The SNARC effect is commonly taken as an index of spatial-numerical associations. It is not difficult to detect a SNARC effect in single groups but it has been quite unusual to find group differences. Is the SNARC effect largely insensitive to group differences and are there design parameters especially sensitive to group differences? Using numerical simulations, we provide evidence that both sample size and the number of stimulus repetitions contribute in a substantial way to the probability of detecting an existing SNARC effect. Inadequate choice of either sample size or repetitions cannot be easily compensated by the other. Moreover, repeated failures to find significant group differences in the SNARC effect can be explained to a large extent by a chronic lack of power. Fortunately, increasing the number of repetitions to about 20 and testing at least 20 participants in most cases provides sufficient sensitivity to reliably detect the SNARC effect as well as group differences. Power tables are provided, which may be valuable for improving both economy and sensitivity of experimental design in future SNARC experiments.