An apparatus comprises a data processing unit (100) which receives electroencephalographic data based on a measurement of a person (110) exposed to anesthetic drug substance which has one or more estimated or measured non-zero levels of concentration in the body (108) of the person (110) as a function of time； and determines and present activity of slow waves of the electroencephalographic data at the one or more estimated or measured levels of the non-zero concentration of the anesthetic drug substance.
