Principles and Statistics of Individualized Live and Static Z-Scores

Abstract

This report describes and briefly characterizes a method for computing quantitative EEG (qEEG) z-scores based on a modification of the typical methods used for qEEG reporting. In particular, it describes using a sample of EEG from a single individual, and creating a reference database from the individual sample, in contrast to usinga population of individuals as the source data. The goal of this method is to quantify and localize within-subject changes that may arise due to time or various factors. We refer to this approach as “z-builder,” because the z-score reference is constructed or “built” on a per-subject basis in the office or laboratory and is not derived from a reference obtained from an outside source. It is confirmed that z-scores for EEG acquired during a test period can be calculated based on a single previously recorded reference sample from an individual, and that the resulting z-scores obey the expected statistical distribution. Reference data can be calculated using samples in the 1- to 5-minute range, and subsequent static or dynamic z-scores for a test sample can then be computed using this reference data in lieu of a population database. It is confirmed that, in the absence of systematic change in the EEG, z-scores generally fall well within the range of ±1.0, providing a sensitive indicator whenchanges do occur. It is shown that this method has value in assessing individual stability of EEG parameters and for quantifying changes that may occur due to time effects, aging, disorders, medications, or interventions.