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The extent of intra-individual and inter-individual variability is animportant factor in determining the statistical, and hence possibly clinical,significance of observed differences in the EEG. This study investigates thechanges in classical quantitative EEG (qEEG) measures, as well as of parametersobtained by fitting frequency spectra to a continuum model of brain electricalactivity. These parameters may have extra variability due to model selectionand fitting. Besides estimating levels of intra-individual and inter-individualvariability, we determine approximate time scales for change in qEEG measuresand model parameters. This provides an estimate of the recording length neededto capture a given percentage of the total intra-individual variability. Also,if more precise time scales can be obtained in future, these may aid thecharacterization of physiological processes underlying various EEG measures.Heterogeneity of the subject group was constrained by testing only healthymales in a narrow age range (mean = 22.3 years, sd = 2.7). Resting eyes-closedEEGs of 32 subjects were recorded at weekly intervals over an approximatelysix-week period. Of these, 13 subjects had follow-up recordings spanning up toa year. QEEG measures, computed from Cz spectra, were powers in five frequencybands, alpha peak frequency, and spectral entropy. Of these, theta, alpha, andbeta band powers were most reproducible. Of nine model parameters obtained byfitting model predictions to experiment, the most reproducible ones quantifiedthe total power and the time delay between cortex and thalamus. About 95% ofthe maximum change in spectral parameters was reached within minutes ofrecording time, implying that repeat recordings are not necessary to capturethe bulk of the variability in EEG spectra likely to occur in the restingeyes-closed state on the scale of a year.

Variability of model-free and model-based quantitative measures of EEG