. Events 1-4 are retrieved from a small group assessment interview that occurred on March 10, 1992 with four students, Milin, Michelle, Jeff, and Stephanie, and Researcher Maher (R2). Other researchers, a classroom teacher, and mathematics supervisor were observers. Milin, during his third individual interview, expressed interest to researchers about how other students were solving the problems. Each of the four students in this small group assessment had been interviewed at least once after they participated in the fourth-grade classroom session to solve the 5-tall Tower Task. Their approaches varied and so the researchers decided that sharing approaches with each other was timely. During this group assessment, Stephanie provides a justification by cases for the 3-tall Tower task. Specifically, in Events 1-3 she organizes the eight tower outcomes into cases of no blue cubes, one blue cube, two blue cubes “stuck together,” three blues cubes, and two blue cubes separated. In Event 4 she then repeats her argument by cases, with the help of Michelle, a second time in an attempt to convince Jeff that her towers are all different. Event 5 is retrieved from a June 15, 1992 session in which Stephanie and her partner, Milin, work to solve the 3-tall Tower task at the end of the fourth grade. They construct the solution by a modified case organization and color opposite grouping that includes two single-colored towers, the case of two green cubes, and the opposite case of two black cubes. In this solution she does not separate the cases of two of a color together and two of a color separated. The following definitions and background information about the Tower task are offered. Duplicate This occurs when there is a repetition of color cubes in the possible positions. For Towers, this would be two identical towers or images of towers. Guess and Check The strategy of guess and check involves first guessing an outcome then checking that the outcome is applicable to the solution. Students can be observed using the Guess and Check method when building a tower pattern in a random order (or with no observable method) and then double-checking for duplicate towers (Maher & Martino, 1996). This occurred during the construction and generation of possibilities to obtain a solution. Strategies of locally exhaustive, systematic enumeration (Maher & Martino, 1996, 2000, 2013): Color “Opposites” (children’s language) Each element in a combination containing exactly two types of a particular characteristic, such as color, is replaced with the element of the opposite characteristic. In the combinatorics strand this is known as the strategy of symmetry (Janackova & Janacek, 2006). The opposite of a tower in two colors is a tower of the same height where the cube in each position is the opposite color of the cube in the corresponding position of the first tower. For example, a 4-tall tower with YBBB and a tower with BYYY are considered to be opposites. “Elevator” strategy (Jeff’s language; Milin called this “staircases”) The elevator pattern is used when finding all possible towers containing one cube of one color and the remaining cubes of the other color. The single colored cube is placed in the first position of the first tower (either at the top or the bottom level). To create a second tower, the cube is then moved to the second position (descending, if starting at the top level, or ascending, if starting at the bottom level). The cube is recursively raised or lowered to the next available level to create new towers until it is placed in the final position (Maher, Sran & Yankelewitz, 2011). Case organization and/or argument (an example of globally exhaustive systematic enumeration; Maher & Martino, 1996, 2000, 2013; Batanero et al., 1997). In an organization and/or argument by cases, a statement is demonstrated by showing all the smaller subsets of statements that make up the whole. For example, the solution to the 3-tall Tower Task when selecting from two colors (i.e. blue and red) can be justified by separating the towers into cases using a characteristic of the tower. One such characteristic is the number of cubes of a specific color that the towers contain. In this situation, the towers can be broken down into four cases: 1) towers containing no red cubes (towers with a single color blue); 2) three towers containing one red (towers with exactly one of a particular color); 3) three towers containing 2 red (within cases (2) and (3) can be cubes of same color adjacent to or separated from each other); 4) one tower containing three red, i.e. with all red. An argument by cases would include an exhaustive enumeration of the total number of towers in each case. Equivalent cases The case when selecting m times of a color to place into n positions is equivalent to the case when selecting n–m of the opposite color into n positions. In the example for Towers combinations, selecting two blues to place into five available positions is equivalent to the case of selecting three reds to place into five available positions. The towers with the attribute of exactly two of a color in towers 5-tall and the towers with the attribute of exactly three of the opposite color result in duplicates (e.g., RRBBB have both two reds and three blues). In the figure the first and last tower of each set are duplicates and occur when the strategy of (color opposite) symmetry is applied for each case of adjacent blues and then exhausting all adjacent blue cases, thereby repeating combinations. To avoid repetition, the strategies in combination must be taken with caution. Three-tall Tower Task (selecting from 2 colors): You have plastic cubes of 2 colors available to build towers. Your task is to make as many different looking towers as possible, each exactly 3 cubes high. Find a way to convince yourself and others that you have found all possible towers 3 cubes high, and that you have no duplicates [repetition of same color and order]. Record your towers below and provide a convincing argument why you think you have them all. After completing the Task for Towers 3-tall, describe and justify the approach you have chosen. (The Tower Task can be generalized to towers of any height “n-tall”). Video and Transcript References (in chronological order of Stephanie’s journey): B41, The Gang of Four (Jeff and Stephanie view), Grade 4, March 10, 1992, raw footage. Retrieved from: https://doi.org/doi:10.7282/T3CV4FWP B75, Towers Assessment, WV, Grade 4, Jun 15, 1992, raw. Retrieved from: https://doi.org/doi:10.7282/t3-tpqc-b719 References Batanero, C., Godino, J. 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