<p>There is no absolute sample size that is best for a QEEG database because statistically sample size is related to the “effect size” and “power” (Hayes, 1973; Winer, 1971). The smaller the effect size then the larger the sample size necessary to detect that effect. The power of a statistical measure varies as a function of sample size and the effect size (Cohen, 1977). Another issue related to sample size is the degree to which a sample approximates a Gaussian distribution. Increased sample size is often necessary in order to achieve closer approximations to Gaussian which in turn is related to the accuracy of cross-validation. Thus, the sample size is one of several inter-related issues in all normative databases and the sample size should not be singled out as being the most important factor in a QEEG normative database.</p><p>In regard to pediatric EEGs, there are rapid changes in EEG measures during early childhood. For this reason, a relatively large sample sizes is required during the period of time when the brain is changing most rapidly, as well as during old age when potential rapid declines in neural function may occur.</p>