The structure of each trial was as follows: (1) a fixation point was presented for 500 ms; (2) the minuend was presented for 1500 ms; (3) the subtraction sign
was presented for 300 ms; (4) the subtrahend was presented for 2500 ms; (5) the proposed solution was presented for 2500 ms or until the response; and (6) a question about strategy use was presented and remained on the screen until the participant gave an answer.
For single digit subtraction problems, 45% of the pilot group said they took the second number away from both sides of the subtraction sign
. For example, with the problem 5-3 a typical explanation was: "I took 3 away from the 5 and the 3." As shown in Appendix A, this is how subtraction was taught.
Unclear how to use the chips to show this, she abandons the model and demonstrates how to solve -1 - (-3) = 2 by showing that the minus sign in front of the 3 and the subtraction sign
combine to make addition of a positive.
A great deal of conversation focused on whether the numbers 14, 13, and 12 could be related to 15 using a subtraction sign
. Ultimately the class determined that the situation could be defined as described in figure 4.