The Bispectral Index or" BIS" is a single number, composed from different EEG features, using multivariate statistical methods. It was designed for monitoring the depth of hypnosis during anaesthesia and sedation. Empirically derived from the EEGs of more than 5,000 anaesthetised patients, the BIS reflects the effect of anaesthetics on the brain. Unlike univariate measures, such as Spectral Edge Frequency or Median Frequency, the Bispectral Index is drug independent and changes with increasing doses of hypnotics in an almost linear way. Though sophisticated technology, the A-2000 BIS monitor and the BIS sensor are easy to use and also allow non-experts to monitor the depth of hypnosis and sedation on a routine basis.