This study examines the mental computation strategies used by pupils at the end of primary school. In 1998 a new strategy for teaching mathematics was introduced into British schools (the National Numeracy Strategy) that specifically focused on mental calculation. The first part of the study took place in 1999, one year after the implementation of the National Numeracy Strategy. These results were compared with those obtained in 2005, six years later. The study shows that the National Numeracy Strategy has improved pupils' performance both mentally and when given the option to use written working. But some discrepancies deserve further study; for example, the greater use of the "rounding and adjusting" method for subtraction than for addition. (Contains 3 figures and 5 tables.)