Electroencephalography (EEG) forms the basis for most depth of anaesthesia (DOA) monitors. Raw EEG has limited use in the measurement of DOA. Thus, efforts were made to compress and simplify the analysis. Numerous monitors have been marketed for this purpose; the most popular include Narcotrend™ (MonitorTechnik, Bad Bramstedt, Germany), M-Entropy™ (GE Healthcare, Helsinki, Finland) and Bispectral Index (BIS™ Medtronic-Covidien, Dublin, Ireland).