Interference of subtraction strategies
This study concerns a particular kind of mistake that a number of pupils made when subtracting two positive whole numbers. The aim was to analyse the cause behind this particular mistake. According to the pupils, the difference was equal to the subtrahend. It was found that the pupils counted down to the subtrahend. But instead of finding the answer as the number of steps between the two terms, the pupils applied the last-number-word rule and gave the subtrahend, which was the last mentioned number word, as the result. When seeing subtraction as a concept, it could be assumed that the lack of experience of subtraction as a comparison and as equalization played a decisive role for this mistake. A comparable mistake described in previous research is also analysed.
Per-Olof Bentley is senior lecturer at Gothenburg University. He has a Ph D degree in mathematics education and has been responsible for the TIMSS project in Sweden from 2004 to 2008. He has made three indepth analyses of the Swedish pupils mathematical knowledge exposed in TIMSS 2007 and TIMSS Advanced 2008. During three and a half years he had also carried out a large interview study in which about 500 pupils were in-depth interviewed.