Although the within subject standard deviation (the standard deviation of repeated observations in the same individual) is unknown for studies with levetiracetam, data from similar research with anti-epileptic medications and published normative data for these tests suggests the within subject standard deviation varies from a standardized 0.1 to 0.6 units. A minimum power of 0.80 ( was selected for the study. A minimum detectable difference in neuropsychological tests scores was set at a 0.5 units (a half a standard deviation in change for a neuropsychological test score). A standardized measure of mean difference was selected as the raw scores of neuropsychological instruments vary, and converting raw scores to standard scores allow all test scores to be compared using a common metric. Based on a two-tailed significance of 0.05 (= .05), the minimum N needed is 18 subjects (Z = 1.96 and  = 0.84). With 20 participants, assuming a within subject standard deviation of 0.5 units and  = 0.05, the minimum detectable difference will be 0.467 standard deviation units (slightly less than a ½ a standard deviation of change). It should be noted that for several neuropsychological tests the within subject change is less than 0.2 SD units. With a smaller within subject SD, and using the same parameters above, N = 20 yields a minimum detectable difference of 0.187 standard deviation units (small effect size). The change in neuropsychological measures found in other antiepileptic medications (e.g., carbamazepine and topiramate) has been from 0.2 to 2.5 units (e.g., Meador et al., 2001).