Abstract To analyse human reliability on performing a specific action, human reliability analysis (HRA) event trees are used involving the breakdown of an action into small elementary steps (events). An example of a specific action, the adjustment of a two-position switch, is analysed and the theory of event trees is explained briefly. The application of event trees in human reliability analysis involves more difficulties than in the case of technical systems, where event trees have been mainly used up until now. The main problem is that the operator is able to rectify his incorrect action, where memory effects play a significant role. In this study these difficulties are dealt with theoretically.