<p>The statistics of replication and independent cross-validation of normative QEEG databases was first applied by E. Roy John and collaborators in 1974 to 1977 (John, 1977; John et al, 1977; 1987). The importance of approximation to a Gaussian distribution was emphasized by both Dr. E. Roy John and Dr. Frank Duffy a Harvard Neurologist in the 1970s and 1980s. In 1994 the American EEG Association produced a position paper in which the statistical standards of replication, cross-validation, reliability and Gaussian approximation were iterated as acceptable basic standards to be met by any normative QEEG database (Duffy, 1994). The American EEG Society included the same standards.</p><p>Matousek and Petersen (1973) computed means and standard deviations in one-year age groups and were the first to use Z scores to compare an individual to the&nbsp;</p><p>normative database means and standard deviations.&nbsp; &nbsp;The Z score is an excellent statistic defined as the difference between the value from an individual and the mean of the population divided by the standard deviation of the population or&nbsp;</p><p style="text-align: center;"><img src="@@PLUGINFILE@@/cross-validation.png" alt="" width="102" height="65" role="presentation" class="img-responsive atto_image_button_text-bottom"><br></p><p style="text-align: center;">This translates to Z= [normative value - observed value]/normative standard deviation</p><p style="text-align: center;"><br></p>